A method for pit-in-pit construction in a mucky soil area

Abstract

The application discloses a kind of mucky soil area pit-in-pit construction methods, the method comprises: one, construction pit bottom reinforcing construction structure;Two, construction steel sheet pile;Three, excavate top layer earthwork;Four, excavate and support inside earthwork excavation area;Five, excavate first layer annular soil pit and third layer inside soil pit;Six, excavate second layer annular soil pit and fourth layer inside soil pit;Seven, pour pit-in-pit pile cap and remove steel sheet pile.The application is excavated before whole earthwork, and steel sheet pile is struck, and pit-in-pit and steel sheet pile outside earthwork excavation are formed under the premise of guaranteeing construction safety, and the construction period of whole raft is greatly shortened, and large foundation pit whole supporting system is formed faster, and the exposure time of deep foundation pit is reduced, and the construction operation risk of mucky soil deep foundation pit is reduced;Through the process of pre-striking steel sheet pile, steel sheet pile whole pile is completed once, and the strength of steel sheet pile body and water-resisting effect are guaranteed.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of foundation construction, and particularly relates to a pit-in-pit construction method in a silt soil area. BACKGROUND

[0002] Deep silt layers are widely deposited in coastal areas, which have the characteristics of high natural moisture content, large void ratio, low shear strength and high compressibility, and bring great difficulties to pit-in-pit construction. The silt has the characteristics of flow plasticity and high water content, and the design and construction quality of foundation pit support and water stop are extremely high. The existing technology for soil excavation, pit-in-pit support and core tube foundation construction under the premise of deep foundation pit and deep silt soil layer has the following shortcomings:

[0003] 1. The overall soil excavation construction of the foundation pit is completed before the pit-in-pit construction, which affects the construction period of the pit-in-pit. According to this method, the construction progress of the pit-in-pit is affected by the overall soil excavation of the foundation pit, which further affects the construction of the bottom plate and the upper structure, and has a great influence on the construction period, and at the same time, the quality and safety risks caused by the long-term non-closed foundation pit bottom;

[0004] 2. According to the existing method, the steel sheet pile construction area and the horizontal projection range of the inner support of the foundation pit support overlap, vertical construction operation surfaces conflict, the whole pile of the steel sheet pile cannot be completed at one time, and local segment jointing is required, which has a great influence on the strength of the steel sheet pile body, the water isolation effect and the construction period;

[0005] 3. According to the existing method, the pit-in-pit excavation is carried out after the soil excavation to the foundation pit bottom elevation, and the silt soft soil layer at the pit bottom cannot bear the heavy machinery for pit-in-pit construction, and needs to be reinforced. SUMMARY

[0006] The technical problem to be solved by the present application is to provide a pit-in-pit construction method in a silt soil area, which can solve the problems in the prior art. The steel sheet pile is driven before the overall soil excavation, the pit-in-pit and the soil excavation outside the steel sheet pile are constructed in a staggered manner under the premise of ensuring construction safety, which greatly shortens the completion period of the overall raft, forms the overall support system of the deep foundation pit faster, reduces the exposure time of the deep foundation pit, and reduces the construction operation risk of the silt soil deep foundation pit. Through the process of pre-driving the steel sheet pile, the whole pile of the steel sheet pile is completed at one time, which avoids the overlap of the construction area and the horizontal projection range of the inner support of the foundation pit support in the existing method, and the vertical construction operation surface conflict, effectively improves the construction efficiency, and ensures the strength of the steel sheet pile body and the water isolation effect.

[0007] To solve the above technical problems, the technical scheme adopted by the present application is as follows: a pit-in-pit construction method in a silt soil area, characterized in that the method comprises the following steps:

[0008] Step one, construction pit bottom reinforcement construction structure: according to the design requirements to carry out construction lofting work, using rotary jet grouting pile machine to drill grouting hole unit and grouting into the grouting hole unit, the slurry solidifies to form a pit bottom reinforcement construction structure under the ground; wherein, the pit bottom reinforcement construction structure comprises a plurality of arrayed pit bottom reinforcement rotary jet grouting piles;

[0009] Step two, construction of steel sheet pile, the process is as follows:

[0010] Step 201, excavate a construction slot on the ground above the pit bottom reinforcement construction structure, the construction slot is a frame structure, and the pit bottom reinforcement construction structure is arranged in the frame structure;

