A construction and design method of a new foundation pit enclosure structure adjacent to an existing foundation pit

Abstract

A construction and design method of a new foundation pit enclosure structure adjacent to an existing foundation pit, the new foundation pit enclosure structure uses the enclosure pile adjacent to the completed foundation pit as a part of the new foundation pit enclosure pile; the enclosure pile serves as the enclosure structure of the excavated foundation pit (1) and also serves as the enclosure structure of the new foundation pit (4); the enclosure pile is connected with the enclosure structure of the new foundation pit through an inner support; and the enclosure pile is connected with the structural column of the completed foundation pit through a support inclined beam. The application provides a design and calculation method of the shared enclosure pile inclined support axial force. The construction method of the application uses the enclosure pile adjacent to the completed foundation pit as the enclosure pile of the new foundation pit, the shared enclosure pile (2) is connected with the new foundation pit enclosure system through a horizontal support (3) and is connected with the structural column of the excavated foundation pit through a support inclined beam. The application has the characteristics of cost saving, convenient construction and being conducive to the safety and stability of the foundation pit.

Description

TECHNICAL FIELD

[0001] The present application relates to a foundation pit retaining structure and construction, in particular to a construction and design method of a new foundation pit retaining structure adjacent to an existing foundation pit, and belongs to the technical field of building engineering. BACKGROUND

[0002] With the rapid development of underground space excavation, the surrounding environment of the foundation pit is increasingly complex, and the simultaneous excavation of multiple adjacent foundation pits is more common. At the same time, the excavation area of the foundation pit is increasing, and the method of dividing the large foundation pit into small foundation pits for separate construction has become a common method.

[0003] Affected by factors such as excavation unloading, these foundation pits will have a significant impact on each other during construction. In the past design and construction, measures such as "isolation" are often taken for adjacent foundation pits to minimize disturbance to adjacent foundation pits. This not only is not economical, but sometimes also makes the foundation pit group unreasonably stressed.

[0004] Therefore, it is particularly important to propose a new foundation pit retaining structure adjacent to an existing foundation pit and its construction method for the design and construction of adjacent foundation pits. SUMMARY

[0005] The technical problem to be solved by the present application is to solve the mutual influence problem of new foundation pits adjacent to the surrounding foundation pits, and to propose a construction and design method of a new foundation pit retaining structure adjacent to an existing foundation pit.

[0006] The technical solution realized by the present application is as follows: a new foundation pit retaining structure adjacent to an existing foundation pit, the new foundation pit retaining structure uses the retaining piles of the adjacent excavated foundation pit as part of the new foundation pit retaining piles; the retaining piles are used as common retaining piles of the excavated foundation pit and the new foundation pit; the new foundation pit retaining structure adopts a row pile combined with a reinforced concrete support scheme, and the new foundation pit adopts an angle support and an inclined counter support arrangement; the common retaining piles and the new foundation pit retaining structure are connected through internal support; the common retaining piles and the structure column of the excavated foundation pit are connected through inclined support, one end of the inclined support is connected with the top beam of the common retaining pile, and the other end of the inclined support is connected with the structure column.

[0007] The excavated foundation pit and the new foundation pit are both internal support type retaining structures; the top beam elevation of the retaining piles of the excavated foundation pit is consistent with the top beam elevation or support elevation of the new foundation pit; the excavated foundation pit does not start the construction of the main structure above ±0.0m.

[0008] The row pile is combined with a reinforced concrete support scheme, the row pile adopts a cast-in-situ bored pile, a row of three-axis cement mixing piles are arranged outside the row pile to stop water and soil, the southeast corner and the southwest corner of the foundation pit adopt a full-rotation interlocking pile combined with three rows of reinforced concrete support and protection, and a row of high-pressure jet grouting piles are arranged outside the interlocking pile to stop water.

[0009] The new foundation pit enclosure structure transmits the force of the shared enclosure pile to the floor slab by arranging an inclined support at a proper position of the foundation pit floor slab and supporting the upper end of the inclined support under the corbel, and a bracket is arranged at the contact node of the inclined support and the floor slab to prevent stress concentration at the connection position of the floor slab and to strengthen the structural safety strength.

