A design method of a foundation pit pit-in-pit support and dewatering system

Abstract

The application provides a design method of a foundation pit pit-in-pit supporting and dewatering system. The pit-in-pit supporting and dewatering system uses Larsen steel sheet piles as a supporting structure and simultaneously considers the continuity of the steel sheet pile structure and serves as a cut-off curtain. By increasing a seepage path or penetrating an impervious layer to cut off the hydraulic connection between the inside and outside of the pit, the stability of the pit bottom against sudden gushing or soil flow is ensured. The supporting of the pit-in-pit bottom sealing can be cancelled, the engineering cost is reduced, and the method has good popularization value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of foundation pit support engineering, in particular to a design method of a foundation pit pit-in-pit support and dewatering system. BACKGROUND

[0002] In the design of foundation pit support of water-rich layer, when the foundation pit is excavated to the water-bearing layer or the thickness of the overburden layer does not meet the anti-seepage condition, the foundation pit needs to be designed for dewatering. The pit-in-pit of the elevator shaft or the fire water collecting well needs to be over-excavated. For the dewatering and support of the elevator shaft, the bottom of the pit is generally sealed by mixing piles or high-jet piles, and the side wall is supported by methods such as slope, cement-soil retaining wall, and cantilever pile support according to the over-excavation depth. The existing pit-in-pit support and dewatering are considered separately, which is high in construction cost and the effect of support and anti-seepage is not ideal. SUMMARY

[0003] In view of the above shortcomings of the prior art, the present application provides a design method of a foundation pit pit-in-pit support and dewatering system, which can not only provide support for pit-in-pit construction, but also combine with foundation pit dewatering, use the closed support structure as a partition to disconnect the hydraulic connection inside and outside the pit or as a suspended curtain to increase the seepage path, prevent seepage at the bottom of the pit, cancel the support for sealing the bottom, and reduce the support cost.

[0004] The technical scheme provided by the present application is a design method of a foundation pit pit-in-pit support and dewatering system, comprising the following steps:

[0005] (1) determining the over-excavation depth d of the pit-in-pit, the parameters of the soil layer and the soil layer buoyant density , the water density , the water level under the foundation pit after large-area dewatering is higher than the bottom elevation of the pit by Ah, and the pressure water head elevation of the confined water-bearing layer top surface after large-area dewatering of the foundation pit ;

[0006] (2) evenly arranging steel sheet piles along the foundation pit boundary line of the pit-in-pit, and calculating the pile length L according to the pit-in-pit depth and the stratum condition

[0007] (3) according to the two cases that the bottom of the pit-in-pit after over-excavation is located in the permeable layer and the impermeable layer, checking whether the steel sheet pile setting meets the seepage stability requirement;

[0008] (3.1) If the bottom of the pit is located in the impervious layer after the pit-in-pit over-excavation, check the anti-inrush stability of the pit bottom. If the anti-inrush stability safety factor meets the requirements, no separate dewatering design is needed for the pit-in-pit. If the anti-inrush stability safety factor does not meet the requirements, when the confined water aquifer under the impervious layer of the pit bottom is thin, the steel sheet pile can penetrate the confined water layer, the steel sheet pile separates the hydraulic connection between the pit-in-pit and the basement pit, and the pit bottom does not need to check the anti-inrush stability. When the confined water aquifer under the impervious layer of the pit bottom is thick, the steel sheet pile cannot penetrate the confined water layer, the steel sheet pile cannot separate the hydraulic connection between the pit-in-pit and the basement pit, and the dewatering well is increased to reduce the confined water head height to meet the anti-inrush requirements.

[0009] (3.2) If the bottom of the pit-in-pit is located in the pervious layer after the pit-in-pit over-excavation, check the anti-flow-soil stability of the pit-in-pit over-excavation. If the anti-flow-soil stability safety factor meets the requirements, no separate dewatering design is needed for the pit-in-pit. If the anti-flow-soil stability safety factor does not meet the requirements, when the pervious layer under the pit bottom is shallow, the steel sheet pile can penetrate the pervious layer and enter the impervious layer, the steel sheet pile separates the hydraulic connection between the pit-in-pit and the basement pit, and the pit bottom does not need to check the anti-flow-soil stability. When the impervious layer under the pit bottom is deep, the steel sheet pile cannot penetrate the pervious layer, the steel sheet pile cannot separate the hydraulic connection between the pit-in-pit and the basement pit, and the dewatering well is increased to reduce the water head height to meet the anti-flow-soil stability requirements.

