Keywords: deep excavation, TRD method, soft soil foundation, waterproof curtain
Abstract The retaining structure with a waterproof curtain outside the retaining soldier piles has been widely used in engineering projects due to its economic efficiency. However, as excavations become larger and deeper, ordinary triple-axis mixing piles as waterproof curtains struggle to meet construction quality requirements. Combining an engineering case, this paper introduces the TRD method as a new type of cement-soil mixing wall, which has good adaptability and water tightness. The feasibility and reliability of TRD construction in soft soil foundations are verified.
1 Key Construction Points of Ultra-Deep TRD Method
1.1 Obstruction Removal
To ensure the smooth progress of the TRD trial section and the quality of TRD construction, obstruction removal must first be carried out in the TRD construction area. A pneumatic breaker is used to break obstacles, and then an excavator removes the broken obstacles and debris, which are transported away promptly. After obstruction removal, to ensure subsequent TRD construction quality, equipment stability, and wall verticality, the cleared area is backfilled with 10% cement-soil and compacted.
1.2 TRD Construction
1.2.1 Ground Treatment
Underground obstacles are treated first. After treatment, 10% cement-soil is used for backfilling. After leveling the site, steel plates are laid. This ensures the flatness and bearing capacity of the construction site meet the requirements for large machinery movement.
1.2.2 Surveying and Positioning
Guide trenches are excavated and steel plates laid according to the axis of the TRD mixing wall. Positioning steel beams for the mixing piles are placed beside the trench, and the mixing pile positions are marked on the positioning steel beams. During TRD construction, the wall position should be centered. Before wall construction, two theodolites are used to check the verticality of the main machine's guide frame to ensure a wall verticality of 1/250.
1.2.3 Trial Wall Construction
Before construction, a trial wall should be constructed to determine technical parameters, pile formation techniques and steps. For strata with large soil property differences, layered technical parameters need to be verified based on geological reports. Based on the trial wall results, construction parameters such as bentonite content in the excavation fluid, cement content in the curing fluid, and water-cement ratio of the cement slurry are determined; wall quality, cement mixing uniformity, bonding degree and strength are determined; the verticality of the column guide and the wall verticality are determined.
1.2.4 Emergency Measures
For ultra-deep TRD pile construction, due to long construction time, large thrust and pressure on the cutting box, and high cutting resistance of the cutter head, the cutting box may lose power due to insufficient power supply or unstable voltage. Therefore, a backup generator set must be provided on site. In case of abnormal power supply, slurry supply, air pressure, and normal mixing operations can be restored promptly to avoid buried drill accidents caused by delays.
2 Quality Control of Ultra-Deep TRD
2.1 Surveying and Positioning
After the TRD main machine is in place, the wall position should be re-measured. If there is any error or deviation, the main machine must be adjusted and repositioned. Only when the wall position is accurately centered and the verticality deviation of the main machine's guide frame is ≤1/250 can TRD construction proceed.
2.2 Verticality Control
2.2.1 Calibrating the Verticality of the Main Machine's Guide Rod
After the TRD machine is assembled and relocated, a theodolite is used to calibrate the verticality of the pile driver's column guide frame from the front and side. During the driving of the cutting box, the verticality of the cutting box is checked at any time.
2.2.2 Installing Inclinometer
After the cutting box is driven to the design depth, an inclinometer is installed inside the cutting box to monitor the in-plane and out-of-plane deviations in real time. Adjustments are made promptly through the operator's control to ensure the verticality of the TRD equal-thickness cement-soil mixing wall meets design requirements, as shown in Figure 1.
2.3 Mixing System Control
Strictly control various parameters of the automatic mixing system, including cement slurry mix ratio, air pressure, and slurry pressure. Once parameters are input into the system, they must not be changed arbitrarily.
2.4 Cutting Speed Control
Strictly follow the three-process wall construction sequence for trial wall construction. Control the lateral advance speed of the cutting box and the rotation speed of the cutting chain to ensure uniform mixing of cement-soil. During wall mixing, keep mixing records and strengthen cement content control; meanwhile, control the grouting rate and coordinate with the cutting box advance speed for grouting.