[0010] To ensure the safe operation of the foundation pit enclosure structure, the axial force design calculation method of the shared enclosure pile inclined support is provided.

[0011] The multi-row support equivalent beam method is adopted to perform stress analysis and calculation, the axial force on the horizontal support transmitted by the non-shared enclosure pile is calculated first, and then the shared enclosure pile is subjected to stress analysis and solution.

[0012] The weighted average density gamma, the weighted average internal friction angle phi and the weighted average cohesion c of the soil layer are obtained by weighting and averaging each soil layer before calculation.

[0013] The internal force calculation steps of the three rows of horizontal supports of the non-shared enclosure pile are as follows:

[0014] (1) the earth pressure coefficients Ka and Kp are calculated.

[0015] (2) the earth pressure intensity acting on the pile body is calculated, and the earth pressure distribution diagram is drawn.

[0016] (3) the distance y of the earth pressure zero point (approximate zero bending moment point) from the bottom of the foundation pit is calculated, at y, the passive earth pressure in front of the pile body is equal to the active earth pressure behind the pile body.

[0017] (4) the bending moment of each fixed end of the continuous beam and the reaction force of each support point are calculated, at this time, the bending moment and support reaction force diagram can be solved by means of structural mechanics method or using structural mechanics solver, midas, mathcad and other software.

[0018] The calculation steps of the shared enclosure pile inclined support are as follows:

[0019] (1) after the axial forces of the three rows of horizontal supports are obtained, they can be applied to the shared enclosure pile, and the calculation diagram of the shared enclosure pile is drawn.

[0020] (2) According to the method of equivalent continuous beam, the bending moment of each fixed end of the continuous beam and the reaction diagram of each support point can be obtained by means of structural mechanics method or structural mechanics solver, midas, mathcad and the like software.

[0021] Thus, the horizontal component of the axial force of the inclined support can be obtained; if the angle between the inclined support and the shared retaining pile is alpha, the axial force of the inclined support can be further calculated. In addition, in order to better consider the asymmetry of the foundation pit, the support fixed point adjustment coefficient can be introduced when using the support stiffness to adjust it, so that the calculation result is more accurate and reliable.

[0022] The application discloses a construction method of a new foundation pit retaining structure adjacent to an existing foundation pit, which comprises the following steps:

[0023] Step 1, leveling the site, constructing the foundation pit retaining pile to be excavated, and pouring and maintaining the C30 concrete to reach the design strength.

[0024] Step 2, constructing the inclined support of the shared retaining pile of the excavated foundation pit, anchoring one end of the inclined support into the shared retaining pile compression beam, and anchoring the other end into the structural column.

[0025] Step 3, constructing the first horizontal support, so that the end of the horizontal support is adjacent to the new foundation pit retaining pile and the shared retaining pile.

[0026] Step 4, constructing the first surrounding purlin, connecting the end of the horizontal support with the retaining pile by pouring concrete and maintaining to reach the design strength.

[0027] Step 5, excavating the earthwork, excavating the area of the new foundation pit to the height of the second horizontal support.

[0028] Step 6, constructing the second horizontal support, so that the end of the horizontal support is adjacent to the new foundation pit retaining pile and the shared retaining pile.

[0029] Step 7, constructing the second surrounding purlin, connecting the end of the horizontal support with the retaining pile by pouring concrete and maintaining to reach the design strength.

[0030] Step 8, excavating the earthwork, excavating the area of the new foundation pit to the height of the third horizontal support.

[0031] Step 9, constructing the third horizontal support, so that the end of the horizontal support is adjacent to the new foundation pit retaining pile and the shared retaining pile.

[0032] Step 10, constructing the third surrounding purlin, connecting the end of the horizontal support with the retaining pile by pouring concrete and maintaining to reach the design strength.

[0033] Step 11, excavating the earthwork, excavating the area of the new foundation pit to the bottom of the foundation pit.

[0034] Step 12, after the new building is completed, the bottom plate of the pourable area is poured and maintained to reach the design strength.

