Design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas

Abstract

The present invention discloses a design method for a pile-slab anti-deformation foundation structure of a dense building group in a shallow old empty area: the relative position relationship between the planned dense building group area and the influence range of the shallow old empty area is analyzed to determine the governance range of the planned dense building group; the width of the enclosure belt is determined based on the protection level of the dense building group, and the reinforcement area of ​​the dense building group is delineated; the number and distribution position of the cast-in-place piles in the reinforcement area are determined based on the load distribution of the reinforcement area of ​​the dense building group; the relative relationship between the critical mining depth of the shallow old empty area, the height of the water-conducting fracture zone and the critical depth of the load influence of the dense building group is comprehensively considered to determine the depth of the cast-in-place piles in different reinforcement areas, and construct the cast-in-place piles; the concrete cap is arranged according to the distribution of the dense building group to form a "cast-in-place pile + cap" pile-slab anti-deformation foundation structure. The present invention can solve the long-term stability of the overlying rock strata and the surface of the shallow old empty area and its hidden dangers to the safe operation of the dense building group.

Description

Technical Field

[0001] The invention relates to the technical field of anti-deformation of densely packed buildings in old empty areas, and more specifically to a design method for pile-slab anti-deformation foundation structure of densely packed buildings in shallow old empty areas. Background Art

[0002] The long-term exploitation of shallow coal resources has formed a large area of ​​old coal mine voids. The old voids have the characteristics of small deformation, long duration, and difficulty in monitoring. At the same time, with the continuous development of the coal industry, towns and engineering construction, the area of ​​coal mining subsidence areas has gradually increased, and the land available for construction in the mining area is extremely tight, or even there is no land to build. The contradiction between the increasing area of ​​coal mining subsidence areas and the tight land for engineering construction has become increasingly prominent, and has become a constraint factor for engineering construction and urban planning in my country. Dense building clusters refer to groups of buildings that appear in the form of contiguous pieces or zones, or groups of buildings of the same type that appear in the form of groups. When a building (group) is built above the old void, the additional stress caused by the building load will inevitably break the balanced stress state of the old void and the overburden, resulting in the "activation" of the old void, the instability of the foundation, and it is difficult to ensure the stability of the building (group). The existing anti-deformation measures for buildings above the goaf include: changing the building type, setting deformation joints, setting buffer trenches, anti-deformation integral foundations and using jack systems. These measures can increase the foundation stability of buildings (groups) in old goafs, but for densely populated surface buildings (groups) with large loads and poor foundation stability, the reinforcement effect of this method is still limited.

[0003] With the depletion of shallow coal resources, coal mining has further developed into deeper areas, and a large number of legacy old empty areas have been formed in shallow mining areas. At the same time, shallow old empty areas are easier to activate under ground loads, have low costs, and are convenient for anti-deformation engineering design. Therefore, the design of anti-deformation foundations for buildings (groups) in shallow old empty areas is particularly important.

[0004] The patent with application number 201810265966.6 discloses an anti-deformation composite raft foundation suitable for buildings in goaf areas and its construction method. The anti-deformation composite raft foundation includes a cushion layer, a rigid cushion layer, a viscous sliding layer and a double-layer raft, and anchor rods are arranged inside the foundation; the raft foundation has the characteristics of large overall stiffness and strong horizontal deformation resistance, and is effective in resisting uneven settlement; however, the measure of using the raft foundation to resist the foundation deformation caused by the goaf does not consider the demarcation of the protection range of the house, and due to the use of the raft foundation, this method is effective for general foundation reinforcement. For conditions where there is a loose layer (especially a quicksand layer with strong fluidity), the cost of excavating the foundation too deep is too high.

[0005] The patent application number 201811056816.0 discloses a high-rise building foundation reinforcement protection structure, which arranges multiple cast-in-place piles around the high-rise building, and uses reinforced concrete floor to tie the cast-in-place piles and the foundation of the high-rise building together, and sets concrete peripheral beams on the outside of the cast-in-place piles to apply lateral pre-stress to the cast-in-place piles. This reinforcement method is simple and fast to construct, which is conducive to the construction progress and can avoid the relocation of high-rise buildings; although the use of cast-in-place piles and pouring reinforced concrete floor can effectively improve the bearing capacity of the foundation, the protection effect is poor when applied to large and dense buildings (groups), and it is difficult to bear the load of the building (group) by relying solely on a single reinforced concrete floor. Summary of the invention

[0006] To this end, the technical problem to be solved by the present invention is to provide a design method for a pile-plate type anti-deformation foundation structure for densely packed building groups in shallow old empty areas. This design method connects the pile-plate foundation with the coal seam bottom plate to achieve the goal of stabilizing the foundation, thereby ensuring the long-term stability of densely packed surface buildings (groups) in shallow old empty areas from the root, so as to solve the problem of foundation instability caused by "activation" in shallow old empty areas, as well as the problem of weak bearing capacity of the foundation of densely packed building groups in shallow old empty areas and poor deformation resistance of the foundation of densely packed building groups.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas includes the following steps:

[0009] Step (1): Analyze the relative position relationship between the proposed densely built area and the impact range of the shallow old empty area, and determine the governance scope of the proposed densely built area;

[0010] Step (2): Determine the width of the enclosure belt based on the protection level of the dense building group and delineate the reinforcement area of ​​the dense building group;

[0011] Step (3): Taking the size of the reinforcement area of ​​the dense building group as the technical indicator and based on the load distribution of the reinforcement area, determine the theoretical number and distribution position of the cast-in-place piles in the reinforcement area;

[0012] Step (4): comprehensively considering the relative relationship among the critical mining depth of the shallow old empty area, the height of the water-conducting fracture zone and the critical depth of the load impact of the dense building group, determine the depth of the bored piles in different reinforcement areas, and construct the bored piles;

[0013] Step (5): Arrange the concrete cap according to the distribution of densely packed buildings, so that the concrete cap and the bored piles form a "bored pile + cap" pile-plate type anti-deformation foundation structure, thereby improving the overall structural strength of the foundation.

[0014] In the above-mentioned design method for pile-slab anti-deformation foundation structure of densely packed buildings in shallow old empty areas, in step (1), the influence range of the shallow old empty area is calculated by the following formula:

[0015]

[0016]

[0017] d q1 =h 下 *cot(β-Δβ); (1-3)

[0018] d q2 =h 上 *cot(γ-Δγ); (1-4)

[0019] In formula (1-1), A is the length of the influence range of the shallow old void area, a is the strike length of the shallow old void area, h j is the thickness of bedrock in the shallow old void area, h s is the thickness of the loose layer in the shallow old empty area, c is the extension outside the influence range considering the safety factor, and c=1.0*(h j +h s ), σ is the strike movement angle of the shallow old goaf area, Δσ is the correction value of the strike movement angle of the shallow old goaf area; It is the movement angle of loose layer in shallow old void area;

[0020] In formula (1-2), B is the width of the influence range of the shallow old void area, b is the dip length of the shallow old void area; d q1 is the horizontal distance from the edge of the downhill mining area to the boundary of the shallow old void area, d q2 h is the horizontal distance from the edge of the uphill mining area to the boundary of the shallow old void area, s is the thickness of the loose layer in the shallow old empty area, c is the extension outside the influence range considering the safety factor, and c=1.0*(h j +h s ), It is the movement angle of loose layer in shallow old void area;

[0021] In formula (1-3): d q1 h is the horizontal distance from the edge of the downhill mining area to the boundary of the shallow old void area, 下 is the thickness of bedrock in the downhill direction along the shallow old empty area; β is the downhill movement angle of the shallow old empty area along the coal seam, and Δβ is the correction value of the downhill movement angle of the shallow old empty area along the coal seam;

[0022] In formula (1-4): d q2 h is the horizontal distance from the edge of the uphill mining area to the boundary of the shallow old void area, 上is the thickness of bedrock in the upward direction along the shallow old void area; γ is the upward movement angle of the shallow old void area along the coal seam; Δγ is the correction value of the upward movement angle of the shallow old void area along the coal seam.