[0010] (4) When the anti-inrush or anti-flow-soil stability does not meet the requirements, dewatering wells are added around the pit-in-pit. According to the dewatering characteristics of the dewatering well, the dewatering well arrangement is uniformly arranged according to the shape of the pit-in-pit. To ensure the dewatering effect, the pit-in-pit plane is ensured to be within the range of any two dewatering well connecting lines. The anti-inrush stability is checked to ensure that the pit-in-pit center position and corner point position meet the anti-inrush stability after the water level is lowered, and the entire pit-in-pit anti-inrush or anti-flow-soil meets the design requirements.

[0011] Further, in step (2), the pile length L is calculated by Tianhan software, 0~2 layers of H-shaped steel support are set, and the steel support spacing is controlled at 3~6m according to the pit-in-pit plane shape.

[0012] Further, in step (3.1), the anti-inrush stability checking formula is as follows,

[0013] (Formula 1)

[0014] In the formula: - Anti-inrush stability safety factor, not less than 1.1;

[0015] - The thickness of the soil layer from the top surface of the confined water aquifer to the pit bottom (m); - The natural density of the soil layer from the top surface of the confined water aquifer to the pit-in-pit pit bottom (KN / m3 - the water head height (m) of the top surface of the confined aquifer after the foundation pit dewatering.

[0016] Further, the checking formula of the soil flow resistance stability in the step (3.2) is as follows, (Formula 2)

[0017] In the formula, - the safety factor of the soil flow resistance stability, not less than 1.6, 1.5, 1.4, respectively corresponding to the foundation pit safety levels one, two and three; - the soil layer thickness (m) from the top surface of the phreatic water or the confined water aquifer to the bottom of the foundation pit, if there is a water-impermeable layer in the steel sheet pile side wall, it is the soil layer thickness from the bottom of the water-impermeable layer to the bottom of the pit-in-pit; - the soil layer buoyant specific weight (KN / m 3 ).

[0018] Further, the arrangement method of the dewatering well in the step (4) is as follows:

[0019] S1, according to the shape of the pit-in-pit, find out the shape center of the pit-in-pit plane, and set the dewatering well one at the shape center;

[0020] S2, draw a circle according to the shape center as the circle center, wrap the entire pit-in-pit plane in the circle, and draw a perpendicular line along the long side direction of the pit-in-pit plane to determine the positions of the dewatering well two and the dewatering well three by intersecting with the circle;

[0021] S3, draw a ray from the centers of the dewatering well two and the dewatering well three to the intersection points of the short sides of the pit-in-pit plane, and determine the dewatering well four and the dewatering well five at the intersection points on both sides.

[0022] Further, when no support is set in the step (2), the steel sheet pile length L is not less than 1.0 times the overbreak depth d; when the support is set, the steel sheet pile length L is not less than 1.3 times the overbreak depth d.

[0023] Further, when the support is set in the step (2), the support and the steel sheet pile are connected by H-shaped steel welding.

[0024] Further, in the step (3), when the steel sheet pile plays a role of cutting off the hydraulic connection between the pit-in-pit and the basement foundation pit, in order to ensure the water isolation effect, 1-2 rows of mixing piles or high-pressure rotary jet piles are used on the outside of the steel sheet pile to strengthen the water isolation effect.

[0025] Further, in the step (2), the steel sheet pile is set continuously and forms a closed support system.