[0035] The construction method of the new foundation pit enclosure structure adjacent to the existing foundation pit, by connecting the horizontal support of the new foundation pit as an inner support with the enclosure pile of the excavated foundation pit, makes the enclosure pile of the excavated foundation pit become a shared enclosure pile, and sets an inclined support at the structural column of the excavated foundation pit, the intersection of the excavated foundation pit and the new foundation pit is extended to the support-changing purlin with a structural beam, the structural beam is connected with the support-changing purlin with a channel steel connecting structure plate, the both ends of the channel steel are fastened with steel plate gaskets, the support-changing structure of the excavated foundation pit is strengthened, thereby forming a whole to resist and transfer the water and soil pressure behind the enclosure wall of the new foundation pit, the influence of the horizontal displacement on the support can be further reduced by setting multiple inner supports and strengthening the inclined support, so that the safety of the foundation pit engineering is better ensured.

[0036] The beneficial effects of the present application are that the present application utilizes the stress of the adjacent foundation pit enclosure pile, which can reduce the stress of the new foundation pit itself, increase the pile spacing, and reduce the support cross-sectional area, thereby reducing the cost. The foundation pit enclosure structure is safe and stable, and by using the stress of the adjacent foundation pit enclosure pile, the stress system of two or more foundation pits is considered, which can make the stress system more reasonable and the foundation pit more stable. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 is a plan view of the present application using the adjacent foundation pit enclosure pile as the new foundation pit enclosure pile;

[0038] Figure 2 is a cross-sectional view of the present application using the adjacent foundation pit enclosure pile as the new foundation pit enclosure pile;

[0039] Figure 3 is a plan view of the support-changing of the excavated foundation pit;

[0040] Figure 4 is a plan view of the inclined support of the excavated foundation pit;

[0041] Figure 5 is a pile body soil pressure distribution diagram for the embodiment;

[0042] Figure 6 is a bending moment and support reaction diagram of each fixed end of the continuous beam of the embodiment;

[0043] Figure 7 is a calculation diagram of the enclosure pile of the embodiment;

[0044] Figure 8 is a reaction diagram of each fixed end of the continuous beam of the embodiment;

[0045] In the figure, 1 is the excavated foundation pit; 2 is the shared retaining pile; 3 is the horizontal support; 4 is the newly built foundation pit; 5 is the inclined support; 6 is the original top beam; 7 is the top beam; 8 is the structural floor; 9 is the replacement support purlin; 10 is the channel steel; 11 is the structural beam; 12 is the replacement support beam; 13 is the structural column. DETAILED DESCRIPTION

[0046] The technical solution of the present invention will be further described below with reference to the accompanying drawings and through specific implementation methods.

[0047] Figure 1 The figure shows a plan view of a new foundation pit retaining pile constructed by using retaining piles adjacent to the foundation pit. As can be seen from the figure, this embodiment is a new foundation pit retaining structure adjacent to the existing foundation pit, including an excavated foundation pit 1, shared retaining piles 2, supports 3 for the new foundation pit and a new foundation pit 4.

[0048] like Figure 2 As shown, the right side of the common retaining pile 2 in this embodiment is connected to the newly built foundation pit retaining system through the horizontal support 3, and the left side of the common retaining pile 2 is connected to the main structure of the excavated foundation pit 4 through the inclined support 5; one end of the inclined support 5 is connected to the top pressure beam 7, and the other end is fixed on the structural floor 8; the top pressure beam 7 is installed on the original top pressure beam 6.

[0049] like Figure 3 The figure shows a schematic plan view of the replacement support of the excavated foundation pit. In this embodiment, since the newly built foundation pit retaining structure needs to be converted through the retaining piles of the excavated foundation pit, the replacement support structure of the excavated foundation pit needs to be strengthened.

[0050] The specific strengthening measures are as follows: at the junction of the excavated foundation pit and the newly built foundation pit, the structural beam 11 is extended to the replacement support purlin 9, and channel steel is used between the structural beams 11 to connect the structural plate 8 and the replacement support purlin 9.

[0051] like Figure 4 As shown, in this embodiment, one end of the inclined support 5 is connected to the top beam of the common retaining pile 2 , and the other end of the inclined support 5 is connected to the structural column 13 .

[0052] The non-public retaining piles are connected to the excavated foundation pit structure and the retaining piles via a replacement support beam 12. A replacement support beam 12 is installed between every three retaining piles. One end of the replacement support beam 12 is fixed to the retaining pile, and the other end is anchored to the main reinforcement of the basement wall.