[0023] The above-mentioned pile-slab anti-deformation foundation structure design method for densely packed buildings in shallow old empty areas and the method for determining the scope of treatment of densely packed buildings to be built are as follows:

[0024] When the area where densely packed buildings are planned to be built is outside the influence range of the shallow old empty area: the area where densely packed buildings are planned to be built does not need to be reinforced;

[0025] When part of the area where densely packed buildings are planned to be built is located within the influence range of the shallow old empty area: the part of the densely packed buildings located within the influence range of the shallow old empty area shall be the governance scope;

[0026] When the area where a densely populated group of buildings is planned to be built is entirely within the influence range of the shallow old empty area, the area where a densely populated group of buildings is planned to be built is the scope of governance.

[0027] The above-mentioned pile-slab anti-deformation foundation structure design method for densely packed buildings in old empty areas in shallow areas is that when the protection levels of the densely packed buildings are level I, level II, level III and level IV, the widths of the retaining belts are 20m, 15m, 10m and 5m respectively.

[0028] In the above-mentioned design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas, the reinforcement range of densely packed buildings is calculated by the following formula:

[0029] l=2f+d; (2-1)

[0030] w=2f+e; (2-2)

[0031] In formula (2-1) and formula (2-2): d is the length of the dense building group within the influence range of the shallow old empty area, e is the width of the dense building group within the influence range of the shallow old empty area; l is the length of the reinforcement range of the dense building group to be reinforced, w is the width of the reinforcement range of the dense building group to be reinforced; f is the width of the protective belt.

[0032] In the above-mentioned design method for pile-slab anti-deformation foundation structure of densely packed buildings in shallow old empty areas, in step (3), the method for determining the number and layout positions of cast-in-place piles at each position in the reinforcement area is as follows:

[0033] (3.1) A total of m cast-in-place piles are arranged at the important corner points outside the reinforcement area and the center of the reinforcement area; the criteria for judging the important corner points outside the reinforcement area are: the corner points of two adjacent buildings outside the reinforcement area and the positions of the load-bearing columns of the buildings located at the corner points of the reinforcement area are all important corner points outside the reinforcement area; the method for defining the center of the reinforcement area is: the intersection of the diagonals is the center of the regular reinforcement area, and the approximate geometric center is the center of the irregular reinforcement area;

[0034] (3.2) m0 bored piles are arranged at non-important corner points outside the reinforcement area. The bored piles arranged outside the reinforcement area are arranged continuously and evenly according to the grout penetration range of a single bored pile, that is, the grout penetration ranges of two adjacent bored piles are continuous and do not overlap, so that the m0 bored piles can be connected to the bored piles at the important corner points outside the reinforcement area as a whole;

[0035] (3.3) The theoretical total number n of cast-in-place piles arranged in the reinforcement area except the central position is determined by the following calculation formula:

[0036] Q uk =Q sk +Q pk ; (3-1)

[0037]

[0038]

[0039] In formula (3-1): Q uk is the standard value of the vertical ultimate bearing capacity of a single pile; Q sk is the standard value of total limit lateral resistance; Q pk is the standard value of total limiting end resistance;

[0040] In formula (3-2): R a is the characteristic value of the vertical bearing capacity of a single pile, in kN; K is the safety factor; Q uk is the standard value of the vertical ultimate bearing capacity of a single pile;

[0041] In formula (3-3): n is the initial estimated number of piles, which is an integer; F k is the vertical force on the top surface of the pile foundation cap under different standard combinations of load effects, in kN; R a is the characteristic value of the vertical bearing capacity of a single pile, in kN; m is the total number of cast-in-place piles at important corner points outside the reinforcement area and at the center of the reinforcement area;

[0042] The n cast-in-place piles in the reinforcement area are distributed on one or more arcs in the reinforcement area, and the spacings between adjacent cast-in-place piles on the same arc are equal, and the spacings between two adjacent arcs are equal.

[0043] In the above-mentioned design method for pile-slab anti-deformation foundation structure of densely packed buildings in shallow old empty areas, in step (4), the critical mining depth H of shallow old empty areas is compared. 临 , height of water-conducting fracture zone H 导 and the critical depth H affected by the load of densely packed buildings 荷 The relative relationship between the three; when the height of the water-conducting fracture zone H 导 Less than the bedrock thickness h in the shallow old void area j , that is, H 导 <h j hour:

[0044] ①If H 导 +H 荷 <h j When the thickness of the bedrock in the shallow old empty area is greater than the sum of the height of the water-conducting fracture zone and the critical depth of the load influence of the dense building group, the arrangement method of the cast-in-place pile depth in the reinforcement area is:

[0045] a. The depth of the m cast-in-place piles located at the important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area;

[0046] b. m0 cast-in-place piles are arranged at non-important corners outside the reinforcement area, with the pile depth penetrating the loose layer to the bedrock;

[0047] c. The depth of the cast-in-place piles located in the reinforcement area except the center position penetrates the loose layer to the bedrock position; at this time, the number of cast-in-place piles in the reinforcement area except the center position is n. z is (1 / 5~1 / 4)n, further method n z Take an integer, n z The cast-in-place piles are distributed on one or more arcs, and the spacing between adjacent cast-in-place piles on the same arc is equal;

[0048] ②If h j <H 导 +H 荷 <H 临 When the sum of the height of the water-conducting fracture zone and the critical depth of the load influence of dense buildings exceeds the bedrock thickness of the shallow old void area, but is less than the critical mining depth of the shallow old void area, the arrangement method of the cast-in-place pile depth in the reinforcement area is:

[0049] a. The depth of the m cast-in-place piles located at the important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area;

[0050] b. m0 cast-in-place piles are arranged at non-important corner points outside the reinforcement area, and the pile depths are respectively through the loose layer to the bedrock position and to the position where the movement angle of the old void area is demarcated. The cast-in-place piles with a pile depth of through the loose layer to the bedrock position and the cast-in-place piles with a pile depth of to the position where the movement angle of the old void area is demarcated are arranged alternately;

[0051] c. Cast-in-place piles located outside the center of the reinforcement area: The pile depths are respectively from the loose layer to the bedrock and from the old empty area to the defined range of the moving angle, and the cast-in-place piles from the loose layer to the bedrock and from the old empty area to the defined range of the moving angle are arranged alternately; at this time, the number of cast-in-place piles constructed outside the center of the reinforcement area is n. z is (1 / 3~1 / 2)n, further method n z Take an integer, n z The cast-in-place piles are distributed on one or more arcs in the reinforcement area, and the spacing between adjacent cast-in-place piles on the same arc is equal;

[0052] ③If H 临 <H 导 +H 荷 When the critical mining depth of the shallow old empty area is less than the sum of the depth of the water-conducting fracture zone and the critical depth of the load influence of the dense building group, the arrangement method of the pile depth of the reinforced area is:

[0053] a. The depth of the m cast-in-place piles located at the important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area;

[0054] b. m0 cast-in-place piles are arranged at non-important corner points outside the reinforcement area, with a pile depth to the position defined by the moving angle of the old empty area;

[0055] c. Cast-in-place piles located outside the center of the reinforcement area: The pile depths are respectively from the loose layer to the bedrock and from the old empty area to the defined range of the moving angle, and the cast-in-place piles from the loose layer to the bedrock and from the old empty area to the defined range of the moving angle are arranged alternately; at this time, the number of cast-in-place piles constructed outside the center of the reinforcement area is n. z is the theoretical value n, and n in the reinforcement area z The cast-in-place piles are distributed on one or more arcs in the reinforcement area, and the spacing between adjacent cast-in-place piles on the same arc is equal, and the spacing between two adjacent arcs is equal;

[0056] When the height of the water-conducting fracture zone H 导 Greater than the thickness of bedrock in the shallow old void area h j , that is, H 导 >h jWhen: the height of the water-conducting fracture zone has far exceeded the thickness of the bedrock in the shallow old void area, the rock layer and the surface of the shallow old void area are severely damaged. The arrangement of the cast-in-place piles in the reinforcement area is consistent with the critical mining depth H of the shallow old void area. 临 Less than the depth H of the water-conducting fracture zone 导 The critical depth H affected by the load of densely packed buildings 荷 The arrangement is the same when the sum is .

[0057] The above-mentioned pile-slab anti-deformation foundation structure design method for densely packed buildings in shallow old empty areas, the critical mining depth H of shallow old empty areas 临 , height of water-conducting fracture zone H 导 and the critical depth H affected by the load of densely packed buildings 荷 The calculation methods of the three are as follows:

[0058] H 临 =h j +h s ; (4-1)

[0059]

[0060]

[0061] In formula (4-1): h j is the thickness of bedrock in the shallow old void area, h s is the thickness of loose layer in shallow old void area;

[0062] In formula (4-2): M is the thickness of coal seam;

[0063] In formula (4-3): B is the width of the roadway, γ is the density of the overlying rock layer above the roadway, and P0 is the unit pressure at the base of the building; is the internal friction angle of the overlying rock strata of the roadway.

[0064] The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas mentioned above, the construction process of cast-in-place piles is as follows:

[0065] The bored piles are constructed according to the determined bored pile depth; the soil-extracting drill rod is inserted into the steel pipe pile, and the soil is continuously excavated and extracted with the soil-extracting drill rod; the vibration equipment at the bottom of the steel pipe pile will shatter the gravel and broken stones encountered during the pile foundation, so as to facilitate the entry of the soil at the head of the steel pipe pile; through holes are provided at the bottom of the steel pipe pile and on the steel pipe body, so as to facilitate the cast bored pile to be bonded with the surrounding soil as a whole to increase the stress strength; after the construction of the steel pipe pile is completed, the steel cage is lowered so that the steel cage reserved at the top of the bored pile is higher than the ground, which is convenient for connection with the concrete cap; after the steel cage is lowered, the concrete is poured; the steel bar model of the steel cage is HPB300; the concrete is C30 concrete or other concrete with strength greater than C30 concrete.

[0066] In the above-mentioned design method of pile-slab anti-deformation foundation structure for densely packed building groups in shallow old empty areas, in step (5), the construction method of the "cast-in-place pile + cap" pile-slab anti-deformation foundation structure is as follows: concrete caps are arranged based on the distribution characteristics of the densely packed building groups as basic units, and deformation joints are left between two adjacent concrete caps to achieve a relatively stable state; the steel frame of the concrete cap is connected to the steel cage reserved above the cast-in-place piles by binding to increase the overall structural strength;

[0067] The construction method of the concrete cap is as follows: excavate the soil in the concrete cap construction area with the dense building group distribution characteristics as the basic unit; the distance between the pile foundation excavation soil outside the reinforcement area and the concrete cap construction area is greater than 2000mm, and the depth of the excavation soil in the concrete cap construction area is 2500-3500mm; set a cushion layer at the bottom of the excavated soil in the concrete cap construction area, and the cushion layer thickness is 800-1200mm; above the cushion layer is a steel cage reserved for binding with the top of the cast-in-place pile, and the steel model is HPB300; use cement concrete with a flexural strength greater than 4.5MPa for pouring, and the template height is 20cm higher than the uppermost part of the steel cage during construction; after the concrete cap is poured, cultivate the edge of the concrete cap pouring area with plain soil, and the compaction degree of the plain soil is greater than or equal to 90%, to ensure the overall stability of the integrated "pile + slab" foundation;

[0068] The cushion layer is composed of the following graded crushed stone: crushed stone with a particle size of 20 to 40 mm, crushed stone with a particle size of 10 to 20 mm and stone chips with a particle size of less than 5 mm, and the mass ratio of the three is 4:4:2; the soil content in the crushed stone is less than 5wt%, and the content of stone powder in the stone chips is 15 to 25wt%; during the cushion layer construction process, the various components of the cushion layer are laid and then rolled until the compaction degree reaches more than 93%.

[0069] The technical solution of the present invention achieves the following beneficial technical effects:

[0070] 1. The present invention provides a method for designing pile-slab anti-deformation foundations for densely packed building groups in shallow old empty areas based on the characteristics of small mining depth and low management cost in shallow old empty areas, so as to achieve the goal of strengthening the bearing capacity of the foundation of densely packed building groups in shallow old empty areas, ensuring the deformation resistance of the foundation of densely packed building groups, and ensuring the safe operation of densely packed building groups above shallow old empty areas, and effectively solves the problems of long-term stability of the overlying rock strata and the surface of the shallow old empty areas and their hidden dangers to the safe operation of densely packed building groups in the prior art.

[0071] 2. Aiming at the problem that the foundations of densely packed buildings in shallow old empty areas are easily affected by "activation" and thus become unstable, the present invention adopts anti-deformation measures of combining piles and plates on the foundations, which effectively increases the overall strength of the foundation structure. Specifically: 1) It reduces the hidden dangers of coal mining in shallow old empty areas to the safe operation of densely packed buildings on the surface; 2) It increases the bearing capacity of the foundations of shallow old empty areas for densely packed buildings on the surface; 3) Compared with the method of changing the foundation type of the anti-deformation frame structure, it greatly increases the stability of the foundation, providing a new method and idea for the management and utilization of coal mining subsidence areas.

[0072] 3. The present invention defines the reinforcement scope of dense building groups according to the relationship between mining parameters, dense building group management scope and retaining belt in mining subsidence; by determining the number, distribution and pile depth of the bored piles in the pile-plate type anti-deformation foundation structure, and by controlling the casting conditions of the bored piles and the concrete cap, an anti-deformation foundation structure of "bored piles + concrete cap" is designed, which ensures the stability of dense building groups in shallow old empty areas to the greatest extent.

