New method of caisson construction in large flow pump house in Tianjin flow plastic silt
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Tianjin Economic and Technological Development Zone Sewage Treatment Plant is upgrading its investment environment and establishing environmental infrastructure. The sewage pump room was originally built in an area known as Salt Wang Zi Sea sedimentation tank, located about 2 meters below the surface of the earth fill. Over many years, this area has been filled with silt-like sediment, making it difficult to construct traditional open-cut foundations. Therefore, a deep underground pumping station was designed, reaching a depth of 13 meters with a convex shape. Due to the presence of nearby substations, dehumidifier rooms, and mud cake storage pits, it was not feasible to use conventional foundation pit excavation methods. Instead, a polygonal caisson structure was chosen. However, controlling the sinking speed and direction of the caisson proved challenging, and this issue remained unresolved for some time.
To address these challenges, the 18th Bureau of China Railway Construction implemented a design that involved prefabricating DJM piles at the base of the caisson to form an underground continuous wall. This approach provided support for the shaft wall, turning the sinking process into a controlled operation and successfully solving the construction difficulties.
**1. Construction Preparation**
**1.1 Excavation and Reduction of Initial Sinking Elevation**
Based on geological surveys, to ensure balanced sinking in the initial phase, the artificial fill layer was excavated. The precast caisson’s initial settlement elevation was set to 0.48m. This created two advantages: first, the soft silt had uniform moisture and bearing capacity, making it easier to stabilize; second, by reducing the total sinking depth by 2.5m, the top third (0.5m) of the caisson could be avoided during construction, minimizing weight and shortening the sinking depth.
**1.2 Powder Spray Pile Construction**
DJM piles were used to reinforce the soft soil beneath the caisson, forming a continuous diaphragm wall that supported the sinking process. Key considerations included:
- The outer edge of the pile should be tangent to the shaft wall to prevent cement soil from squeezing outward during sinking.
- The pile bottom should reach 16m below the caisson edge.
- The outer ring pile’s top 1.0m range should have 10% cement content, while the rest should have 7%.
- The inner pile should have 10% cement content.
- The spacing between inner and outer piles should be 10cm to avoid damage during excavation.
**2. Caisson Prefabrication**
**2.1 Site Layout**
The caisson was prefabricated at the location of the pump pool. To facilitate construction and concrete pouring, the site surface was compacted with 15cm of 8% lime soil, and a 10cm limestone platform was laid under the support wall.
**2.2 Prefabrication Method**
Originally designed as three sections, after excavation, the ground level matched the second section’s top. Therefore, the third section was no longer considered part of the caisson structure. The first section was poured in two stages, with the first 1.0m poured until 70% of the design strength before completing the rest.
**2.3 Blade Foot Mold and Support Base Mold**
Two rows of DJM piles were used to reinforce the soft soil under the blade foot. The original ground had a bearing capacity of 80kPa, and after reinforcement, the unconfined compressive strength reached up to 800–1000kPa. A supporting base mold was added to share the caisson’s weight during prefabrication.
**3. Caisson Sinking**
**3.1 Preparation**
Before sinking, the caisson’s concrete must reach design strength. Observational signs were marked on the well walls, and standard points were set at the corners to monitor subsidence and balance. Modules were removed, and the top layer of soil was excavated.
**3.2 Sinking Coefficient Calculation**
The sinking coefficient formula K = Q / (f · h · L) > 1 was used, where Q is the caisson’s gravity, f is the friction coefficient (9.8–11.76 kN/m²), h is the maximum sinking depth, and L is the perimeter. With a typical coefficient of 3.0, the sinking process was manageable.
**3.3 DJM Pile Wall Control Mechanism**
The DJM piles formed a continuous wall that guided the caisson and prevented sudden sinking or soil infiltration. When the caisson reached the design elevation, the blade edge rested on the DJM pile top, ensuring accurate depth control.
**3.4 Excavation Method**
Surface soil was manually excavated, while the rest was washed away using high-pressure water jets. Soil was removed in layers, with careful attention to avoid bending moments on the support wall.
**3.5 Caisson Correction**
Due to the guidance of DJM piles, the caisson rarely deviated. If necessary, adjustments were made through monitoring and manual excavation or filling.
**4. Caisson Back Cover**
After sinking, the caisson’s deviation must meet standards: axis displacement ≤1% of well depth, elevation ±40mm to -60mm, and inclination ≤0.7%. Once stable, backfilling was performed with seepage pipes and hole placements to manage groundwater.
**5. Conclusion**
In soft silt, sudden sinking and over-sinking are common. By using DJM piles to create a continuous bearing wall, the sinking process became controllable. Adjustments in pile length, diameter, and cement content allowed for precise control of the caisson’s position, speed, and depth. This method is simple, cost-effective, and highly successful for similar projects.
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