3 Engineering Case
3.1 Project Overview
A project in Shanghai has a total excavation area of about 46,090 m2, a total perimeter of about 890 m, and an excavation depth of about 17 m. The surrounding retaining structure uses cast-in-place pile soldier piles combined with an external TRD waterproof curtain. Three levels of concrete struts are installed vertically inside the excavation. Outside the cast-in-place pile soldier piles, a single-row 800 mm thick TRD waterproof curtain is installed, using a three-cycle cement-soil mixing wall construction process for continuous wall formation. The elevation range is -0.5 to -40.5 m (-45.5 m, -49.5 m), with 25% cement content using P.O42.5 cement, and pile lengths of 40 m, 45 m, and 49 m.
3.2 Construction Planning
The original design of this project used a triple-axis mixing pile waterproof curtain. After expert review and discussions among the participating parties, the owner, and the designer, the construction process was improved to use the new TRD method for the ultra-deep waterproof curtain. To ensure the construction quality of the TRD waterproof curtain, an 8 m long TRD trial section was first constructed, with a depth of 52 m and a thickness of 800 mm.
3.3 Analysis of Monitoring Results
3.3.1 Surface Vertical Displacement Monitoring
Analysis of monitoring data shows that surface deformation caused by wall construction is relatively small, with variation within ±5.0 mm. The surface deformation during the trial section construction is as follows: from the start of trenching to the end of wall formation, the ground surface experienced slight uplift, with relatively obvious uplift points at 2.2–4.4 mm; after wall completion, the ground surface deformation was recovering and settling, with the fastest recovery within 24 hours after wall completion, collectively showing a transition from uplift to settlement. After 36 hours, although surface deformation developed slightly, it basically stabilized over time.
3.3.2 Deep Soil Inclinometer Monitoring
Analysis of deep horizontal displacement monitoring results shows that the soil lateral displacement pattern caused by wall construction is as expected. From the start of trenching to the end of wall formation, due to the extrusion of grout from the wall, deep horizontal soil displacement is mostly away from the wall. The closer to the trial wall section, the larger the displacement amplitude; the maximum displacement reached -10.4 mm at a depth of 35 m. Based on the depth analysis of deep horizontal displacement at each monitoring point, the larger displacement values occur at depths between 30 and 50 m. 24 hours after wall completion, the displacement rate slowed down, and deformation gradually stabilized.
3.3.3 Soil Layered Settlement
Analysis of the summary table and curves of soil layered settlement monitoring results shows that the vertical displacement pattern at different depths caused by wall construction is as expected. During the trenching and wall formation stages of the trial section, the layered vertical displacement of the soil generally showed uplift, with a maximum cumulative uplift of 7.1 mm. After wall completion, each soil layer generally showed settlement, with a maximum cumulative settlement of 10.2 mm. From the recovery settlement amplitude at each monitoring point, the closer to the excavation, the larger the recovery settlement amplitude, and vice versa.
3.3.4 Monitoring Conclusions
(a) The surrounding soil deformation pattern caused by TRD construction is consistent with that of conventional mixing pile construction, both including two stages: extrusion deformation and deformation recovery. However, the soil deformation values caused by this process are smaller and recovery is faster. This regularity has significant guiding significance for the subsequent large-scale TRD retaining construction. (b) The various soil deformations are not large in numerical magnitude, which is beneficial to the safety of the surrounding environment during formal TRD construction. However, the factor of small soil deformation values due to the limited scale of the trial section should also be considered. (c) The presence of thick loose miscellaneous fill at the surface can largely dissipate the instantaneous increase in extrusion stress in the soil, which is beneficial for controlling deformation of surrounding pipelines and the ground surface.
4 Conclusion
This paper provides a detailed introduction to the construction technology, key points, and quality control measures of the ultra-deep TRD cement-soil mixing wall. Combined with its application in a project in Shanghai, a further understanding of its applicability in solving confined water problems in ultra-deep excavation projects has been achieved.