In the construction of super high-rise buildings in cities, difficulties are often increased due to the narrow site of deep foundation pits, large excavation depths, and various problems existing in slopes with complex soil conditions. To improve the construction quality of deep foundation pits in narrow sites, attention must be paid to the following three aspects. Here is a summary of key points for foundation pit construction. Take a look.

I. Construction Dewatering Treatment

Deep foundation pit dewatering needs to meet certain water control requirements. To meet structural and water-stop requirements, the joints between diaphragm wall panels adopt cross-shaped rigid connections, with water-stop steel plates 14mm thick and two joint steel plates 10mm thick.

1. Dewatering Scheme

(1) The hole is formed by drilling with original soil mud wall protection. After positioning, drill to the required depth and use the drilling rig for hole cleaning. After drilling and cleaning, lower the sand-free cement pipe into the tube well and position it, and promptly backfill medium-coarse sand at the bottom and small gravel between the well pipe and hole wall as a filter layer, and perform well washing until the water is clear.

(2) Before excavation, dewater using submersible pumps with a diameter of 40mm, head of 25m, and flow rate of 15m³/h in the dewatering tube wells, with one pump per well. After dewatering, the groundwater level should be 0.5m below the base. During dewatering, observe the water levels in the dewatering wells and observation wells at all times. If the water level in the observation wells outside the diaphragm wall drops and affects surrounding buildings, stop pumping or inject water into the observation wells. After pumping for 7 days, if the groundwater level remains at this elevation for more than 2 days, earthwork excavation can proceed.

2. Earthwork Excavation and Support Scheme

Earthwork excavation can adopt layered excavation, construction of soil transport ramps, and final grabbing and transport. The foundation pit has three steel supports and one reinforced concrete support. Following the principle of supporting before excavating and supporting as excavating, the earthwork is divided into four excavation stages, with earthwork and reinforced concrete support alternating, i.e., excavate one layer and install one layer of support; after the support reaches 100% strength, excavate the next layer and install the next support. In addition, to avoid over-excavation, when excavation reaches 20cm above the base, manual excavation begins until the design elevation. The manually excavated 20cm thick soil and broken pile head concrete are cleared as excavated and transported off-site as the excavator retreats. Blind ditches are excavated to connect the dewatering wells, and large-sized gravel is filled in the ditches. After the base is excavated to the elevation and a considerable plane is formed, organize relevant personnel to inspect the trench immediately, and then pour C15 concrete cushion.

II. Construction Method of Underground Diaphragm Wall

1. Form and Fabrication of Guide Wall

The guide walls are all inverted L-shaped reinforced concrete guide walls. The clear width between the two guide walls is 850mm (sections AB, BC, HI, IA) or 1050mm (sections CD, DE, EF, FG, GH). The depth of the guide wall is not less than 1.800m or 2.100m (section FG), and must penetrate into undisturbed soil. The guide wall flange is 1.0m, thickness 0.2m, with double-layer bidirectional A12@200 mesh reinforcement.

2. Preparation and Use of Mud

(1) Mud Preparation

Bentonite mud must be fully stirred, otherwise it will affect the water loss and viscosity of the mud. The feeding order is generally water, bentonite, CMC, dispersant, and other additives.

To fully utilize the role of mud in underground diaphragm wall construction, the mud should preferably be used after the bentonite is fully hydrated. Newly prepared mud should be left standing for more than 3 hours; if site conditions permit, standing for 24 hours before use is even better.

(2) Mud Treatment

During construction, the mud contacts the soil layer and becomes contaminated, causing deterioration. The contaminated and deteriorated mud can be reused after physical and chemical treatment.

(3) Mud Use

The performance indicators of all types of mud should comply with national standards, and can only be used after sampling tests meet the qualified standards. During trenching, the mud level in the trench should be maintained at least 0.5m above the groundwater level and not lower than 0.3m below the top of the guide wall. Replenish mud promptly if the level drops to prevent trench wall collapse.‍

3. Trenching and Excavation

(1) For straight trench sections, adopt the method of grabbing both sides first and then the middle; for corner trench sections, grab the short side first and then the long side. During trenching, the grab bucket should be lowered vertically along the centerline of the guide wall.

(2) The excavation speed of the trenching machine should be controlled at about 15m/h. The grab bucket should not excavate too fast to prevent trench wall instability. When excavating to 2~3m above the trench bottom, use a measuring rope to measure the depth to prevent over-excavation and under-excavation, and control the sediment thickness within 10cm.

(3) After trenching to the design elevation, for connection and closure sections, first remove the soil on the side wall of the previously constructed diaphragm wall, then clean the trench by moving the grab bucket about 50cm each time.

(4) During trenching, large machinery must not frequently move along the edge of the trench section to ensure trench wall stability.

(5) After trenching, use a combination of grabbing and mud circulation for bottom cleaning to ensure the sediment at the bottom is not more than 100mm.

4. Trench Cleaning and Bottom Cleaning

Remove sediment at the trench bottom by sedimentation method.