[0073] 4. The present invention proposes a method for defining the reinforcement range of densely packed buildings and a basis for determining the number and depth of cast-in-place piles in the pile-sheet combined grouting reinforcement anti-deformation measures. This method can better reinforce the foundation of densely packed buildings and ensure the stability of the anti-deformation foundation structure of densely packed buildings in shallow old empty areas. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1a Schematic diagram (trend) of determining the influence range of the shallow old empty area in the embodiment of the present invention;

[0075] Figure 1b Schematic diagram (tendency) of determining the influence range of the shallow old void area in the embodiment of the present invention;

[0076] Figure 2a Schematic diagram of the situation in which "part of the area to be built with densely packed buildings is located within the influence range of the shallow old empty area" in the embodiment of the present invention;

[0077] Figure 2b Schematic diagram of the situation in which "all areas where densely packed buildings are planned to be built are located within the influence range of shallow old empty areas" in an embodiment of the present invention;

[0078] Figure 3 Schematic diagram of determining the reinforcement area of ​​a densely packed building complex in an embodiment of the present invention;

[0079] Figure 4 A schematic diagram of the distribution of the depths of the cast-in-place piles at the corners and the center of the reinforcement area in an embodiment of the present invention;

[0080] Figure 5Schematic diagram of uniform distribution of cast-in-place piles in the reinforcement area according to the embodiment of the present invention;

[0081] Figure 6a Schematic diagram of the depth distribution of cast-in-place piles in the reinforcement area according to the embodiment of the present invention (H 导 +H 荷 <h j );

[0082] Figure 6b Schematic diagram of the depth distribution of cast-in-place piles in the reinforcement area according to the embodiment of the present invention (h j <H 导 +H 荷

[0083] <H 临j );

[0084] Figure 6c Schematic diagram of the depth distribution of cast-in-place piles in the reinforcement area according to the embodiment of the present invention (H 导 >h j );

[0085] Figure 7a Schematic diagram of deep cross section of cast-in-place piles in the reinforcement area according to an embodiment of the present invention (inclination);

[0086] Figure 7b A schematic diagram of a deep cross section of a bored pile in a reinforcement area according to an embodiment of the present invention (direction);

[0087] Figure 8 Schematic diagram of the pile-plate type anti-deformation foundation structure of "cast-in-place piles + cap" in the reinforcement area in an embodiment of the present invention;

[0088] Fig. 9 Schematic diagram of the design of a pile-slab anti-deformation foundation structure for a densely packed building group in an embodiment of the present invention. DETAILED DESCRIPTION

[0089] In this embodiment, the design method of pile-slab anti-deformation foundation structure of densely packed buildings in shallow old empty areas is used to design pile-slab anti-deformation foundation structure of densely packed buildings in shallow old empty areas of a certain mining area, specifically including the following steps:

[0090] Step (1): Analyze the relative position relationship between the proposed densely built area and the impact range of the shallow old empty area, and determine the governance scope of the proposed densely built area;

[0091] First, according to the size of the shallow old void area and the geological mining conditions, the impact range of the shallow old void area is calculated using the following formula:

[0092]

[0093]

[0094] d q1 =h 下 *cot(β-Δβ); (1-3)

[0095] d q2 =h 上 *cot(γ-Δγ); (1-4)

[0096] In formula (1-1), A is the length of the influence range of the shallow old void area, a is the strike length of the shallow old void area, h j is the thickness of bedrock in the shallow old void area, h s is the thickness of the loose layer in the shallow old empty area, c is the extension outside the influence range considering the safety factor, and c=1.0*(h j +h s ), σ is the strike movement angle of the shallow old goaf area, Δσ is the correction value of the strike movement angle of the shallow old goaf area; It is the movement angle of loose layer in shallow old void area;

[0097] In formula (1-2), B is the width of the influence range of the shallow old void area, b is the dip length of the shallow old void area; d q1 is the horizontal distance from the edge of the downhill mining area to the boundary of the shallow old void area, d q2 h is the horizontal distance from the edge of the uphill mining area to the boundary of the shallow old void area, s is the thickness of the loose layer in the shallow old empty area, c is the extension outside the influence range considering the safety factor, and c=1.0*(h j +h s ), It is the movement angle of loose layer in shallow old void area;

[0098] In formula (1-3): d q1 h is the horizontal distance from the edge of the downhill mining area to the boundary of the shallow old void area, 下 is the thickness of bedrock in the downhill direction along the shallow old empty area; β is the downhill movement angle of the shallow old empty area along the coal seam, and Δβ is the correction value of the downhill movement angle of the shallow old empty area along the coal seam;

[0099] In formula (1-4): d q2 h is the horizontal distance from the edge of the uphill mining area to the boundary of the shallow old void area, 上 is the thickness of bedrock in the upward direction along the shallow old void area; γ is the upward movement angle of the shallow old void area along the coal seam; Δγ is the correction value of the upward movement angle of the shallow old void area along the coal seam.

[0100] like Figure 1a , Figure 1b and Figure 2aAs shown in the example, the strike length a of the shallow old empty area is 2000m, the dip length b of the shallow old empty area is 500m, and the thickness of the loose layer in the shallow old empty area is h. s is 100m, the bedrock thickness of the shallow old empty area is h j =200m, calculated c is 300m; the strike movement angle σ of the shallow old empty area is 68°, and the strike movement angle correction value Δσ is 20°; the loose layer movement angle of the shallow old empty area is Take 45°; the length A of the influence range of the shallow old empty area is calculated to be 3142m;

[0101] In this embodiment, the bedrock thickness h along the downhill direction of the shallow old empty area 下 The horizontal distance d from the edge of the mining range to the boundary of the shallow old empty area is calculated. q1 307m; the thickness of bedrock along the shallow old empty area in the uphill direction is h 上 The horizontal distance d from the edge of the mining range to the boundary of the shallow old empty area along the coal seam is calculated. q2 is 300m; the final calculation shows that the width B of the impact range of the shallow old empty area is 1707m.

[0102] Then, according to the determined influence range of the shallow old empty area and the relative position relationship of the planned dense building group, the governance range of the planned dense building group is delineated; according to the relationship between the two, the determination method of the governance range of the planned dense building group is divided into the following three cases:

[0103] ① When the area where the densely populated buildings are planned to be built is outside the influence range of the shallow old empty area: the densely populated buildings planned to be built do not need to be reinforced;

[0104] ② When part of the area to be built with densely packed buildings is located within the influence range of the shallow old empty area: the part of the densely packed buildings within the influence range of the shallow old empty area shall be regarded as the governance scope of the densely packed buildings, see Figure 2a ;

[0105] ③ When the area where the densely built group is planned to be built is all within the influence range of the shallow old empty area, the area where the densely built group is planned to be built is the governance area, see Figure 2b .

[0106] In this embodiment, it is found through calculation that part of the area where the densely packed buildings are planned to be built is located within the influence range of the shallow old empty area, which belongs to the second case mentioned above. Therefore, part of the densely packed buildings within the influence range of the shallow old empty area are taken as the treatment range.

[0107] Step (2): Determine the width of the enclosure belt based on the protection level of the dense building group and delineate the reinforcement area of ​​the dense building group;

[0108] The protection belt is delineated according to the protection level of the dense buildings above the shallow old empty area. According to the importance, purpose and consequences of mining, the buildings and structures within the mining area can be divided into four protection levels. Different levels of buildings use different widths of protection belts, see Table 1 for details. The reinforcement area of ​​the dense buildings above the shallow old empty area is delineated according to the width of the protection belt and the range of the dense buildings.

[0109] Table 1 Protection levels of buildings and structures in mining areas

[0110]

[0111] Since the densely packed building complex to be built in the mining area in this embodiment is a six-story densely packed residential building, the protection level of the densely packed building complex is determined to be Level II, and the width of the enclosure belt is determined to be 15m.