[0011] Step 202, vertically insert a plurality of steel sheet piles at the bottom of the construction slot by using a hydraulic vibrating hammer, a plurality of the steel sheet piles are sequentially connected end to end, a plurality of the steel sheet piles are arranged along the circumferential direction of the construction slot, and a plurality of the steel sheet piles enclose a closed frame structure, the inner side of the frame structure is an inner side earth excavation area, and the outer side of the frame structure is an outer side earth excavation area;

[0012] Step three, excavate top layer earthwork: vertically excavate top layer earthwork from the ground, the height of the excavated top layer earthwork is equal to the height of the construction slot;

[0013] Step four, excavate and support the inner side earth excavation area, the process is as follows:

[0014] Step 401, vertically excavate a first layer of inner side soil pit in the inner side earth excavation area, the excavation depth of the first layer of inner side soil pit is 1.8m~2m;

[0015] A plurality of inner support structures are horizontally installed in the first layer of inner side soil pit, and a plurality of the inner support structures are uniformly arranged along the width direction of the first layer of inner side soil pit; wherein, the end of the inner support is fixed on the inner side of the steel sheet pile exposed outside after excavating the first layer of inner side soil pit;

[0016] Step 402, vertically excavate a second layer of inner side soil pit from the bottom of the first layer of inner side soil pit, the first layer of inner side soil pit and the second layer of inner side soil pit are connected, and the first layer of inner side soil pit and the second layer of inner side soil pit form an inner side pit cavity; wherein, the excavation depth of the second layer of inner side soil pit is 1.8m~2m;

[0017] A plurality of inner support structures are horizontally installed in the second layer of inner side soil pit, and a plurality of the inner support structures are uniformly arranged along the width direction of the second layer of inner side soil pit; wherein, the end of the inner support is fixed on the inner side of the steel sheet pile exposed outside after excavating the second layer of inner side soil pit;

[0018] Step five, excavate the first layer of annular pit and the third layer of inner pit, the process is as follows:

[0019] Step 501, excavate the first layer of annular pit vertically downward from the outer earthwork excavation area, the excavation depth of the first layer of annular pit is equal to the excavation depth of the first layer of inner pit;

[0020] Step 502, repeat step 402 at the same time, excavate and support the third layer of inner pit vertically downward from the second layer of inner pit, the third layer of inner pit is connected with the inner pit cavity; wherein the excavation depth of the third layer of inner pit is 1.8m~2m;

[0021] Step 503, after the first layer of annular pit is excavated, remove the inner support structure in the first layer of inner pit;

[0022] Step six, excavate the second layer of annular pit and the fourth layer of inner pit, the process is as follows:

[0023] Step 601, excavate the second layer of annular pit vertically downward from the bottom of the first layer of annular pit, the excavation depth of the second layer of annular pit is equal to the excavation depth of the first layer of annular pit, the second layer of annular pit is connected with the first layer of annular pit, and the second layer of annular pit and the first layer of annular pit form an outer pit cavity;

[0024] Step 602, repeat step 402 at the same time, excavate and support the fourth layer of inner pit vertically downward from the third layer of inner pit, the fourth layer of inner pit is connected with the inner pit cavity; wherein the excavation depth of the fourth layer of inner pit is 1.8m~2m;

[0025] Step 603, after the second layer of annular pit is excavated, remove the inner support structure in the second layer of inner pit;

[0026] Step seven, pour the pit-in-pit pile cap and remove the steel sheet pile, the process is as follows:

[0027] Step 701, a brick formwork is erected in the middle of the bottom of the fourth layer of inner pit, and concrete is poured into the brick formwork to form a pit-in-pit pile cap;

[0028] Step 702, install a support replacement structure between the brick formwork and the steel sheet pile; after the strength of the bottom cushion layer reaches the design requirement, remove the inner support structure in the fourth layer of inner pit;

[0029] Step 703, backfill a support layer between the pit-in-pit pile cap and the inner support structure in the third layer of inner pit; remove the inner support structure in the third layer of inner pit;

[0030] Step 704, pull out a plurality of steel sheet piles, complete the pit-in-pit construction.

[0031] The pit-in-pit construction method in the muddy soil area has the characteristics that in step one, the grouting hole unit comprises a plurality of arrayed grouting holes, the grouting holes are vertically arranged, the top of the grouting hole is flush with the ground, and the depth of the grouting hole ranges from 4m to 5m.

