First of all, a brief introduction.
At present, the commonly used composite soil nailing wall support in engineering is mainly the joint application of cement-soil mixing pile and soil nailing wall. Its principle is mainly as follows: using cement-soil mixing pile to strengthen slope soil, to solve the problems of self-reliance, water resistance and bonding between sprayed surface layer and soil; Horizontal compaction grouting and secondary pressure grouting are used to solve the problems of soil reinforcement and soil nailing uplift; Through the deep insertion depth of mixing pile, the problems of anti-uplift, piping and leakage at the bottom of the pit are solved, and a composite soil nailing support composed of impervious curtain, advanced support and soil nailing is formed. Therefore, the composite soil nailing wall is suitable for sandy soil, muddy soil, cohesive soil, muddy soil and muddy soil. In all kinds of strata in Shanghai, there are successful engineering examples of using composite soil nailing wall.
Second, the project overview
Qingpu No.2 Water Plant is located on Xidaying River in the southwest corner of Qingpu District. This project consists of 16 monomers, including sedimentation tank, filtration tank, waste water tank, clear water tank and other large-scale pool structures, all of which use sand cushion to replace and fill the foundation, and the foundation is a large raft foundation. The elevation of sand cushion foundation is -5m, the elevation of raft foundation is -0.3m, and the thickness of sand cushion is 4.7m Spanish Deliman Company is responsible for the design of all installation systems and equipment supply for this project. Due to the late drawing of the system, after the structure of sedimentation tank and filtration tank is completed, the foreign design requires adding a φ 1000mm mud drain pipe with a buried depth of -3.8m, and setting a valve well every 15m along the length of the mud drain pipe. In order to bury this mud discharge pipe, a ditch must be dug on the sand cushion foundation between the sedimentation tank and the filter. The groundwater level is high, which is-0.7m; The sand cushion adopts medium-coarse sand, and its compactness is 1.65t/m3. Therefore, the foundation pits are all in the sand cushion foundation, and under the action of head pressure difference, sand flow and piping are easy to occur; Under the self-weight load of two monomers near the foundation pit, the sand has no possibility of self-supporting and is easy to collapse. Therefore, in this foundation, the scheme selection of foundation pit enclosure is very cautious.
Third, the foundation pit enclosure scheme
According to the functional characteristics of foundation pit support, combined with the actual situation of this project, the sand cushion is first considered for soil reinforcement, and the reinforcement scope is around the excavation face. Through soil reinforcement, on the one hand, the permeability coefficient of the reinforced soil is reduced to 10-5~ 10-8cm/sec to ensure the impermeability of the soil; On the one hand, the strength of the reinforced soil can reach more than 1Mpa, ensuring the self-reliance and strength of the soil. Because the distance between the sedimentation tank and the filtration tank is only 4m, the overhead height is only about 2.7m, and there is no place for large pile drivers to work around the foundation pit, it is impossible to use construction techniques such as cast-in-place pile, cement-soil mixing pile and jet grouting pile to reinforce the sand cushion foundation. The only feasible way is to adopt double liquid grouting, and the slurry and sand will become cement mortar blocks after solidification, which has good strength and impermeability. In view of the high groundwater level and good sand permeability, in order to ensure the grouting effect, it is decided to adopt layered compaction grouting technology to strengthen the sand cushion foundation. Due to the limitation of site and the consideration of cost, it is decided to adopt soil nailing wall as the supporting form of foundation pit: three rows of soil nails are arranged along the depth direction with the spacing of 1m, and plum blossom layout is adopted; The length of soil nail is 6.5m, and the steel pipe is welded by 48 * 3.5mm national standard.
In a sense, the combination of layered compaction grouting and soil nailing wall forms a new type of composite soil nailing support.
Fourth, the foundation pit enclosure construction
Construction technology of composite soil nailing wall;
1, layered compaction grouting
(1) Positioning and setting-out: According to the design scheme, determine the position of grouting holes for layered compaction and mark them with short steel bars;
(2) Drilling: after the hole position is determined, the drilling rig is in place and placed firmly, and then drilling is started, with a dry drilling method and a drilling diameter of 50mm；;
(3) Grouting: after the drill bit reaches the design elevation, connect the upper part of drill pipe (drill pipe is Ф 50 seamless steel pipe) with grouting pump, start grouting from the bottom, and inject cement slurry into the soil through hydraulic grouting pump; The drill bit is tubular, with a closed top and grouting holes with a diameter of 8 mm around it. After each layer of grouting is completed, lift the drill pipe by 0.3m, and inject it while lifting it until it reaches the orifice, pull out the drill bit, and seal the hole and set it;
(4) Maintenance: The maintenance time is 28 days, and the construction of soil nailing wall can only begin after 28 days of grouting. Before excavation, the compressive strength of grouting and solid is tested by static cone penetration test.
