Abstract

The main types of foundation pit support structures for utility tunnels include sloped excavation, soil nail walls, gravity cement-soil walls, steel sheet piles, precast concrete sheet piles, steel-reinforced cement-soil mixing walls, cast-in-place pile rows, precast concrete pile rows, steel supports, concrete supports, large-diameter recyclable jet grouting anchors, groundwater control, etc. This atlas includes the layout, structural details, and design requirements of the above support forms. During design, the appropriate foundation pit support form should be selected based on local actual conditions.‍

I. Principles for Selecting Support Systems

1. For utility tunnel foundation pits that are relatively deep, have limited site space, or have high requirements for surrounding environment protection, a sheet-pile support system composed of steel sheet piles, precast reinforced concrete sheet piles, steel-reinforced cement-soil mixing walls, cast-in-place pile rows, precast concrete pile rows, etc. as the retaining body, combined with internal supports or anchors, is applicable. 2. In soft soil layers, when steel sheet piles are used, the excavation depth should not exceed 10m; when steel-reinforced cement-soil mixing walls are used, the excavation depth should not exceed 15m; when precast reinforced concrete sheet piles or precast reinforced concrete pile rows are used, the excavation depth should not exceed 10m; when cast-in-place pile rows are used, the excavation depth should not exceed 20m.‍

II. Technical Requirements for Steel Sheet Piles

1. Steel sheet pile support consists of steel sheet pile retaining bodies driven into the soil and internal support or anchor systems. When selecting steel sheet piles as the foundation pit retaining body, the impact of steel sheet pile driving and extraction on the surrounding environment should be considered.

2. The specifications of steel sheet piles should be determined through calculation of internal forces, deformation, and stability of the support structure. Their shape, interlock dimensions, and section properties are detailed in the national standard "Hot-rolled Steel Sheet Piles" GB/T 20933.

3. When steel sheet piles adopt interlock-type waterproof construction, butter, asphalt, or other sealing waterproof materials should be embedded in the interlocks before pile driving. For high anti-seepage requirements or when large-groove steel sheet piles have anti-seepage requirements, an additional water cutoff curtain should be installed outside the pit.

4. When internal supports or anchors are used, steel walings should be installed on the steel sheet piles. The connection between steel walings and steel sheet piles should preferably be by welding or by providing steel brackets. The steel walings should fit tightly against the steel sheet piles; if gaps exist, they should be filled with fine stone concrete.

5. The steel sheet pile bodies should not be bent, and the interlocks should not be damaged or deformed; the interlocks of adjacent steel sheet piles should be checked by interlocking before use.

6. The allowable deviations for steel sheet pile construction are shown in the table below.

7. Before steel sheet pile construction, appropriate techniques and methods should be adopted to reduce soil squeezing and vibration effects during pile driving. The verticality of the pile frame and steel sheet piles should be controlled during driving; previously driven steel sheet piles should be protected during driving.

8. Steel sheet pile driving can adopt various methods such as single pile driving (also called screen method) and stepped driving. The final closure of sheet piles should preferably use the screen method. For steel sheet piles adjacent to buildings (structures) and underground pipelines, the static pile pressing method should be used for construction, and the pressing rate should be controlled based on monitoring results.

9. Before extraction, the steel sheet piles should be vibrated with a vibratory hammer. During extraction, measures such as simultaneous extraction and grouting backfill should be adopted to promptly fill the voids left after extraction.‍

III. Layout of Steel Sheet Piles

▲Steel sheet pile support section

▲Plan node of small-groove water stop for steel sheet piles

▲Steel sheet pile corner layout 1, steel sheet pile corner layout 2

Notes:

1. The specifications and pile lengths of steel sheet piles should be determined based on soil conditions, various stability requirements, and calculations of internal forces and deformation.

2. Steel sheet piles should preferably use small-groove overlapping to form a closed soil-retaining and water-stop structure. Before pile driving, butter, asphalt, or other sealing waterproof materials should be embedded in the interlocks.

3. Steel sheet piles should be inspected before driving; the pile bodies should not be bent, and the interlocks should not be damaged or deformed.

4. Steel sheet piles at foundation pit corners can be made into corresponding special-shaped corner sheet piles according to the plane shape of the corner, and the pile lengths of corner piles and guide piles should preferably be 2.0m longer than other sheet piles.

5. The clear distance S between the steel sheet pile and the outer wall of the utility tunnel should meet the construction requirements of the tunnel structure and waterproofing, and should not be less than 800mm.‍