[0032] The pit-in-pit construction method in the muddy soil area has the characteristics that in step 201, the section of the construction groove is a reverse trapezoidal structure, and the bottom width of the construction groove is 2m.

[0033] The pit-in-pit construction method in the muddy soil area has the characteristics that in step 202, the steel sheet pile is close to the side of the inner side earth excavation area and the edge of the pit bottom reinforcement construction structure, and the bottom of the steel sheet pile extends to the bottom of the pit bottom reinforcement construction structure.

[0034] The pit-in-pit construction method in the muddy soil area has the characteristics that in steps 401, 402, 502 and 602, the inner support structure comprises two symmetrically arranged steel waist beams in the inner side earth excavation area and a support steel pipe horizontally arranged between the two steel waist beams; the support steel pipe and the steel waist beam are detachably connected; the steel waist beam comprises a support member mounted on the edge of the inner side earth excavation area and arranged on the steel sheet pile, and a plurality of connecting members horizontally arranged between the steel sheet pile and the support steel pipe.

[0035] The pit-in-pit construction method in the muddy soil area has the characteristics that in step 701, the concrete pump truck is used to pour concrete into the brick mold to form a pit bottom cushion layer in the fourth layer of inner side pit; then a waterproof roll is laid on the top of the pit bottom cushion layer to form a pit-in-pit pile cap.

[0036] The pit-in-pit construction method in the muddy soil area has the characteristics that in step 702, the support changing structure comprises a support changing plate horizontally mounted between the brick mold and the steel sheet pile, the bottom of the support changing plate is filled with backfilling material, and the backfilling material is arranged in the gap between the brick mold and the steel sheet pile.

[0037] The pit-in-pit construction method in the muddy soil area has the characteristics that in step 703, the width of the support layer is equal to the width of the support changing plate, and the support layer is a structure composed of gravel and sand.

[0038] Compared with the prior art, the present application has the following advantages:

[0039] 1. The application forms pit-in-pit and soil excavation outside the steel sheet pile under the premise of ensuring construction safety by driving the steel sheet pile before overall earth excavation, greatly shortens the completion period of the overall raft, forms the overall support system of the large foundation pit faster, reduces the exposure time of the deep foundation pit, and reduces the construction operation risk existing in the deep foundation pit of the muddy soil.

[0040] 2. The application completes the driving of the steel sheet pile at one time through the process of driving the steel sheet pile in advance, avoids the situation that the construction area overlaps with the horizontal projection range of the inner support of the foundation pit support in the existing method when the steel sheet pile is constructed at the pit bottom, effectively improves the construction efficiency, and ensures the strength of the steel sheet pile body and the water isolation effect.

[0041] 3. The application prepositions the process of driving the steel sheet pile, avoids the safety risk that the muddy soil layer at the pit bottom cannot bear the heavy construction machinery in the pit-in-pit construction in the existing method, avoids the reinforcement treatment of the construction operation surface, and saves the construction period and cost.

[0042] 4. The application effectively solves the problem that the pit-in-pit construction affects the overall construction progress of the structure bottom plate by adjusting the construction deployment of the pit-in-pit construction and overall earth excavation.

[0043] In summary, the application forms pit-in-pit and soil excavation outside the steel sheet pile under the premise of ensuring construction safety by driving the steel sheet pile before overall earth excavation, greatly shortens the completion period of the overall raft, forms the overall support system of the large foundation pit faster, reduces the exposure time of the deep foundation pit, and reduces the construction operation risk existing in the deep foundation pit of the muddy soil; the application completes the driving of the steel sheet pile at one time through the process of driving the steel sheet pile in advance, avoids the situation that the construction area overlaps with the horizontal projection range of the inner support of the foundation pit support in the existing method when the steel sheet pile is constructed at the pit bottom, effectively improves the construction efficiency, and ensures the strength of the steel sheet pile body and the water isolation effect.

[0044] The technical solutions of the application will be further described in detail through the accompanying drawings and embodiments. DRAWINGS

[0045] Figure 1 It is a construction state schematic diagram of the construction steel sheet pile of the application.

[0046] Figure 2 It is a construction state schematic diagram of the excavation and support of the second layer inside soil pit of the application.

[0047] Figure 3 It is a construction state schematic diagram of the excavation of the first layer annular soil pit of the application.

[0048] Figure 4 It is a construction state schematic diagram of the excavation of the second layer annular soil pit of the application.

[0049] Figure 5 A construction state diagram of a pit support platform in a pouring pit of the application.

