Core tip: Project overview. The xx district is located on the east bank of the Fen River, belonging to the alluvial-proluvial Grade I terrace landform. It is mainly composed of Quaternary Holocene artificial fill, Quaternary Holocene fluvial alluvial-proluvial deposits (Q4al+pl) and Upper Pleistocene

Project Overview

　　The xx district is located on the east bank of the Fen River, belonging to the alluvial-proluvial Grade I terrace landform. It is mainly composed of Quaternary Holocene artificial fill, Quaternary Holocene fluvial alluvial-proluvial deposits (Q4al+pl) and Upper Pleistocene fluvial alluvial-proluvial deposits (Q3al+pl), dominated by artificial fill, silty clay, silt and sandy soil. The TISCO 1.5 million-ton stainless steel steelmaking project is located on this stratum. The foundation treatment mainly uses Φ600 high-strength prestressed concrete pipe piles, with the bearing stratum being the 7th layer of dense medium sand. The soil conditions of each layer are as follows: 1—1 miscellaneous fill, loose; 1—2 plain fill, loose; 2—1 silt, medium dense to dense; 2—2 fine sand, loose to slightly dense; 2 silty clay, soft plastic to plastic; 3—1 silty clay, plastic to soft plastic; 3 silt, medium dense to dense; 4—1 medium sand, medium dense; 4—2 silty clay, soft plastic; 4 silt, medium dense to dense; 5—1 silt, medium dense to dense; 5 fine sand, medium dense to dense, standard penetration test measured blow count N of 20.0-50.0 blows, average 35.4 blows, thickness 0.60—5.20m; 6—1 silty clay, plastic; 6 silt, medium dense to dense, SPT blow count N of 20.0-45.0 blows, average 31.4 blows, thickness 1.00-10.60m; 7—1 silt, local lens, dense; 7—2 round gravel, local lens, dense; 7 medium sand, SPT blow count N of 43.0-50.0 blows, average 48.4 blows, thickness 5.00-11.90m, dense. The project was designed by CISDI. The PHC piles are end-bearing plus friction piles, requiring the pile tip to penetrate into the bearing stratum (7th layer of dense medium sand) ≥2.5d (d is the pile diameter). Depending on the burial depth of the 7th layer, piles of different lengths from 18m∼25m were selected, with closed pile tips, using crawler-type pile drivers and D80 diesel hammers. The steelmaking main plant building was designed with a total of 3,269 PHCAB600a piles, with a total length of 73,216 linear meters. Construction started on January 18, 2005, and was completed on April 17. Before pile driving began, the owner formulated the ≪PHC Pile Construction Work Instruction≫ based on test pile results. The hammer stop criterion followed the dual-control principle of elevation and penetration, with penetration for the last three sets of ten blows each ≤40mm, and anchoring into the bearing stratum as required by design. However, in actual construction, piles generally could not be driven to the required depth, so the standard was raised to the last three sets of penetration ≤30mm, yet a considerable number of piles still could not reach the required depth. Piles that did not reach the required depth accounted for 90.8% of the total driven piles, with pile tips generally 1.6m∼4m higher than design, and many piles had their heads shattered during driving. After analysis, two main reasons were identified. First, because the ○5 layer of fine sand has very high density, with an average SPT of 35.4 blows and a general thickness of 3 m to 5.2m, and the ○6 layer of silt has an average SPT blow count of 31.4 blows with a general thickness of 2.7 m to 5m, forming two hard interlayers (sometimes merged into one depending on the stratum), it is very difficult for pipe piles to penetrate, and even more difficult to penetrate into the ○7 layer. Second, dewatering during earlier construction of surrounding projects increased the foundation density, making pile driving difficult. 2 Owner's scheme. The owner proposed using a dry jet mixing machine to inject clean water for pre-boring, with the aim of disturbing the stratum within a Φ600∼800mm range along the pile depth through high-pressure water to reduce pile driving resistance and achieve the required pile depth. However, this machine model has a maximum pre-boring depth of 19m and cannot penetrate the ○5 layer. At the same time, it requires disturbing the soil over the full height and cannot selectively disturb the hard interlayers. As verification, two holes were pre-bored using this method. After pre-boring, a vertical hole of 500mm diameter was formed, with thin mud slurry in the upper part and thicker mud slurry in the lower part. When driving the first pile, an open pile tip was used, and the first pile section burst due to instantaneous high pressure caused by the mud slurry in the pile core hole. The second pile used a closed pile tip, but during driving the thin mud slurry splashed everywhere, making normal construction impossible. It was decided to abandon this scheme. Others proposed using an auger drill for pre-boring, but because it still involves full-height pre-boring and discharges soil, it was rejected. 