[0026] ​The pit-in-pit support and dewatering system provided by the application provides certain water resistance based on the actual engineering and the continuous performance of the steel sheet pile support system, and can be used as water resistance or a suspended curtain to solve the sudden gushing or soil stability problems caused by over-excavation of the pit-in-pit, save the cost of pit-in-pit support and bottom sealing, and effectively reduce the engineering cost. BRIEF DESCRIPTION OF DRAWINGS

[0027] Fig. 1 is the plan layout of the application;

[0028] Fig. 2 is the sectional view of the application;

[0029] Fig. 3 is the plan layout of the dewatering well of the application;

[0030] Fig. 4 is the case where the pit bottom is an impermeable layer and the underlying confined aquifer is thin

[0031] Fig. 5 is the case where the pit bottom is an impermeable layer and the confined aquifer is thick

[0032] Fig. 6 is the case where the pit bottom is a permeable layer and the underlying impermeable layer has a shallow depth

[0033] Fig. 7 is the case where the pit bottom is a permeable layer and the underlying impermeable layer has a deep depth

[0034] In the figure: 1-steel sheet pile, 2-support pile, 3-H-shaped steel enclosing purlin, 4-mixed pile or high-pressure jet pile. DETAILED DESCRIPTION

[0035] The application will be further described below in conjunction with the drawings and examples. The drawings of the examples are drawn in a simplified manner and are only used for the purpose of clearly and concisely illustrating the embodiments of the application. The technical solutions shown in the drawings are specific solutions of the embodiments of the application, and are not intended to limit the scope of the claimed application. Based on the embodiments in the application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of the application. Figs. 1-2 The drawings of the examples are drawn in a simplified manner and are only used for the purpose of clearly and concisely illustrating the embodiments of the application. The technical solutions shown in the drawings are specific solutions of the embodiments of the application, and are not intended to limit the scope of the claimed application. Based on the embodiments in the application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of the application.

[0036] A design method of a pit-in-pit support and dewatering system, specifically comprising the following steps:

[0037] Step one: determining the over-excavation depth d of the pit-in-pit, the parameters of the soil layer and the soil layer buoyant specific gravity , the specific gravity of water , the phreatic water level after the large-area dewatering of the foundation pit is higher than the bottom elevation of the pit by Ah, and the pressure water head elevation of the top surface of the confined water aquifer under the foundation pit after the large-area dewatering of the foundation pit , the parameters in the following formula are matched with the above parameters;

[0038] Step two: the pit-in-pit support includes uniformly arranging steel sheet piles along the foundation pit boundary line of the pit-in-pit; according to the pit-in-pit depth and stratum conditions, the pile length L is calculated by Tianhan software, 1-2 layers of H-shaped steel supports are set, and the steel support spacing is controlled at 3-6 m according to the pit-in-pit plane shape.

[0039] Step three: according to the appendix C of the foundation pit specification “Technical Specification for Building Foundation Pit Support JGJ 120-2012”, the steel sheet pile setting is calculated to see whether it meets the seepage stability requirements according to whether the pit-in-pit bottom after over-excavation is located in the permeable layer or the impermeable layer:

[0040] (1) If the pit-in-pit bottom after over-excavation is located in the impermeable layer, the pit bottom anti-burst stability needs to be calculated at this time: the pit-in-pit groundwater seepage burst stability is calculated according to the formula, and the known parameters are brought in according to the above:

[0041] (Formula 1)

[0042] In the formula: - the anti-burst stability safety factor, not less than 1.1;

[0043] - the soil layer thickness from the top surface of the confined water aquifer to the pit bottom (m); - the soil layer natural density from the top surface of the confined water aquifer to the pit-in-pit pit bottom (KN / m 3 ); - the pressure water head height of the top surface of the confined water aquifer after the foundation pit dewatering (m);

[0044] (1.1) If the burst stability safety factor requirement is met at this time, no separate dewatering design is needed for the pit-in-pit;

[0045] (1.2) If the burst stability safety factor requirement is not met at this time, the impermeable layer thickness does not meet the anti-burst stability requirement, and the confined water aquifer under the pit bottom is thin: the steel sheet pile can penetrate the confined water layer, so the steel sheet pile disconnects the hydraulic connection between the pit-in-pit and the basement foundation pit, and the pit bottom does not need to be calculated for anti-burst stability at this time.

[0046] (1.3) If the stability safety factor of the sudden gushing is not met at this time, the bottom of the pit is an impermeable layer, the thickness of the impermeable layer does not meet the stability requirement of the sudden gushing, and the thick pressurized water-bearing layer under the bottom of the pit is thick: the steel sheet pile cannot penetrate the pressurized water layer, so the steel sheet pile cannot cut off the hydraulic connection between the pit and the underground pit, and a dewatering well needs to be added to reduce the height of the pressurized water head to meet the requirement of the sudden gushing.