[0050] Figure 6 A flow chart of the application.

[0051] Legend:

[0052] DETAILED DESCRIPTION

[0053] As Figures 1 to 6 shown in a kind of mucky soil area pit-in-pit construction method, the method includes the following steps:

[0054] Step one, construction pit bottom reinforcement construction structure: according to design requirement, construction lofting work is carried out, grouting hole unit is drilled using rotary jetting pile machine and grouting is injected into the grouting hole unit, and pit bottom reinforcement construction structure is formed under ground 2 after grout solidification;Wherein, the pit bottom reinforcement construction structure includes a plurality of pit bottom reinforcement rotary jetting piles 1 arranged in an array;

[0055] Step two, construction steel sheet pile, as follows:

[0056] Step 201, excavate construction slot 3 on the ground 2 above the pit bottom reinforcement construction structure, the construction slot 3 is a frame structure, and the pit bottom reinforcement construction structure is arranged in the frame structure;

[0057] Step 202, a plurality of steel sheet piles 4 are vertically inserted at the bottom of the construction slot 3 using a hydraulic vibrating hammer, a plurality of the steel sheet piles 4 are sequentially connected head to tail, a plurality of the steel sheet piles 4 are arranged along the circumferential direction of the construction slot 3, and a plurality of the steel sheet piles 4 enclose a closed frame structure, the inner side of the frame structure is an inner side earth excavation area, and the outer side of the frame structure is an outer side earth excavation area;

[0058] Step three, excavate top layer earthwork: excavate top layer earthwork vertically from ground 2, and the height of the excavated top layer earthwork is equal to the height of the construction slot 3;

[0059] Step four, excavate and support the inner side earth excavation area, as follows:

[0060] Step 401, vertically excavate a first layer of inner side soil pit in the inner side earth excavation area, and the excavation depth of the first layer of inner side soil pit is 1.8m~2m;

[0061] A plurality of inner support structures are horizontally installed in the first layer inner side pit, and the plurality of inner support structures are uniformly arranged along the width direction of the first layer inner side pit; wherein the end of the inner support is fixed on the inner side of the steel sheet pile 4 exposed after the first layer inner side pit is excavated;

[0062] Step 402, vertically downwardly excavate the second layer inner side pit from the bottom of the first layer inner side pit, the first layer inner side pit and the second layer inner side pit are communicated, and the first layer inner side pit and the second layer inner side pit form an inner side pit cavity 5; wherein the excavation depth of the second layer inner side pit is 1.8m~2m;

[0063] A plurality of inner support structures are horizontally installed in the second layer inner side pit, and the plurality of inner support structures are uniformly arranged along the width direction of the second layer inner side pit; wherein the end of the inner support is fixed on the inner side of the steel sheet pile 4 exposed after the second layer inner side pit is excavated;

[0064] Step five, excavate the first layer annular pit and the third layer inner side pit, the process is as follows:

[0065] Step 501, vertically downwardly excavate the first layer annular pit 10 from the outer side earthwork excavation area, and the excavation depth of the first layer annular pit 10 is equal to the excavation depth of the first layer inner side pit;

[0066] Step 502, simultaneously repeat step 402, vertically downwardly excavate and support the third layer inner side pit from the second layer inner side pit, and the third layer inner side pit and the inner side pit cavity 5 are communicated; wherein the excavation depth of the third layer inner side pit is 1.8m~2m;

[0067] Step 503, after the first layer annular pit 10 is excavated, the inner support structure in the first layer inner side pit is removed;

[0068] Step six, excavate the second layer annular pit and the fourth layer inner side pit, the process is as follows:

[0069] Step 601, vertically downwardly excavate the second layer annular pit from the bottom of the first layer annular pit 10, and the excavation depth of the second layer annular pit is equal to the excavation depth of the first layer annular pit 10, the second layer annular pit and the first layer annular pit 10 are communicated, and the second layer annular pit and the first layer annular pit 10 form an outer side pit cavity 11;

[0070] Step 602, simultaneously repeat step 402, vertically downwardly excavate and support the fourth layer inner side pit from the third layer inner side pit, and the fourth layer inner side pit and the inner side pit cavity 5 are communicated; wherein the excavation depth of the fourth layer inner side pit is 1.8m~2m;

[0071] Step 603, after the second layer annular pit is excavated, the inner support structure in the second layer inner pit is removed;

