Although foundation pit support technology has accumulated considerable successful experience across different regions and geological conditions—some even reaching highly advanced levels—there remain issues that require further consideration or improvement to meet the needs of current construction.

The common problems and corresponding countermeasures during the support construction of deep foundation pit projects are mainly as follows:

Common quality problem 1: Displacement

The support structure displaces toward the inner side of the foundation pit, causing ground settlement behind the piles and cracks in nearby buildings, as well as slope sliding and loss of stability.

Preventive measures:

1. The cross-section and embedment depth of the retaining piles in the support structure shall be strictly calculated to prevent omission of additional loads such as soil stockpiled on the ground at the pile top, traveling machinery, transport vehicles, and stored materials;

2. The cast-in-place piles and the water-blocking jet grouting piles must be tightly combined to form a closed water-stop curtain, preventing large amounts of soil behind the piles from flowing into the foundation pit under dynamic water pressure;

3. Before excavation of the foundation pit, the entire support system, including soil anchors and pile-top ring beams, shall be completed, and the retaining piles shall reach the required strength to ensure the strength and overall stiffness of the support structure and reduce deformation;

4. Anchor construction must ensure that the anchors can penetrate into a reliable anchorage layer;

5. During construction, management shall be strengthened to avoid large surcharges, parking of excavation machinery, and transport vehicles on the support structure;

6. Dewatering shall be carried out before excavation of the foundation pit to reduce lateral earth pressure on the piles and the inflow of water into the pit that causes pile displacement;

7. When displacement is detected through monitoring, unloading and additional piling shall be carried out at the locations with larger displacement, or cement grouting shall be used at those locations to reinforce the soil layer.

Common quality problem 2: Piping and flowing sand

During excavation of the foundation pit, the soil at the bottom of the pit flows and, together with groundwater, surges into the pit from the bottom or surrounding sides, causing settlement of the surrounding ground and cracks in buildings.

Preventive measures:

1. Geological investigation shall be strengthened before construction to ascertain the soil conditions;

2. The retaining piles should preferably penetrate through the silty fine sand layer at the bottom of the foundation pit;

3. When there are gaps between the retaining piles, water-blocking jet grouting piles shall be installed on the back side to block water, avoiding water-flow gaps that cause soil and water loss and inflow into the foundation pit;

4. The depth of pile embedment into the pit bottom shall be determined by calculation, ensuring that the submerged density of soil particles is greater than the upward seepage dynamic water pressure on the pile side;

5. The water-stop piles shall be designed to be tangent to the retaining piles and maintain tight combination, so as to improve support stiffness and act as a curtain wall;

6. During construction, wellpoint or deep well dewatering shall first be used for effective dewatering of the foundation pit;

7. Traveling of large machinery and mechanical excavation shall prevent damage to water supply and drainage pipelines; any rupture found shall be repaired promptly.

Common quality problem 3: Collapse

Failure of the support structure during excavation of the foundation pit, resulting in local large-scale instability and collapse of the slope.

Preventive measures:

1. The retaining piles shall be designed with sufficient stiffness and strength and connected into an integral whole with top ring beams;

2. Soil anchors shall penetrate into firm soil layers and be grouted densely;

3. The retaining piles shall have sufficient embedment depth and be embedded into firm soil layers to ensure the overall stability of the support structure;

4. Before excavation of the foundation pit, effective dewatering methods shall first be used to lower the groundwater to 0.5 m below the excavation base;

5. The practice of supporting as excavation proceeds shall be prevented; in particular, construction shall follow the procedures specified in the design, and the stress state of the support structure shall not be arbitrarily changed, nor shall large construction loads not considered in the support design be arbitrarily added to the support structure.

Necessary monitoring shall be carried out for important foundation pit projects, including:

1. Deformation of the support structure;

2. Ground deformation around the foundation pit;

3. Deformation of adjacent projects and underground facilities;

4. Groundwater level;

5. Conditions such as leakage, water emergence, scouring, and piping.

The following documents shall be available for acceptance of foundation pit support:

1. Construction records and as-built drawings;

2. Location relationship diagram between the slope project and surrounding buildings;

3. Factory certificates of conformity and re-inspection reports for raw materials;

4. Concrete strength test reports and compressive strength test reports of mortar test blocks;

5. Anchor pull-out test reports;

6. Deformation monitoring reports of the slope and surrounding buildings;

7. Design change notices, documents for handling major issues, and technical consultation records.