5. Reinforcement Cage Lifting

Before lifting, consider the thickness of the underground diaphragm wall, the width of the heaviest reinforcement cage, the heaviest weight of the reinforcement cage, and the length of the reinforcement cage. Select lifting equipment based on actual conditions.

6. Construction Quality Assurance Measures

(1) Seepage prevention measures for underground wall joints. Based on project characteristics, the following measures can be taken: After trenching with hydraulic grab, install special scraper blades on the teeth of the hydraulic grab to closely adhere to the end of the underground wall, and move up and down repeatedly to remove mud at the joint using the power of the hydraulic grab. Strengthen hole cleaning by pumping out mud with high sand content to reduce the sand content in the mud.

(2) Control during concrete pouring. Strictly control the depth of the tremie pipe embedded in concrete to always remain between 2~6m, not exceeding 6m, and absolutely avoid the pipe being pulled out. If the pipe is accidentally pulled out, immediately measure the concrete surface elevation, remove the mud on the concrete surface, and then reinsert the pipe with a ball to continue pouring concrete.

(3) Ensure the supply of ready-mixed concrete. Construction technicians must review the concrete mix design provided by the batching plant and test the slump of the concrete upon arrival at the site to ensure the quality of commercial concrete supply.‍

7. Measures to Reduce Sediment Thickness

If the foundation pit is very deep, each process takes a long time, and often although after trench cleaning and bottom cleaning the sediment and mud indicators meet requirements. For this, determine the mud viscosity to be not less than 30s, and strengthen trench cleaning and bottom cleaning to reduce sediment thickness.

8. Technical Measures to Ensure Trench Stability

The geological conditions at the project site are extremely complex. The silt layer and gravel layer revealed in the survey report are prone to trench collapse under dynamic water conditions. How to ensure the stability of the trench during trenching is a major challenge for such projects.

III. Construction Key and Difficult Points and Solutions

1. Construction Key and Difficult Points

(1) Difficulty in controlling trench verticality. If the deepest underground wall of the project exceeds 800mm in thickness, high requirements are placed on trenching equipment. Such deep trench walls place extremely high demands on the weight of the grab bucket, grab height, opening/closing pressure, bucket hinge capacity, and equipment monitoring and precision control systems.

(2) Difficulty in controlling mud wall stability. The geological conditions at the project site are relatively complex. The saturated silt layer and gravel layer within the excavation range basically meet the conditions for quicksand occurrence. Under dynamic water pressure, diameter reduction or quicksand may occur, which are unfavorable factors for diaphragm wall construction.

(3) Difficulty in fabrication, lifting, and installation of reinforcement cages. Due to the large weight and excessive length of reinforcement cages in each trench section, such ultra-long, heavy, and irregular structures always present challenges in deformation control and lifting safety control during installation.

(4) Difficulty in seepage control of the enclosure structure. The seepage resistance of the enclosure structure directly affects the seepage resistance of the permanent structure. Usually, once leakage occurs in the enclosure structure, the probability of leakage at that location in the permanent structure is very high. Due to the large depth of the enclosure in such projects, leakage is very likely to occur at the joints between adjacent trench walls and can easily cause engineering accidents.‍

2. Solutions

The construction of underground diaphragm walls in deep foundation pit projects is one of the most critical technical issues. Measures include the configuration of trenching and lifting equipment, control of mud mix ratio, and setting up heavy-duty roads.

(1) After trenching, first press the teeth of the hydraulic grab against the end, then repeatedly scrape up and down to remove large mud blocks adhering to the joint.

(2) Determine lifting points and lifting methods by calculation to improve safety while reducing deformation of the reinforcement cage.

(3) Seepage control of the enclosure structure: Carefully perform wall brushing work to ensure the quality of the brushing process. Arrange tremie pipes according to regulations to avoid excessive distance from the end causing mud inclusion in joints. Strictly follow requirements when lifting the lock pipe to avoid concrete collapse due to premature lifting.‍

IV. Scheme Innovation

1. Problems

(1) Due to the narrow site, the road width around the foundation pit is only 1.5~3m, and earthwork excavation and transport machinery cannot park or operate.

(2) Due to the limitation of the circular internal support of the foundation pit needing to be closed to form an integral load-bearing structure, the transportation of earthwork is restricted.‍

2. Proposal of Temporary Steel Trestle Scheme

(1) Steel trestles can be installed quickly, and production and installation can be carried out in flow with earthwork excavation, with almost no impact on the schedule;

(2) They can be constructed section by section according to the process, and can be used without completing the entire structure, in sync with earthwork excavation;

(3) They can be quickly removed section by section according to the basement progress.‍

3. Deepening of Scheme Content

After confirming the earthwork transportation method, a careful analysis was conducted on how to effectively improve earthwork excavation efficiency, and a series of scientific optimizations were made to the original earthwork excavation scheme, determining a series of excavation principles: "layered, block-wise, symmetrical excavation, maintaining balanced forces on the foundation pit."