[0047] (2) If the bottom of the pit is in a permeable layer after over-excavation, the stability of the groundwater flow soil of the pit needs to be calculated at this time, and the above known parameters are brought in according to the formula: (Formula 2)

[0048] In the formula:

[0049] - the stability safety factor of the flow soil, not less than 1.6, 1.5, 1.4 (corresponding to the safety levels of the pit, one, two, and three, respectively); - the thickness of the soil layer from the top surface of the phreatic water or the pressurized water-bearing layer to the bottom of the pit (if the steel sheet pile side wall has an impermeable layer, it is the thickness of the soil layer from the bottom of the impermeable layer to the bottom of the pit) (m); - the soil layer buoyant density (KN / m 3 );

[0050] (2.1) If the stability safety factor of the flow soil is met at this time, no separate dewatering design is needed for the pit.

[0051] (2.2) If the stability safety factor of the flow soil is not met at this time, the bottom of the pit is a permeable layer, and the stability of the flow soil is not met, and the permeable layer under the bottom of the pit is shallow: the steel sheet pile can be appropriately extended to penetrate the permeable layer and enter the impermeable layer, so the steel sheet pile cuts off the hydraulic connection between the pit and the underground pit, and the bottom of the pit does not need to be calculated for the stability of the flow soil.

[0052] (2.3) If the stability safety factor of the flow soil is not met at this time, the bottom of the pit is a permeable layer, and the stability of the flow soil is not met, and the impermeable layer under the bottom of the pit is deep: the steel sheet pile cannot penetrate the permeable layer, so the steel sheet pile cannot cut off the hydraulic connection between the pit and the underground pit, and a dewatering well needs to be added to reduce the water head height to meet the stability requirement of the flow soil.

[0053] (3) When the stability of the sudden gushing or the stability of the flow soil is not met, a dewatering well needs to be added around the pit, according to the dewatering characteristics of the dewatering well, and the dewatering well is uniformly arranged according to the shape of the pit, and to ensure the dewatering effect, the pit plane needs to be within the range of any two dewatering well connecting lines, and the arrangement principle is as follows:

[0054] 1. According to the shape of the pit, the shape center of the pit plane is found, and a dewatering well is set at the shape center;

[0055] ​2. Draw a circle with the centroid as the center of the circle, wrap the whole pit-in-pit plane in the circle, and draw a perpendicular line along the long side direction of the pit-in-pit plane to determine the positions of the dewatering well two and the dewatering well three by intersecting with the circle;

[0056] 3. Draw a radial line from the centers of the dewatering well two and the dewatering well three to the intersection point of the short side of the pit-in-pit plane, and determine the positions of the dewatering well four and the dewatering well five by the intersection points on both sides.

[0057] (4) When the anti-burst or anti-soil flow stability is not satisfied, dewatering wells need to be added around the pit-in-pit, and according to the uniform distribution setting principle of the dewatering wells, as long as the calculation ensures that the water level after lowering in the center position and the corner point position of the pit-in-pit satisfies the anti-burst stability, the whole pit-in-pit anti-burst or anti-soil flow can satisfy the design requirements.

[0058] In step two, when no support is set, the steel sheet pile length L is not less than 1.0 times the over-excavation depth d; when the support is set, the steel sheet pile length L is not less than 1.3 times the over-excavation depth d.

[0059] In step two, when the support is set, the support and the steel sheet pile are connected by H-shaped steel welding.

[0060] In step three, when the steel sheet pile plays a role in separating the pit-in-pit and the basement foundation pit in water power connection

[0061] When the steel sheet pile plays a role in separating the pit-in-pit and the basement foundation pit in water power connection, in order to ensure the water isolation effect, 1-2 rows of mixing piles or high-pressure rotary jet piles can be used outside the steel sheet pile to strengthen the water isolation effect.