[0072] Step seven, the pit-in-pit pile cap is poured and the steel sheet pile is removed, and the process is as follows:

[0073] Step 701, a brick formwork is erected at the middle of the bottom of the fourth layer inner pit, concrete is poured into the brick formwork, and a pit-in-pit pile cap 12 is formed;

[0074] Step 702, a support replacement structure is installed between the brick formwork and the steel sheet pile 4; after the strength of the pit bottom cushion layer reaches the design requirement, the inner support structure in the fourth layer inner pit is removed;

[0075] Step 703, a support layer 15 is backfilled between the pit-in-pit pile cap 12 and the inner support structure in the third layer inner pit; the inner support structure in the third layer inner pit is removed;

[0076] Step 704, the steel sheet piles 4 are pulled out, and the pit-in-pit construction is completed.

[0077] The present application punches the steel sheet pile 4 before overall earth excavation, forms the pit-in-pit and the earth excavation outside the steel sheet pile 4 under the premise of ensuring construction safety, greatly shortens the completion period of the overall raft, forms the overall support system of the large foundation pit faster, reduces the exposure time of the deep foundation pit, and reduces the construction operation risk of the muddy soil deep foundation pit.

[0078] The present application completes the steel sheet pile 4 whole pile punching through the process of pre-punching the steel sheet pile 4, avoids the situation that the construction area and the horizontal projection range of the inner support of the foundation pit support overlap when the steel sheet pile at the pit bottom is constructed in the prior art, and the vertical construction operation surface conflicts, effectively improves the construction efficiency, and ensures the strength and waterproof effect of the steel sheet pile body.

[0079] The present application pre-punches the process of the steel sheet pile, avoids the safety risk that the pit bottom muddy soil layer cannot bear the heavy construction machinery of the pit-in-pit construction in the prior art, avoids the reinforcement treatment of the construction operation surface, and saves the construction period and cost.

[0080] The present application adjusts the construction deployment of the pit-in-pit construction and the overall earth excavation, effectively solves the problem that the pit-in-pit construction affects the overall construction progress of the structure bottom plate.

[0081] It should be noted that before construction, the BIM technology is used to perform three-dimensional modeling of the overall construction structure in geological layers; the rock-soil calculation software is used to perform stress deformation analysis and calculation on the pit-in-pit support system, and the pit-in-pit support design drawings are drawn and determined; the BIM technology is used again to simulate the pit-in-pit support construction process to ensure the smooth progress of the construction. In step 202, after the construction of the steel sheet pile 4 is completed, the peripheral deformation monitoring points are arranged around the steel sheet pile 4 to monitor the steel sheet pile 4.

[0082] In step five, a ring-shaped soil pit is excavated on the outside of the frame structure, and an inner soil pit is also excavated in the inner side earthwork excavation area. The vertical distance between the ring-shaped soil pit on the outside and the inner soil pit during excavation does not exceed 5 m. The width of the first layer of ring-shaped soil pits 10 is determined by the overall pit-in-pit structure design requirement. In addition, the first layer of ring-shaped soil pits 10, the second layer of ring-shaped soil pits, the third layer of ring-shaped soil pits, and the fourth layer of ring-shaped soil pits need to be supported during excavation, and the support method is conventional support.

[0083] In this embodiment, in step one, the grouting hole unit includes a plurality of grouting holes arranged in an array, the grouting holes are vertically arranged, the top of the grouting hole is flush with the ground 2, and the depth of the grouting hole ranges from 4 m to 5 m.

[0084] In step one, the height of the pit bottom reinforcement jet grouting pile 1 is preferably 4 m, the diameter Φ of the pit bottom reinforcement jet grouting pile 1 is 500 mm, and the spacing between adjacent two pit bottom reinforcement jet grouting piles 1 is 400 mm. Since the soil layer below the pit-in-pit bottom surface is silt soft soil, the passive earth pressure is insufficient, and the pit bottom reinforcement jet grouting pile 1 is used to reinforce the passive zone soil body of the pit-in-pit bottom.

[0085] As shown in Figure 1 , in this embodiment, in step 201, the cross section of the construction slot 3 is a reverse trapezoidal structure, and the bottom width of the construction slot 3 is 2 m.

[0086] As shown in Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5 , in this embodiment, in step 202, the steel sheet pile 4 is close to the side of the inner side earthwork excavation area and the edge of the pit bottom reinforcement construction structure, and the bottom of the steel sheet pile 4 extends to the bottom of the pit bottom reinforcement construction structure.