[0112] The size of the reinforcement area for the proposed densely packed building complex is obtained by the following formula:

[0113] l=2f+d; (2-1)

[0114] w=2f+e; (2-2)

[0115] In formula (2-1) and formula (2-2): d is the length of the dense building group within the influence range of the shallow old empty area, and the measured result in this example is 800m; e is the width of the dense building group within the influence range of the shallow old empty area, and the measured result in this example is 770m; l is the length of the reinforcement range of the dense building group to be reinforced, w is the width of the reinforcement range of the dense building group to be reinforced; f is the width of the enclosure belt, that is, 15m, see Figure 3 ; In this embodiment, the final calculated l and w are 830m and 800m respectively.

[0116] Step (3): Taking the size of the reinforcement area of ​​the dense building group as the technical indicator and based on the load distribution of the reinforcement area, determine the theoretical number and distribution position of the cast-in-place piles in the reinforcement area;

[0117] According to the size of the reinforcement area of ​​the densely packed building group determined in step (1) and step (2), and based on the load distribution in the reinforcement area, the number of cast-in-place piles at each position in the reinforcement area is determined:

[0118] (3.1) A total of m cast-in-place piles are arranged at important corner points outside the reinforcement area and at the center of the reinforcement area (e.g. Figure 4As shown); the criteria for judging important corner points on the periphery of the reinforcement area are: the vicinity of the turning points of two adjacent buildings on the periphery of the reinforcement area, the load-bearing columns of the buildings located at the corner points of the reinforcement area, and other important positions in the reinforcement area; the method for defining the center position of the reinforcement area is: the regular reinforcement area takes the intersection of the diagonals as the center position, and the irregular reinforcement area takes the approximate geometric center as the center position; in this embodiment, m is determined to be 5;

[0119] (3.2) m0 cast-in-place piles are arranged at non-important corner points on the periphery of the reinforcement area. The cast-in-place piles arranged on the periphery of the reinforcement area are arranged continuously and evenly according to the grout penetration range of a single cast-in-place pile, that is, the grout penetration ranges of two adjacent cast-in-place piles are continuous and non-overlapping, so that the m0 cast-in-place piles and the cast-in-place piles at the important corner points on the periphery of the reinforcement area can be connected as one; in this embodiment, m0 is determined to be 18;

[0120] (3.3) The total number of cast-in-place piles to be laid out in the reinforcement area, excluding the center, is calculated as follows:

[0121] Q uk =Q sk +Q pk ; (3-1)

[0122]

[0123]

[0124] In formula (3-1): Q uk is the standard value of the vertical ultimate bearing capacity of a single pile; Q sk is the standard value of the total ultimate lateral resistance, which is 2192.38 kN in this example; Q pk is the standard value of the total ultimate end resistance, which is 1760.8 kN in this example;

[0125] In formula (3-2): Calculated according to the "Highway Bridge and Culvert Foundation and Substructure Design Code-2019", R a is the characteristic value of the vertical bearing capacity of a single pile, in kN, which is 2000 kN in this example; K is the safety factor, which is 2 in this example; Q uk is the standard value of the vertical ultimate bearing capacity of a single pile. The calculation result in this example is 4000kN;

[0126] In formula (3-3): n is the initial estimated number of piles, which is an integer; F k is the vertical force on the top surface of the pile foundation cap under different standard combinations of load effects, in kN. The different load effects in this embodiment mainly refer to the two effects of axial load and eccentric load. k The value is 40000 kN [In other embodiments, F kAccording to the actual load effect, the value is taken]; m is the total number of cast-in-place piles at important corner points outside the reinforcement area and the center of the reinforcement area; the difference between the dead weight of the pile body and the weight of the replaced soil (when the dead weight is included in the buoyancy, the replaced soil is also included in the buoyancy) is included in the effect;

[0127] After calculation, in this embodiment, m = 5 cast-in-place piles need to be arranged at important corner points and the center position of the outer periphery of the reinforcement range, m0 = 18 cast-in-place piles need to be arranged at non-important corner points of the outer periphery of the reinforcement area, and n = 19 cast-in-place piles need to be arranged in other parts of the reinforcement area in theory, and the 19 cast-in-place piles in the reinforcement area are evenly arranged, that is, assuming that the area of ​​the reinforcement area is An, there is only one cast-in-place pile in the reinforcement area with an area of ​​An / n; in this embodiment, the reinforcement area is approximately fan-shaped, and the 19 cast-in-place piles in the approximately fan-shaped reinforcement area are distributed in an "O" shape, such as Figure 5 As shown, that is: 5 circles of cast-in-place piles are distributed in equal intervals in the reinforcement area from the vertex of the approximate fan shape to the periphery of the arc, each circle of cast-in-place piles is on an arc with the vertex of the approximate fan shape as the center, and the numbers of the 5 circles of cast-in-place piles are 1, 2, 4, 5 and 7 respectively; each circle of cast-in-place piles is distributed at equal intervals on the arc in the approximate fan-shaped reinforcement area.

[0128] Step (4): comprehensively considering the relative relationship among the critical mining depth of the shallow old empty area, the height of the water-conducting fracture zone and the critical depth of the load of the dense building group, determine the depth of the bored piles at different positions in the reinforcement area and the final number of bored piles in the reinforcement area, and construct the bored piles;

[0129] After the working face is mined, the overlying rock strata form caving zones, crack zones and bending zones; among them, the caving zones and crack zones are collectively called caving crack zones, also known as water-conducting fracture zones. After the coal resources are mined, the internal structure of the rock mass is destroyed. Although it has been compacted for many years, there are still inevitable cracks. Its compressive and shear strengths are significantly lower than those of the original rock mass. If the load of the planned dense building complex affects these two zones, it is bound to increase the residual settlement of the surface. Therefore, when considering the depth of the cast-in-place piles in the reinforced area, it is necessary to consider the following situations: the depth of the five cast-in-place piles located at the important corner points outside the reinforced area and the center of the reinforced area needs to extend to the bottom plate position of the shallow old empty area; and the depth of other cast-in-place piles needs to consider the critical mining depth of the shallow old empty area (H 临 ), height of water-conducting fracture zone (H 导 ) and the critical depth of load influence of densely packed buildings (H 荷 ) The relative relationship between the three factors determines the depth of the cast-in-place pile:

[0130] 4.1: When the height of the water-conducting fracture zone H 导 Less than the bedrock thickness h in the shallow old void area j , that is, H 导 <hj hour:

[0131] ①If H 导 +H 荷 <h j When the thickness of the bedrock in the shallow old empty area is greater than the sum of the height of the water-conducting fracture zone and the critical depth of the load influence of the dense building group, the arrangement method of the depth of each cast-in-place pile in the reinforcement area is:

[0132] a. The depth of m=5 cast-in-place piles located at important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area;

[0133] b. m0 = 18 cast-in-place piles are arranged at non-important corners outside the reinforcement area, with the pile depth penetrating the loose layer to the bedrock;

[0134] c. The pile depth of the cast-in-place piles located in the reinforcement area except the center position penetrates the loose layer to the bedrock position; at this time, the number of cast-in-place piles in the reinforcement area except the center position can be reduced from the theoretical value n = 19 to the final value n z =4, that is, Figure 5 Based on the uniform "O"-shaped distribution, the piles are arranged in intervals, and the piles arranged in the 1st, 3rd and 5th circles are cancelled. Only 2 piles are arranged in the 2nd and 4th circles respectively. The specific arrangement method is shown in Figure 6a ;