[0062] The application will be further described below in combination with specific embodiments: A certain residential plot in Fancheng District of Xiangyang City is composed of one plot, two-story basement is set, and partial one-story basement, the project is about 160m away from Hanjiang River. The site ground elevation is about 65.50m, the excavation depth of the two-story basement pit is 10.69m (absolute elevation 54.81), and the pit-in-pit over-excavation is 3.5m (the absolute elevation of the pit-in-pit bottom is 51.31m). Select the J3-18 borehole (the absolute elevation of the hole mouth is 65.04m) near the elevator shaft of the 7# main building, and according to the geological exploration report, the detailed geological condition information in table 1 can be obtained:

[0063] Table 1 Detailed geological conditions of J3-17 borehole in construction area

[0064]

[0065] For the pit-in-pit support at this place, the water level of the large-area foundation pit has been lowered to 0.5m-1.0m below the negative two-story raft, the measured water level elevation is 54.00m, the pit-in-pit needs to be over-excavated to 51.31m, the pit bottom is located in the permeable layer of round gravel, and combined with the thick round gravel layer below the geological exploration, the steel sheet pile cannot penetrate the water isolation layer, at this time, the pit-in-pit support needs to be considered while the soil flow stability of the underground water is calculated.

[0066] At this time according to condition, pit in pit support can be considered as two levels according to importance level, through the input stratum parameter and considering load condition of Tianhan software, the SP-IV type Larsen steel sheet pile with L=9m can be easily obtained, which can meet the requirement of pit in pit support.

[0067] At this time, it is easy to obtain:

[0068] L=9m, d=3.5m, D=54-51.31=2.69m, KN / m 3

[0069] Δh=D=2.69m, which is brought into (formula 2) to obtain: ≥1.5

[0070] At this time, the steel sheet pile support with L=9m can meet the support and soil stability problems, and it is not necessary to use mixing pile or high jet pile to seal the bottom, only the construction of dewatering well to reduce the confined water below the pit bottom can be constructed. And combined with the site condition, the construction joint position of the steel sheet pile does not leak, and it is not necessary to increase the water stop high pressure jet grouting pile behind the pile.

[0071] The application fully considers the continuity of the steel sheet pile from the reality, and the continuity is used as a water interception curtain while playing a supporting structure role, so that the cost of the dewatering design is optimized, and the engineering cost is reduced.

[0072] The above description is only one embodiment of the application, which is described in detail, but it cannot be understood as the limitation of the scope of the application. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the application, a number of modifications and improvements can be made, which belong to the protection scope of the application. The protection scope of the application should be subject to the appended claims.

Claims

1. A design method for a pit-within-a-pit support and dewatering system, characterized in that... Includes the following steps: (1) Determined over-excavation depth d of the pit within the pit, soil parameters and buoyancy of the soil layer. The density of water After extensive dewatering of the foundation pit, the groundwater level dropped to a height ∆h above the pit bottom elevation, and the pressure head elevation dropped to the top surface of the confined aquifer below the foundation pit. ; (2) Steel sheet piles are evenly laid along the edge of the pit within the pit. The pile length is calculated as L based on the depth of the pit within the pit and the geological conditions. (3) Based on the two cases of the foundation being located in a permeable layer and an impermeable layer after the over-excavation of the pit-in-pit, verify whether the steel sheet pile setting meets the seepage stability requirements; (3.1) If the foundation of the pit-in-pit is located in an impermeable layer after over-excavation, the stability of the pit bottom against sudden surge should be checked. If the requirements of the sudden surge stability safety factor are met, then there is no need to carry out separate dewatering design for the pit-in-pit. If the requirements of the sudden surge stability safety factor are not met, and the confined aquifer under the impermeable layer at the bottom of the pit is thin: the extended steel sheet piles can penetrate the confined aquifer, and the steel sheet piles can isolate the hydraulic connection between the pit-in-pit and the basement pit. At this time, the stability of the pit bottom against sudden surge does not need to be checked. When the confined aquifer under the impermeable layer at the bottom of the pit is thick: the steel sheet piles cannot penetrate the confined aquifer, and the steel sheet piles cannot isolate the hydraulic connection between the pit-in-pit and the basement pit. Increasing the dewatering wells and reducing the height of the confined water head can meet the requirements against sudden surge. (3.2) If the foundation of the pit-within-a-pit is located in a permeable layer after over-excavation, the groundwater flow stability of the pit-within-a-pit should be checked. If the flow stability safety factor requirement is met, then no separate dewatering design is required for the pit-within-a-pit. If the flow stability safety factor requirement is not met, and the bottom of the pit is a permeable layer, when the permeable layer is shallow: the steel sheet piles can be extended to penetrate the permeable layer and enter the impermeable layer. The steel sheet piles isolate the hydraulic connection between the pit-within-a-pit and the basement pit. In this case, the stability of the pit bottom does not need to be checked. When the impermeable layer is deep: the steel sheet piles cannot penetrate the permeable layer and cannot isolate the hydraulic connection between the pit-within-a-pit and the basement pit. Increasing the dewatering wells and reducing the water head height will meet the stability requirements of the pit. (4) When the stability against sudden surge or soil erosion is not met, dewatering wells are added around the pit within the pit. According to the dewatering characteristics of the dewatering wells, the dewatering wells are arranged evenly according to the shape of the pit within the pit. In order to ensure the dewatering effect, the plane of the pit within the pit is guaranteed to be within the range of any two dewatering wells. Verification is performed to ensure that the water level at the center position and the corner position of the pit within the pit is reduced to meet the stability against sudden surge, and to ensure that the entire pit within the pit meets the design requirements for resistance to sudden surge or soil erosion.