[0087] As shown in Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5As shown, in the embodiment, in the steps 401, 402, 502 and 602, the inner support structure comprises two symmetrical steel waist beams 7 arranged in the inner side earthwork excavation area and a support steel pipe 6 horizontally arranged between the two steel waist beams 7; the support steel pipe 6 and the steel waist beam 7 are detachably connected; the steel waist beam 7 comprises a support 8 arranged on the steel sheet pile 4 at the edge of the inner side earthwork excavation area and a plurality of connecting pieces 9 horizontally arranged between the steel sheet pile 4 and the support steel pipe 6.

[0088] In actual use, the support 8 and the connecting piece 9 are both made of profiled steel, and when the support 8 and the connecting piece 9 are installed, the support 8 and the connecting piece 9 can be connected with the steel sheet pile 4 by using a fixing piece or directly connected with the steel sheet pile 4 by welding, and the inner support structure can be disassembled.

[0089] In the embodiment, in the step 701, concrete is poured into the brick mold by using a concrete pump truck to form a pit bottom cushion layer in the fourth layer inner side pit, and then a waterproof roll material is laid on the top of the pit bottom cushion layer to form a pit-in-pit pile cap 12.

[0090] In the embodiment, in the step 702, the support changing structure comprises a support changing plate 13 horizontally arranged between the brick mold and the steel sheet pile 4, and the bottom of the support changing plate 13 is filled with backfill 14 which is arranged in the gap between the brick mold and the steel sheet pile 4.

[0091] In the embodiment, in the step 703, the width of the support layer 15 is equal to the width of the support changing plate 13, and the support layer 15 is a structure composed of gravel and sandy soil.

[0092] The above is only a preferred embodiment of the present application, and does not limit the present application in any way. Any simple modification, change and equivalent structural change made according to the technical essence of the present application to the above embodiment are still within the protection scope of the technical solution of the present application.

Claims

1. A method for constructing a pit-within-a-pit system in silty soil areas, characterized in that, The method includes the following steps: Step 1: Construction of pit bottom reinforcement structure: Construction layout work is carried out according to design requirements. Grouting hole units are drilled using a jet grouting machine and grout is injected into the grouting hole units. After the grout solidifies, a pit bottom reinforcement structure is formed below the ground (2). The pit bottom reinforcement structure includes multiple pit bottom reinforcement jet grouting piles (1) arranged in an array. Step 2: Construct steel sheet piles, the process is as follows: Step 201: Excavate a construction trench (3) on the ground (2) above the pit bottom reinforcement construction structure. The construction trench (3) is a frame structure, and the pit bottom reinforcement construction structure is arranged inside the frame structure. Step 202: At the bottom of the construction trench (3), multiple sheet piles (4) are vertically inserted using a hydraulic vibratory hammer. The multiple sheet piles (4) are connected end to end in sequence. The multiple sheet piles (4) are arranged along the circumferential direction of the construction trench (3), and the multiple sheet piles (4) form a closed frame structure. The inner side of the frame structure is the inner earthwork excavation area, and the outer side of the frame structure is the outer earthwork excavation area. Step 3: Excavate the top layer of soil: Excavate the top layer of soil vertically downward from the ground (2), and the height of the excavated top layer of soil is equal to the height of the construction trench (3); Step 4: Excavate and support the inner excavation area, as follows: Step 401: Vertically excavate the first layer of inner pits within the inner earthwork excavation area. The excavation depth of the first layer of inner pits is 1.8m to 2m. Multiple internal support structures are horizontally installed in the inner pit of the first layer, and the multiple internal support structures are evenly distributed along the width direction of the inner pit of the first layer; wherein, the end of the internal support is fixed on the inner side of the steel sheet pile (4) exposed after the excavation of the inner pit of the first layer. Step 402: Excavate the second inner pit vertically downward from the bottom of the first inner pit. The first inner pit and the second inner pit are connected and form an inner pit cavity (5). The excavation depth of the second inner pit is 1.8m to 2m. Multiple internal support structures are horizontally installed in the inner pit of the second layer, and the multiple internal support structures are evenly distributed along the width direction of the inner pit of the second layer; wherein, the end of the internal support is fixed on the inner side of the steel sheet pile (4) exposed after the excavation of the inner pit of the second layer. Step 5: Excavate the first ring-shaped pit and the third inner pit, as follows: Step 501: Excavate the first layer of annular pit (10) vertically downward from the outer earthwork excavation area. The excavation depth of the first layer of annular pit (10) is equal to the excavation depth of the first layer of inner pit. Step 502: Simultaneously repeat step 402, vertically excavating and supporting the third inner pit from the second inner pit, wherein the third inner pit and the inner pit cavity (5) are connected; wherein the excavation depth of the third inner pit is 1.8m~2m; Step 503: After the first layer of annular pit (10) is excavated, remove the inner support structure inside the first layer of inner pit; Step Six: Excavate the second ring-shaped pit and the fourth inner pit, as follows: Step 601: Excavate a second annular pit vertically downward from the bottom of the first annular pit (10). The excavation depth of the second annular pit is equal to the excavation depth of the first annular pit (10). The second annular pit and the first annular pit (10) are connected. The second annular pit and the first annular pit (10) form an outer pit cavity (11). Step 602: Simultaneously repeat step 402, vertically excavate and support the fourth inner pit from the third inner pit, the fourth inner pit and the inner pit cavity (5) are connected; wherein, the excavation depth of the fourth inner pit is 1.8m~2m; Step 603: After the excavation of the second layer of annular pit is completed, remove the inner support structure inside the inner side of the second layer of pit; Step 7: Pour the foundation stone into the pit and remove the sheet piles. The process is as follows: Step 701: Erect a brick formwork at the middle of the bottom of the pit on the inner side of the fourth layer, and pour concrete into the brick formwork to form a pit-in-pit foundation (12). Step 702: Install a replacement support structure between the brick formwork and the steel sheet pile (4); after the strength of the bottom cushion layer reaches the design requirements, remove the inner support structure inside the fourth layer of the inner soil pit; Step 703: Backfill the support layer (15) between the pit foundation (12) and the inner support structure in the third inner pit; remove the inner support structure in the third inner pit; Step 704: Remove multiple sheet piles (4) to complete the pit-in-pit construction.