[0135] ②If h j <H 导 +H 荷 <H 临 When the sum of the height of the water-conducting fracture zone and the critical depth of the load influence of dense buildings exceeds the bedrock thickness of the shallow old void area, but is less than the critical mining depth of the shallow old void area, the arrangement method of the cast-in-place pile depth in the reinforcement area is:

[0136] a. The depth of m=5 cast-in-place piles located at important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area;

[0137] b. m0 = 18 cast-in-place piles are arranged at non-important corner points outside the reinforcement area, and the pile depths are respectively through the loose layer to the bedrock position and to the position where the movement angle of the old void area is demarcated. The cast-in-place piles with a pile depth of through the loose layer to the bedrock position and the cast-in-place piles with a pile depth to the position where the movement angle of the old void area is demarcated are arranged alternately;

[0138] c. Cast-in-place piles located outside the center of the reinforcement area: their number can be reduced from the theoretical value n = 19 to the final value n z =7, that is, Figure 5On the basis of the uniform "O"-shaped distribution, the intermittent arrangement is adopted, and the cast-in-place piles arranged in the 1st, 3rd and 5th circles are cancelled. Only 2 and 5 cast-in-place piles are arranged in the 2nd and 4th circles respectively; the depth of cast-in-place piles in each circle is respectively through the loose layer to the bedrock position and to the position where the moving angle of the old void area is demarcated, and the cast-in-place piles with a pile depth of through the loose layer to the bedrock position and the cast-in-place piles with a pile depth to the position where the moving angle of the old void area is demarcated are arranged alternately. For the specific arrangement method, see Figure 6b ;

[0139] ③If H 临 <H 导 +H 荷 When the critical mining depth of the shallow old empty area is less than the sum of the depth of the water-conducting fracture zone and the critical depth of the load influence of the dense building group, the arrangement method of the pile depth of the reinforced area is:

[0140] a. The depth of m=5 cast-in-place piles located at important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area;

[0141] b. m0 = 18 cast-in-place piles are arranged at non-important corner points outside the reinforcement area, with a pile depth to the position defined by the moving angle of the old empty area;

[0142] c. Cast-in-place piles located outside the center of the reinforcement area: The theoretical value n of the number of cast-in-place piles is the final value n z = 19, 19 pile layout and theoretical layout Figure 5 The same; the depth of each circle of cast-in-place piles is respectively through the loose layer to the bedrock position and to the position of the old void area movement angle demarcation range, and the cast-in-place piles with a pile depth of through the loose layer to the bedrock position and the cast-in-place piles with a pile depth to the position of the old void area movement angle demarcation range are arranged alternately. The specific layout method is shown in Figure 6c ;

[0143] 4.2: When the height of the water-conducting fracture zone H 导 Greater than the bedrock thickness h in the shallow old void area j , that is, H 导 >h j hour:

[0144] The height of the water-conducting fracture zone has far exceeded the thickness of the bedrock in the shallow old void area. At this time, the rock layer and surface damage in the shallow old void area are serious. The arrangement method of the cast-in-place piles in the reinforcement area is the same as the above-mentioned case ③, that is, the arrangement method is the same as the arrangement method when the critical mining depth of the shallow old void area is less than the sum of the depth of the water-conducting fracture zone and the critical depth of the load influence of the dense building group. The specific arrangement method is shown in Figure 6c .

[0145] In this example, the critical mining depth of the shallow old goaf (H 临 ), height of water-conducting fracture zone (H 导) and the critical depth of load influence of densely packed buildings (H 荷 )Calculation method:

[0146] H 临 =h j +h s (4-1)

[0147]

[0148]

[0149] In formula (4-1): h j is the thickness of bedrock in the shallow old void area, which is 200m in this example; h s is the thickness of loose layer in shallow old empty area, which is 100m in this example; the critical mining depth H of shallow old empty area is calculated 临 300m;

[0150] In formula (4-2), M is the coal seam thickness, which is 6.0m in this example; the height of the water-conducting fracture zone H is calculated as 导 67.4m;

[0151] In formula (4-3), B is the width of the tunnel, which is 10m in this example; γ is the weight of the overlying rock layer above the tunnel, which is 22.0KN / m 3 Calculation: P0 is the unit pressure of the building base, based on the unit pressure of the building base of 108kpa (6-story building); is the internal friction angle of the overlying rock layer of the roadway, calculated as 38.0°; the critical depth H of the load influence of densely packed buildings is calculated 荷 It is 54m.

[0152] Therefore, in this embodiment, the bedrock thickness of the shallow old void area is h j Greater than the height H of the water-conducting fracture zone 导 The critical depth H affected by the load of densely packed buildings 荷 The sum of the above belongs to the first case in "4.1". The OA section and the BB' section are made to represent the depth of the bored piles at different positions (as shown in Figure 7).

[0153] The construction process of bored piles is as follows: construct bored piles according to the determined bored pile depth; insert the soil-taking drill rod into the steel pipe pile, and the soil is continuously excavated and taken out with the soil-taking drill rod; the vibration equipment at the bottom of the steel pipe pile will shatter the gravel and broken stones encountered during the pile foundation, so as to facilitate the entry of the soil at the head of the steel pipe pile; through holes are provided at the bottom of the steel pipe pile and on the steel pipe body, so as to facilitate the cast bored pile to be bonded with the surrounding soil as a whole, thereby increasing the stress strength; after the construction of the steel pipe pile is completed, the steel cage is lowered, so that the steel cage reserved at the top of the bored pile is higher than the ground, so as to facilitate the connection with the concrete cap, and the steel bar model of the steel cage is HPB300; after the steel cage is lowered, concrete is poured, and the concrete is C30 concrete or other concrete with strength greater than C30 concrete.

[0154] Step (5): Arrange concrete slabs according to the distribution of densely packed buildings, so that the concrete cap and cast-in-place piles form a "cast-in-place pile + cap" pile-slab type anti-deformation foundation structure, thereby improving the overall structural strength of the foundation.

[0155] Different cast-in-place piles are connected by pouring concrete caps: concrete caps are arranged based on the distribution characteristics of dense building groups as basic units [i.e., concrete caps are divided according to the planned construction area of ​​dense building groups and the distribution positions of load-bearing pile foundations, while concrete caps may not be arranged in non-load-bearing positions between buildings], and deformation joints are left between two adjacent concrete caps to achieve a relatively stable state; the steel frame of the concrete cap is connected to the steel cage reserved above the cast-in-place piles by binding to increase the overall structural strength;

[0156] The construction method of the concrete cap is as follows: excavating the soil in the concrete cap construction area with the dense building group distribution characteristics as the basic unit; the distance between the outer periphery of the reinforcement area and the pile foundation excavation soil in the outer periphery of the concrete cap construction area is greater than 2000mm, and the depth of the excavation soil in the concrete cap construction area is 2500-3500mm; a cushion layer is set at the bottom of the excavated soil in the concrete cap construction area, and the cushion layer thickness is 800-1200mm; above the cushion layer is a steel cage reserved for binding with the top of the cast-in-place pile, and the steel bar model is HPB300; cement concrete with a flexural strength greater than 4.5MPa is used for pouring, and this embodiment uses C30 commercial concrete for pouring, and the template height is 20cm higher than the uppermost part of the steel cage during construction; after the concrete cap is poured, the edge of the concrete cap pouring area is cultivated with soil, and the compaction degree of the soil is greater than or equal to 90%, to ensure the overall stability of the integrated "pile + plate" foundation ("cast-in-place pile + cap" pile-plate type anti-deformation foundation structure);

[0157] The cushion layer is composed of the following graded crushed stones: crushed stones with a particle size of 20 to 40 mm, crushed stones with a particle size of 10 to 20 mm, and stone chips with a particle size of less than 5 mm, and the mass ratio of the three is 4:4:2; the soil content in the crushed stones is 3.5wt%, and the content of stone powder in the stone chips is 20wt%; during the cushion layer construction process, the various components of the cushion layer are laid and rolled until the compaction degree reaches more than 93% (such as Figure 8 shown).