3 Jet grouting pre-boring and pile driving scheme. In response to the above situation, after multiple studies and discussions with the design unit, supervision unit and owner, it was decided to use high-pressure jet grouting technology for pre-boring and then drive the piles. The specific scheme is as follows: (1) Conduct survey and setting out to accurately locate pile positions; thoroughly understand the stratum conditions at the pile positions, especially the burial depth and thickness of hard interlayers; position and level the high-pressure jet grouting machine and prepare bentonite slurry. (2) Lower the drill pipe of the high-pressure jet grouting machine while injecting clean water at an injection pressure of 20MPa. All jet holes at the bottom and side of the drill bit spray water flow to facilitate drilling. (3) Stop when the drill bit reaches 3m~4m above the pile tip. At this point, the ○5 layer has been penetrated, and the ○6 layer has been penetrated by less than 1m. Then place a steel ball inside the drill pipe core to seal the bottom jet hole so that grout can only be sprayed from the side, and then switch to spraying bentonite slurry while lifting the drill pipe. The grouting pressure is raised to 25∼28MPa, rotation speed 18∼20r/min, lifting speed 0.25 m/min, so as to disturb and suspend the hard interlayer, forming a disturbed cylinder of Φ600∼800mm. The bentonite slurry ratio is 1:1, with an appropriate amount of caustic soda added. The bentonite consumption for hard interlayers is about 35kg/m (300kg/8m). To prevent fine sand from clogging the jet holes, before connecting or disconnecting the drill pipe, move it up and down several times, and connect or disconnect only when the liquid level in the borehole drops slightly. (4) Based on the geotechnical investigation report and the drilling speed in different soil layers, the depth and thickness of hard interlayers can be determined relatively accurately. When the drill bit is lifted above the hard interlayer, switch to spraying bentonite slurry diluted to 1:8~1:10 until it is lifted out of the ground. (5) When injecting bentonite, the grouting speed is fast, the energy is large, and the jet is pulsating. Under the impact force, centrifugal force and pressure difference of the jet flow, mud, sand and soil particles are mixed with the slurry and carried out of the ground through the annular space on the side of the drill pipe. From the amount of fine sand particles carried out of the ground, the size of the void formed in the pre-bored hole can be visually observed, with the principle that it should not exceed 1/2 of the pile volume. The underground part of the bentonite slurry mixes with coarse sand particles and remains suspended in the pre-bored hole. However, due to diffusion effects, pile driving must be carried out immediately after pre-boring is completed, with an interval not exceeding 2 hours. Since the hard soil layer has been disturbed and softened, pile driving will be relatively smooth. (6) After pre-boring is completed, drive the pile immediately without delay. The pile hammer remains unchanged, still using the D80 diesel hammer. Use an open pile tip so that part of the soft soil enters the pile cavity to reduce pile driving resistance. The hammer stop criterion during pile driving is comprehensively controlled by elevation combined with penetration, with elevation as the main factor and penetration as the auxiliary factor. 4 Implementation effect. (1) Pile driving practice has proven that using high-pressure jet grouting pre-boring to disturb hard interlayers, with 25m piles pre-bored to a depth of 22m, can basically drive the piles to the required depth, and the penetration is within 30mm per set for the last three sets of ten blows. After pile driving, the pile tip elevation is mostly within 500mm above the design elevation. Large-strain tests on single pile bearing capacity all met the requirements. (2) After excavation of the diaphragm wall for auxiliary raw materials, the support pipe piles in the foundation pit were also exposed. Observation of the soil around the piles further verified the theoretical analysis. The stratification distribution of the soil around the piles is very obvious. Close to the pile is a 10~28mm bentonite consolidated body, and further out is a consolidated body of bentonite mixed with silty sand and coarse sand, with particle size gradually increasing from inside to outside. The density and strength of the consolidated body are higher than those of the stratum soil, and it bonds very tightly with the pile, overturning some people's concerns about pile bearing capacity. (3) The design unit affirmed this process method and explicitly required its use in subsequent pile designs to meet pile driving requirements.