[0053] The replacement support structure should be installed at the floor elevation of the second underground floor and the first underground floor.

[0054] The length of the main reinforcement of the diagonal brace anchored within the structural column and capping beam must meet relevant code requirements. The width of diagonal brace 5 should be the same as the width of the corresponding structural column 13. Where the column width is locally smaller, the width of diagonal brace 5 should be adjusted to the same width, while the height remains unchanged.

[0055] This embodiment is further illustrated by a 44-storey building.

[0056] The 44-storey building has 3 underground floors, and the basement has a depth of 16.45 m and is an irregular hexagon.

[0057] The adjacent building is a 26-storey building with 3 underground floors, and the basement has a depth of 16.45 m and is a regular quadrilateral.

[0058] The plan view of the foundation pit is shown in Figure 1 This embodiment sets three support systems, the first support is set at the upper part of the basement, the second support is set at the middle part of the basement, and the third support is set at the lower part of the basement. All three supports are reinforced concrete supports.

[0059] Design steps:

[0060] (1) Design of the enclosure structure of the new foundation pit 4: the width of the enclosure structure is 1000 mm, the depth is 32 m, and the concrete used for the enclosure pile is C30.

[0061] (2) Design of the support of the new foundation pit 4: as shown in Figure 1 , angle struts are designed for the EF and BD edges, counter struts are designed for the GF and CD edges, and angle struts are designed for the CG and CD edges. C30 concrete is used for the supports, and the protective layer thickness of the main reinforcement of the supports is 25 mm. The support cushion layer is 10 cm thick C15 plain concrete plus one layer of oil felt, and the width of each edge cushion layer is the width of the support plus 20 cm. When excavating the soil under the support, the bottom cushion layer of the support should be removed to prevent injury from falling.

[0062] (3) Design of the inclined support 5 of the foundation pit that has been excavated: as shown in Figure 2 , one end of the inclined support 5 is anchored into the shared enclosure pile cap beam 7, as shown in Figure 4 , the other end of the inclined support 5 is anchored into the structural column 13. The cross-sectional width of the inclined support is 800×800 mm, and at the local column width of 700 mm, the cross-sectional width of the inclined support 5 is adjusted to 700×800 mm.

[0063] (4) Design of the support replacement of the foundation pit that has been excavated: as shown in Figure 1 and Figure 3 , the structure beam 11 at the intersection of BD and DC is extended to the support replacement surrounding purlin 9, the 20b channel steel connecting structure plate 8 and the support replacement surrounding purlin 9 are used between the structure beams, and 10 mm thick 300×130 mm steel plate spacers are used at both ends of the channel steel to secure it firmly. The support replacement structure should be set at the floor elevation of the second underground floor and the first underground floor.

[0064] (5) Support column design: the vertical column of cast-in-place reinforced concrete support system is bored pile, and the upper part of the column is an I-shaped truss which is 2m deep into the vertical column pile. The concrete strength grade of the newly drilled column pile is C25. The pile used as the column should also meet the construction requirements of the project pile. The over-pouring height of the newly drilled column pile is 1200mm.

[0065] An example is provided below as a method for calculating the axial force of the shared retaining pile diagonal brace. This embodiment adopts the multi-channel support equivalent beam method for stress analysis and calculation. First, the axial force on the horizontal support transferred by the non-shared retaining pile is calculated. Then, the shared retaining pile is analyzed and solved.

[0066] The calculation principle of the multi-channel support equivalent beam method is the same as that of the single channel. To facilitate the calculation of fixed-end bending moment and reaction force at each support, software can be used to solve the fixed-end bending moment and reaction force at each support.

[0067] Calculation assumptions: the wall is a rigid body, the wall back is vertical, the wall back is smooth, there is no friction between the wall back and the fill, and the fill surface behind the wall is horizontal

[0068] Calculation conditions: the first horizontal support is located at 0m underground, the second horizontal support is located at 5.75m underground, the third horizontal support is located at 10.25m underground, the foundation pit bottom is located at 16.45m underground, the overburden load q=20kN / m, the first floor basement floor is located at 5.35m underground, and the second floor basement floor is located at 9.85m underground

[0069] The weighted average density γ, the weighted average internal friction angle φ, and the weighted average cohesion c of the soil layer are obtained by weighting and averaging each soil layer before calculation.