[0158] This embodiment adopts a combination of pile-slab reinforcement measures to strengthen the foundation of densely populated buildings in shallow old empty areas. In the actual construction process, the following alternative solutions can also be used to reinforce the foundation of densely populated buildings in shallow old empty areas: ① The entire densely populated building group is reinforced with cast-in-place piles as deep as the bottom plate of the old empty area to ensure increased foundation stability, but the cost of governance will increase significantly; ② A deformation-resistant frame structure foundation type with greater rigidity is selected, but the reliability is lower, especially for loose layers with greater fluidity, which will still be affected by the "activation" of adjacent mining areas, reducing the reliability of foundation stability.

[0159] This embodiment proposes a design method for pile-slab anti-deformation foundation structure of densely packed building groups in shallow old empty areas, calculates the reinforcement range of densely packed building groups, calculates the number and distribution of bored piles, and comprehensively considers the relative relationship among the critical mining depth, the height of the water-conducting fracture zone, and the depth of influence of the building load, determines the number and depth of bored piles at different positions, so that the actual number of bored piles to be constructed is less than or equal to the theoretical value, greatly reducing the amount of engineering work, and uses the method of pouring concrete caps and the bored piles within the protection range to pour a "pile + slab" foundation, thereby improving the overall structural strength of the foundation.

[0160] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A design method for pile-slab anti-deformation foundation structure of densely packed buildings in shallow old empty areas, characterized in that: The steps include: Step (1): Analyze the relative position relationship between the proposed densely built area and the impact range of the shallow old empty area, and determine the governance scope of the proposed densely built area; Step (2): Determine the width of the enclosure belt based on the protection level of the dense building group and delineate the reinforcement area of ​​the dense building group; Step (3): Taking the size of the reinforcement area of ​​the dense building group as the technical indicator, based on the load distribution of the reinforcement area, determine the theoretical number and distribution position of the cast-in-place piles in the reinforcement area; the method for determining the number and layout position of the cast-in-place piles at each position in the reinforcement area is: (3.1) A total of m cast-in-place piles are arranged at the important corner points outside the reinforcement area and the center of the reinforcement area; the criteria for judging the important corner points outside the reinforcement area are: the corner points of two adjacent buildings outside the reinforcement area and the positions of the load-bearing columns of the buildings located at the corner points of the reinforcement area are all important corner points outside the reinforcement area; the method for defining the center of the reinforcement area is: the intersection of the diagonals is the center of the regular reinforcement area, and the approximate geometric center is the center of the irregular reinforcement area; (3.2) m0 bored piles are arranged at non-important corner points outside the reinforcement area. The bored piles arranged outside the reinforcement area are arranged continuously and evenly according to the grout penetration range of a single bored pile, that is, the grout penetration ranges of two adjacent bored piles are continuous and do not overlap, so that the m0 bored piles can be connected to the bored piles at the important corner points outside the reinforcement area as a whole; (3.3) The theoretical total number n of cast-in-place piles arranged in the reinforcement area except the central position is determined by the following calculation formula: Q uk =Q sk +Q pk ; (3-1) In formula (3-1): Q uk is the standard value of the vertical ultimate bearing capacity of a single pile; Q sk is the standard value of total limit lateral resistance; Q pk is the standard value of total limiting end resistance; In formula (3-2): R a is the characteristic value of the vertical bearing capacity of a single pile, in kN; K is the safety factor; Q uk is the standard value of the vertical ultimate bearing capacity of a single pile; In formula (3-3): n is the initial estimated number of piles, which is an integer; F k is the vertical force on the top surface of the pile foundation cap under different standard combinations of load effects, in kN; R a is the characteristic value of the vertical bearing capacity of a single pile, in kN; m is the total number of cast-in-place piles at important corner points outside the reinforcement area and at the center of the reinforcement area; The n cast-in-place piles in the reinforcement area are distributed on one or more arcs in the reinforcement area, and the spacing between adjacent cast-in-place piles on the same arc is equal, and the spacing between two adjacent arcs is equal; Step (4): Comprehensively consider the relative relationship between the critical mining depth of the shallow old empty area, the height of the water-conducting fracture zone and the critical depth of the load of dense buildings, determine the depth of the bored piles in different reinforcement areas, and construct the bored piles; compare the critical mining depth H of the shallow old empty area 临 , height of water-conducting fracture zone H 导 and the critical depth H affected by the load of densely packed buildings 荷 The relative relationship between the three; when the height of the water-conducting fracture zone H 导 Less than the bedrock thickness h in the shallow old void area j , that is, H 导 <h j hour: ①If H 导 +H 荷 <h j When the thickness of the bedrock in the shallow old empty area is greater than the sum of the height of the water-conducting fracture zone and the critical depth of the load influence of the dense building group, the arrangement method of the depth of each cast-in-place pile in the reinforcement area is: a. The depth of the m cast-in-place piles located at the important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area; b. m0 cast-in-place piles are arranged at non-important corners outside the reinforcement area, with the pile depth penetrating the loose layer to the bedrock; c. The depth of the cast-in-place piles located in the reinforcement area except the center position penetrates the loose layer to the bedrock position; at this time, the number of cast-in-place piles in the reinforcement area except the center position is n. z is (1 / 5~1 / 4)n, further method n z Take an integer, n z The cast-in-place piles are distributed on one or more arcs in the reinforcement area, and the spacing between adjacent cast-in-place piles on the same arc is equal; ②If h j <H 导 +H 荷 <H 临 When the sum of the height of the water-conducting fracture zone and the critical depth of the load influence of dense buildings exceeds the bedrock thickness of the shallow old void area, but is less than the critical mining depth of the shallow old void area, the arrangement method of the cast-in-place pile depth in the reinforced area is: a. The depth of the m cast-in-place piles located at the important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area; b. m0 cast-in-place piles are arranged at non-important corner points outside the reinforcement area, and the pile depths are respectively through the loose layer to the bedrock position and to the position where the movement angle of the old void area is demarcated. The cast-in-place piles with a pile depth of through the loose layer to the bedrock position and the cast-in-place piles with a pile depth of to the position where the movement angle of the old void area is demarcated are arranged alternately; c. Cast-in-place piles located outside the center of the reinforcement area: The pile depths are respectively from the loose layer to the bedrock and from the old empty area to the defined range of the moving angle, and the cast-in-place piles from the loose layer to the bedrock and from the old empty area to the defined range of the moving angle are arranged alternately; at this time, the number of cast-in-place piles constructed outside the center of the reinforcement area is n. z is (1 / 3~1 / 2)n, further method n z Take an integer, n z The cast-in-place piles are distributed on one or more arcs, and the spacing between adjacent cast-in-place piles on the same arc is equal; ③If H 临 <H 导 +H 荷 When the critical mining depth of the shallow old empty area is less than the sum of the depth of the water-conducting fracture zone and the critical depth of the load influence of the dense building group, the arrangement method of the pile depth of the reinforced area is: a. The depth of the m cast-in-place piles located at the important corner points outside the reinforcement area and the center of the reinforcement area needs to extend to the bottom plate position of the shallow old empty area; b. m0 cast-in-place piles are arranged at non-important corner points outside the reinforcement area, with a pile depth to the position defined by the moving angle of the old empty area; c. Cast-in-place piles located outside the center of the reinforcement area: The pile depths are respectively from the loose layer to the bedrock and from the old empty area to the defined range of the moving angle, and the cast-in-place piles from the loose layer to the bedrock and from the old empty area to the defined range of the moving angle are arranged alternately; at this time, the number of cast-in-place piles constructed outside the center of the reinforcement area is n. z is the theoretical value n, and n in the reinforcement area z The cast-in-place piles are distributed on one or more arcs in the reinforcement area, and the spacing between adjacent cast-in-place piles on the same arc is equal, and the spacing between two adjacent arcs is equal; When the height of the water-conducting fracture zone H 导 Greater than the bedrock thickness h in the shallow old void area j , that is, H 导 >h j When: the height of the water-conducting fracture zone has far exceeded the thickness of the bedrock in the shallow old void area, the rock layer and the surface of the shallow old void area are severely damaged. The arrangement of the cast-in-place piles in the reinforcement area is consistent with the critical mining depth H of the shallow old void area. 临 Less than the depth H of the water-conducting fracture zone 导 The critical depth H affected by the load of densely packed buildings 荷 The arrangement is the same when the sum is; Step (5): Arrange the concrete cap according to the distribution of the densely packed buildings, so that the concrete cap and the bored piles form a "bored pile + cap" pile-slab type anti-deformation foundation structure.