2. The design method for the pit-within-a-pit support and dewatering system according to claim 1, characterized in that: In step (2), the pile length is calculated as L using Tianhan software, and 0 to 2 layers of H-shaped steel supports are set. The spacing between the steel supports is controlled at 3 to 6m according to the shape of the pit-in-pit plan.

3. The design method for the pit-within-a-pit support and dewatering system according to claim 1, characterized in that: The formula for verifying the anti-surge stability in step (3.1) is as follows: (Equation 1) In the formula: -Surge resistance stability safety factor Not less than 1.1; - Soil thickness from the top of the confined aquifer to the bottom of the pit (m). -Natural unit weight of soil from the top of the confined aquifer to the bottom of the pit (kN / m³) 3 ); - Height of pressure head (m) at the top of the confined aquifer after dewatering of the foundation pit.

4. The design method for the pit-within-a-pit support and dewatering system according to claim 1, characterized in that: The formula for verifying the stability of the erosion-resistant soil in step (3.2) is as follows: (Equation 2) In the formula: - Safety factor for soil stability Not less than 1.6, 1.5, and 1.4, corresponding to foundation pit safety levels one, two, and three, respectively; - The thickness of the soil layer from the top of the phreatic surface or the confined aquifer to the bottom of the pit (m). If there is an impermeable layer on the sidewall of the sheet pile, it is the thickness of the soil layer from the bottom of the impermeable layer to the bottom of the pit. - Buoyant unit weight of soil layer (KN / m) 3 ).

5. The design method for the pit-within-a-pit support and dewatering system according to claim 1, characterized in that: The arrangement method of the dewatering wells in step (4) is as follows: S1. Based on the shape of the pit within a pit, find the centroid of the planar shape of the pit within a pit, and set up a dewatering well at the centroid. S2. Draw a circle with the centroid as the center, and enclose the entire pit-in-pit plane inside the circle. Draw a perpendicular line along the long side of the pit-in-pit plane and intersect the circle to determine the positions of dewatering well two and dewatering well three. S3. Draw a ray from the center of well 2 and well 3 to the intersection of the short side of the pit plane, and determine well 4 and well 5 by the intersection of the two sides.

6. The design method for the pit-within-a-pit support and dewatering system according to claim 2, characterized in that: In step (2), if no support is provided, the length L of the sheet pile shall not be less than 1.0 times the over-excavation depth d; if support is provided, the length L of the sheet pile shall not be less than 1.3 times the over-excavation depth d.

7. The design method for the pit-within-a-pit support and dewatering system according to claim 6, characterized in that: When setting up the support in step (2), the support and the sheet pile are connected by welding H-beams.

8. The design method for the pit-within-a-pit support and dewatering system according to claim 1, characterized in that: In step (3), when the sheet piles act as a hydraulic barrier between the pit and the basement pit, in order to ensure the water-proofing effect, 1 to 2 rows of mixing piles or high-pressure jet grouting piles are used on the outside of the sheet piles to enhance the water-proofing effect.

9. The design method for the pit-within-a-pit support and dewatering system according to claim 1, characterized in that: In step (2), the sheet piles are set continuously to form a closed support system.

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