2. The method for constructing a pit-within-a-pit in a silty soil area according to claim 1, characterized in that: In step one, the grouting hole unit includes multiple grouting holes arranged in an array. The grouting holes are arranged vertically, and the top of the grouting holes is flush with the ground (2). The depth range of the grouting holes is 4m to 5m.

3. The method for constructing a pit-within-a-pit in a silty soil area according to claim 1, characterized in that: In step 201, the cross-section of the construction trench (3) is an inverted trapezoidal structure, and the bottom width of the construction trench (3) is 2m.

4. The method for constructing a pit-within-a-pit in a silty soil area according to claim 1, characterized in that: In step 202, the steel sheet pile (4) is close to the side of the inner earthwork excavation area and the edge of the pit bottom reinforcement construction structure, and the bottom of the steel sheet pile (4) extends to the bottom of the pit bottom reinforcement construction structure.

5. The method for constructing a pit-within-a-pit in a silty soil area according to claim 1, characterized in that: In steps 401, 402, 502 and 602, the inner support structure includes two steel waist beams (7) symmetrically arranged in the inner earthwork excavation area and a support steel pipe (6) horizontally erected between the two steel waist beams (7); the support steel pipe (6) and the steel waist beams (7) are detachably connected; the steel waist beams (7) include support members (8) installed on the edge of the inner earthwork excavation area and arranged on the steel sheet piles (4), and multiple connectors (9) horizontally arranged between the steel sheet piles (4) and the support steel pipes (6).

6. The method for constructing a pit-within-a-pit in a silty soil area according to claim 1, characterized in that: Step 701: Use a concrete pump truck to pour concrete into the brick mold to form a pit bottom cushion layer in the inner pit of the fourth layer; then lay a waterproof membrane on top of the pit bottom cushion layer to form a pit-in-pit foundation (12).

7. The method for constructing a pit-within-a-pit in a silty soil area according to claim 1, characterized in that: In step 702, the replacement support structure includes a replacement support plate (13) horizontally installed between the brick formwork and the steel sheet pile (4), the bottom of the replacement support plate (13) is filled with backfill material (14), and the backfill material (14) is arranged in the gap between the brick formwork and the steel sheet pile (4).

8. A method for constructing a pit-within-a-pit in a silty soil area according to claim 7, characterized in that: In step 703, the width of the support layer (15) is equal to the width of the replacement support plate (13), and the support layer (15) is a structure composed of gravel and sand.

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