[0070] Table 1 shows the basic physical and mechanical parameters of the soil layer.

[0071] From the table, the calculation results are: γ=18.92kN∙m -3 , c=11.77kPa, and φ=22.35°.

[0072] Table 1 Basic physical and mechanical parameters of soil layer

[0073]

[0074] Non-shared retaining pile three horizontal support internal force calculation steps:

[0075] 1. Calculate the earth pressure coefficients Ka and Kp

[0076]

[0077]

[0078] The calculation results are Ka=0.449 and Kp=2.227.

[0079] 2. Calculate the soil pressure intensity acting on the pile body, and draw the soil pressure distribution diagram, as shown in Figure 5 .

[0080] 3. Calculate the distance y of the zero point of the soil pressure (approximate zero bending moment point) from the foundation pit bottom. At y, the passive soil pressure in front of the pile body is equal to the active soil pressure behind the pile body.

[0081]

[0082] Thus, the position y of the zero point of the soil pressure is calculated as 4.35 m.

[0083] 4. Calculate the bending moment of each fixed end of the continuous beam and the reaction force of each support point according to the multi-span continuous beam. At this time, the structural mechanics method or software such as structural mechanics solver, midas, mathcad, etc. can be used for solving. The bending moment and support reaction force diagram is shown in Figure 6 .

[0084] The calculation results are N1=148.57 kN, N2=229.84 kN, and N3=913.17 kN.

[0085] The calculation steps of the common fender pile diagonal bracing are as follows:

[0086] 1. After obtaining the three horizontal support axial forces, they can be applied to the common fender pile, and the calculation diagram of the common fender pile is drawn, as shown in Figure 7 .

[0087] 2. According to the equivalent continuous beam method, the bending moment of each fixed end of the continuous beam and the reaction force diagram of each support point can be obtained by using the structural mechanics method or software such as structural mechanics solver, midas, mathcad, etc. The reaction force diagram of each support point is shown in Figure 8 .

[0088] The calculation result is F1=151.09 kN. If the angle between the diagonal bracing and the common fender pile is α, the axial force of the diagonal bracing can be calculated as N=F1 / sinα.

[0089] In addition, in order to better consider the asymmetry of the foundation pit, the support fixed point adjustment coefficient can be introduced to adjust the support stiffness, so that the calculation result is more accurate and reliable.

Claims

1. A new foundation pit retaining structure adjacent to an existing foundation pit, characterized in that: The newly built foundation pit retaining structure utilizes the retaining piles adjacent to the already excavated foundation pit as part of the newly built foundation pit retaining piles; the retaining piles serve as shared retaining piles for the retaining structure of the already excavated foundation pit and the retaining structure of the newly built foundation pit; the newly built foundation pit retaining structure adopts a scheme of pile rows combined with reinforced concrete support, and the newly built foundation pit adopts an arrangement of angle braces and diagonal braces; the shared retaining piles are connected to the retaining structure of the newly built foundation pit by internal supports; The common retaining pile is connected to the structural column of the excavated foundation pit through an inclined support, one end of the inclined support is connected to the top beam of the common retaining pile, and the other end of the inclined support is connected to the structural column; The newly constructed foundation pit retaining structure is constructed by providing inclined supports at appropriate locations on the foundation pit floor slab, with the upper ends of the inclined supports resting under the crown beams, thereby transmitting the force of the shared retaining piles to the floor slab. In order to prevent stress concentration at the floor slab connection locations, corbels are provided at the contact points between the inclined supports and the floor slab to enhance the structural safety strength. The axial force calculation method of the oblique support is as follows: First, the weighted average of each soil layer is taken to obtain the weighted average density γ, weighted average internal friction angle Φ, and weighted average cohesion c of the soil layer; The steps for calculating the internal forces of the three horizontal supports of non-shared retaining piles are as follows: (1) Calculate the earth pressure coefficients Ka and Kp; (2) Calculate the soil pressure strength acting on the pile and draw a soil pressure distribution diagram; (3) Calculate the distance y between the zero point of earth pressure and the bottom of the foundation pit. At point y, the passive earth pressure in front of the pile is equal to the active earth pressure behind the pile. (4) Calculate the bending moment at each fixed end and the reaction force at each support of the continuous beam according to the multi-span continuous beam. At this time, use the structural mechanics method or use the structural mechanics solver, MIDAS, MathCAD software to solve the bending moment and support reaction force diagram; The calculation steps for the common retaining pile inclined support are as follows: (1) After obtaining the three horizontal support axial forces, they can be applied to the common retaining piles and a calculation diagram of the common retaining piles can be drawn; (2) According to the method of equivalent continuous beam, with the help of structural mechanics method or structural mechanics solver, MIDAS, MathCAD software, the bending moment at each fixed end of the continuous beam and the reaction diagram of each support can be obtained; Thus, the horizontal component of the axial force of the diagonal brace can be obtained; if the angle between the diagonal brace and the shared retaining pile is taken as α, further calculation can obtain the axial force of the diagonal brace.