2. The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas according to claim 1 is characterized in that: In step (1), the influence range of the shallow old void area is calculated by the following formula: d q1 =h 下 *cot(β-Δβ); (1-3) d q2 =h 上 *cot(γ-Δγ); (1-4) In formula (1-1), A is the length of the influence range of the shallow old void area, a is the strike length of the shallow old void area, h j is the thickness of bedrock in the shallow old void area, h s is the thickness of the loose layer in the shallow old empty area, c is the extension outside the influence range considering the safety factor, and c=1.0*(h j +h s ), σ is the strike movement angle of the shallow old goaf area, Δσ is the correction value of the strike movement angle of the shallow old goaf area; It is the movement angle of loose layer in shallow old void area; In formula (1-2), B is the width of the influence range of the shallow old void area, b is the dip length of the shallow old void area; d q1 is the horizontal distance from the edge of the downhill mining area to the boundary of the shallow old void area, d q2 It is the horizontal distance from the edge of the uphill mining range to the boundary of the shallow old void area; In formula (1-3): h 下 is the thickness of bedrock in the downhill direction along the shallow old empty area; β is the downhill movement angle of the shallow old empty area along the coal seam, and Δβ is the correction value of the downhill movement angle of the shallow old empty area along the coal seam; In formula (1-4): h 上 is the thickness of bedrock in the upward direction along the shallow old void area; γ is the upward movement angle of the shallow old void area along the coal seam; Δγ is the correction value of the upward movement angle of the shallow old void area along the coal seam.

3. The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas according to claim 2 is characterized in that: The method for determining the scope of governance of the proposed densely packed buildings is as follows: When the area where densely packed buildings are planned to be built is outside the influence range of the shallow old empty area: the area where densely packed buildings are planned to be built does not need to be reinforced; When part of the area where densely packed buildings are planned to be built is located within the influence range of the shallow old empty area: the part of the densely packed buildings located within the influence range of the shallow old empty area shall be the governance scope; When the area where a densely populated group of buildings is planned to be built is entirely within the influence range of the shallow old empty area, the area where a densely populated group of buildings is planned to be built is the scope of governance.

4. The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas according to claim 3 is characterized in that: When the protection levels of densely packed buildings are Level I, Level II, Level III and Level IV, the widths of the enclosure belts are 20m, 15m, 10m and 5m respectively.

5. The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas according to claim 4 is characterized in that: The reinforcement range of densely packed buildings is calculated by the following formula: l=2f+d; (2-1) w=2f+e; (2-2) In formula (2-1) and formula (2-2): d is the length of the dense building group within the influence range of the shallow old empty area, e is the width of the dense building group within the influence range of the shallow old empty area; l is the length of the reinforcement range of the dense building group to be reinforced, w is the width of the reinforcement range of the dense building group to be reinforced; f is the width of the protective belt.

6. The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas according to claim 5 is characterized in that: Critical mining depth H of shallow old goaf 临 , height of water-conducting fracture zone H 导 and the critical depth H affected by the load of densely packed buildings 荷 The calculation methods of the three are as follows: H 临 =h j +h s ; (4-1) In formula (4-2): M is the thickness of coal seam; In formula (4-3): B is the width of the roadway, γ is the density of the overlying rock layer above the roadway, and P0 is the unit pressure at the base of the building; is the internal friction angle of the overlying rock strata of the roadway.

7. The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas according to claim 6 is characterized in that: The construction process of cast-in-place piles is as follows: The bored piles are constructed according to the determined bored pile depth; the soil-taking drill rod is inserted into the steel pipe pile, and the soil is continuously excavated and taken out with the soil-taking drill rod; the vibration equipment at the bottom of the steel pipe pile will break the gravel and broken stones encountered in the process of pile foundation, and through holes are provided at the bottom of the steel pipe pile and on the steel pipe body; after the construction of the steel pipe pile is completed, the steel cage is lowered so that the steel cage reserved at the top of the bored pile is higher than the ground; after the steel cage is lowered, concrete is poured; the steel bar model of the steel cage is HPB300; the concrete is C30 concrete or other concrete with strength greater than C30 concrete.

8. The design method of pile-slab anti-deformation foundation structure for densely packed buildings in shallow old empty areas according to claim 7 is characterized in that: In step (5), the construction method of the "cast-in-place pile + cap" pile-slab type anti-deformation foundation structure is: concrete caps are arranged based on the distribution characteristics of densely packed buildings as basic units, and deformation joints are left between two adjacent concrete caps; the steel frame of the concrete cap is connected to the steel cage reserved above the cast-in-place pile by binding; The construction method of the concrete cap is as follows: excavate the soil in the concrete cap construction area with the dense building group distribution characteristics as the basic unit; the distance between the excavated soil of the pile foundation outside the reinforcement area and the concrete cap construction area is greater than 2000mm, and the depth of the excavated soil in the concrete cap construction area is 2500-3500mm; a cushion layer is set at the bottom of the excavated soil in the concrete cap construction area, and the cushion layer thickness is 800-1200mm; above the cushion layer is a steel cage reserved for binding with the top of the cast-in-place pile, and the steel bar model is HPB300; cement concrete with a flexural strength greater than 4.5MPa is used for pouring, and the template height is 20cm higher than the uppermost part of the steel cage during construction; after the concrete cap is poured, the edge of the concrete cap pouring area is cultivated with plain soil, and the compaction degree of the plain soil is greater than or equal to 90%; The cushion layer is composed of the following graded crushed stone: crushed stone with a particle size of 20 to 40 mm, crushed stone with a particle size of 10 to 20 mm and stone chips with a particle size of less than 5 mm, and the mass ratio of the three is 4:4:2; the soil content in the crushed stone is less than 5wt%, and the content of stone powder in the stone chips is 15 to 25wt%; during the cushion layer construction process, the various components of the cushion layer are laid and then rolled until the compaction degree reaches more than 93%.

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