2, soil nailing wall construction
(1) Setting-out: according to the design drawings, determine the boundary line of foundation pit excavation, and mark the excavation line with wooden stakes and lime;
(2) Earthwork excavation: it will be carried out in three times, the first time to-1.8m, the second time to -2.8m, and the third time to-3.8m. Support while excavating, excavate by layers, support by layers, and then support after excavation. Earthwork excavation must be closely coordinated with supporting construction, and the next slope can not be excavated until the grouting of the upper layer of soil nail is completed 1 day. During excavation, the shovel head shall not hit the net wall and the nail head, and the excavation process and the soil nail wall construction form a circular operation;
(3) Slope repair: slope repair and leveling are required to ensure the quality of sprayed concrete;
(4) Making soil nails into holes: Make soil nails according to the design scheme, and make grouting holes around the steel pipe with a diameter of 8 mm, and the holes are spirally arranged on the steel pipe with a spacing of 50 mm There is no grouting hole at the steel pipe mouth within 1m, and the end of the steel pipe is closed. The position, spacing and angle of soil nails should meet the requirements of design drawings, and the processed soil nails should be driven by air compressor and welded into the soil in sections;
(5) Preparation of steel mesh: Straighten φ 6.5 steel bars, and bind steel mesh pieces according to the designed spacing. After the soil nail is drilled, φ 16 threaded steel bar and tic-tac-toe steel bar are welded on the steel mesh at the end, so that the steel mesh and the soil nail are connected into a whole. Soil nails, steel bars and connecting rods are connected by welding, and the weld length meets the specification requirements. The binding length of steel mesh and the staggered length of adjacent lap joints meet the specification requirements. If it can't meet the specification requirements, it must be welded firmly by electric welding.
(6) Spraying concrete: after earthwork excavation and slope repair, after steel mesh welding, spraying concrete, with the total thickness of spraying at one time ≥ 100mm, the stone particle size of 5~ 10mm, the maximum particle size < < 12mm, and the content of special accelerator for spraying concrete not less than 5%. Before each layer and each section of shotcrete construction lap joint, impurities such as dirt at the lap joint should be removed to ensure that the shotcrete lap joint is good and the quality of shotcrete is watertight.
(7) Soil nailing grouting: grouting can only be carried out after the surface sprayed concrete reaches a certain strength. For soil nailing grouting, before grouting, insert grouting pipe into the bottom of soil nailing, grout from the bottom of soil nailing, pull out grouting pipe while grouting, and then carry out pressure grouting at the mouth. The cement slurry is mixed according to the design, fully stirred, filtered with a fine screen, and then injected with an extrusion pump. Soil nailing grouting is controlled in two aspects, one is that the grouting pressure is controlled at 0. 1Mpa, and the other is that the single pipe grouting quantity is controlled at about 80L. In order to prevent water seepage at the end of the soil nail, the soil nail is filled and compacted with clay and cement bags after the soil nail is bored and before the shotcrete construction. When spraying concrete, the soil head should be sprayed and compacted first, and the grouting should be full, so as to avoid water seepage from the soil nail head.
Five, foundation pit monitoring
Information construction is a feature of the new technology of foundation pit support-soil nailing wall support technology. In order to ensure the safety of foundation pit and not affect the surrounding buildings, it is required to grasp the dynamic changes of slope at any time during the whole excavation and support construction process, so information construction must be implemented in the support construction process. And through modifying the design, the obtained information will be fed back to the construction operation to guide the construction. According to the requirements of Grade III foundation pit, this foundation pit mainly measures displacement and settlement. The data are as follows:
1. Slope monitoring of foundation pit: horizontal displacement observation points and settlement observation points are arranged along the periphery of foundation pit, and 1 point is arranged every 2 meters. After 4 days of soil nailing wall construction, the maximum horizontal displacement is only 3.5mm and the maximum settlement is 7.2 mm, and the displacement and settlement tend to be stable.
2. Monitoring of sedimentation tank and filter: 1 settlement observation points are arranged every 2 meters along the upper mouth of sedimentation tank and filter tank wall, and 1 displacement observation points are arranged every 5 meters near the pit surface of tank wall. The maximum settlement is 2.8 mm, and there is no displacement.
3. Before excavation, the average compressive strength of grouting and solid was 1.78Mpa by static cone penetration test, which proved that layered compaction grouting was successful in strengthening sand cushion, and the strength of cement mortar block after reinforcement reached the designed expected strength of 1 Mpa.
Conclusion of intransitive verbs
1, according to different working conditions and geological conditions, choosing the appropriate form is the principle of foundation pit support design. Whether gravity retaining wall or non-gravity retaining wall, the essential point of foundation pit support is to stop water and retain soil in the pit for safe construction, but the calculation method and construction technology are different.
2. The layered compaction grouting successfully solved the problem of sand reinforcement in the sand cushion, which made the sand and double liquid slurry coagulate and solidify into mortar blocks, which not only ensured the strength of the foundation, but also formed a dense waterproof curtain.
3. Soil nailing wall has successfully solved the problems of strength and stability of foundation pit slope. Its construction period is short, and excavation is carried out at the same time, which rarely takes up independent construction period. Excavation and soil nailing construction in layers and blocks give full play to the spatial support function of soil, and are closed within a few hours after excavation, so that the displacement and deformation of slope are limited in time.
4. It can be seen that the new composite soil nail support combined with layered compaction grouting and soil nailing wall has less displacement and deformation in this project, which provides safety guarantee for pit construction and is a successful foundation pit enclosure system.
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