2. A new foundation pit retaining structure adjacent to an existing foundation pit according to claim 1, characterized in that: The excavated foundation pit and the newly built foundation pit are both internally supported retaining structures; the elevation of the retaining pile top beam of the excavated foundation pit is consistent with the elevation of the top beam or support elevation of the newly built foundation pit, and the construction of the main structure above ±0.0m has not started in the excavated foundation pit.

3. The new foundation pit retaining structure adjacent to the existing foundation pit according to claim 1 is characterized in that: The pile rows are combined with reinforced concrete support scheme, and the pile rows are bored cast-in-place piles. A row of three-axis cement mixing piles are set outside the pile rows to stop water and soil. The southeast corner and southwest corner areas of the foundation pit use full-circle interlocking piles combined with three reinforced concrete supports, and a row of high-pressure rotary jet piles are set outside the interlocking piles to stop water.

4. A construction method for a new foundation pit retaining structure adjacent to an existing foundation pit according to any one of claims 1 to 3, characterized in that: The construction method comprises the following steps: Step 1: Level the site and construct the foundation pit retaining piles. The retaining piles are poured with C30 concrete and cured until they reach the designed strength. Step 2: Construct the inclined support for the shared retaining piles in the excavated foundation pit. Anchor one end of the inclined support to the shared retaining pile top beam and the other end to the structural column. Step 3: Construct the first horizontal support so that the end of the horizontal support is adjacent to the newly built foundation pit retaining piles and the shared retaining piles; Step 4: Construct the first purlin, connect the ends of the horizontal supports to the retaining piles with concrete and cure until they reach the designed strength; Step 5: Excavate the new foundation pit to the height of the second horizontal support; Step 6: Construct the second horizontal support so that the end of the horizontal support is adjacent to the newly built foundation pit retaining piles and the shared retaining piles; Step 7: Construct the second purlin, connect the ends of the horizontal supports to the retaining piles with concrete and cure until they reach the designed strength; Step 8: Excavate the new foundation pit to the height of the third horizontal support; Step 9: Construct the third horizontal support so that the end of the horizontal support is close to the newly built foundation pit retaining piles and the shared retaining piles; Step 10: Construct the third purlin, connect the ends of the horizontal supports to the retaining piles with concrete and cure until they reach the designed strength; Step 11: Excavate the new foundation pit to the bottom of the foundation pit; Step 12: After the new construction is completed, cast the base plate in the casting area and maintain it until it reaches the design strength.

5. The construction method of a new foundation pit retaining structure adjacent to an existing foundation pit according to claim 4, characterized in that: The method uses the horizontal support of the newly-built foundation pit as an inner support to connect the retaining piles of the excavated foundation pit so that the retaining piles of the excavated foundation pit become common retaining piles, and sets oblique supports at the structural columns of the excavated foundation pit. The junction of the excavated foundation pit and the newly-built foundation pit adopts the method of extending the structural beam to the replacement support purlin. Channel steel is used to connect the structural plate and the replacement support purlin between the structural beams. Steel plate gaskets are used to support the two ends of the channel steel to strengthen the replacement support structure of the excavated foundation pit, thereby forming a whole to resist and transmit the water and soil pressure behind the retaining wall of the newly-built foundation pit.

Images