Application in Protective Construction

Secant pile walls refer to a type of foundation pit retaining structure formed by mechanical drilling, wall protection, soil extraction, and cast-in-place concrete pouring, with piles arranged in an interlocking (intersecting) pattern. Secant pile walls are divided into hard secant piles and soft secant piles.
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Application in Protective Construction

Secant pile walls refer to a type of foundation pit retaining structure formed by mechanical drilling, wall protection, soil extraction, and cast-in-place concrete pouring, with piles arranged in an interlocking (intersecting) pattern. Secant pile walls are divided into hard secant piles and soft secant piles.

Advantages of hard cutting secant piles:

(01) It avoids the phenomenon of concrete piping in adjacent holes that easily occurs when using super-retarding concrete secant piles during the completion of cutting and interlocking. At the same time, since conventional concrete is used, the requirements for concrete are reduced, saving costs.

(02) Due to the hard cutting process, the disadvantage of being unable to perform B-pile secant construction because of excessive strength of A-pile concrete caused by prolonged single-pile formation time does not occur.

(03) When encountering hard large-area underground obstacles, the full-rotation drilling rig can directly remove underground rock layers and obstacles during pile formation, achieving one-time pile formation.

(04) The full-casing full-rotation drilling rig itself has a water-stop and wall-protection function, requiring no additional wall protection measures. It is an energy-saving and environmentally friendly construction process.

(05) According to the grab bucket soil extraction depth, the soil layers can be accurately distinguished, and a suitable pile tip bearing layer can be selected.

(06) The soil extracted from the hole has low water content, is convenient for transport, and there is no concern about mud polluting the environment; the construction site is clean and civilized, making it extremely suitable for construction in urban areas.

(07) Settlement and displacement are easy to control, and construction can be carried out adjacent to nearby buildings and underground pipelines.

(08) The full-rotation follow-up casing drilling and other excavation methods effectively prevent quicksand and mud inflow in the hole, and can perform rock socketing, ensuring pile formation quality.

(09) The second-constructed pile performs cutting and interlocking between the two already-formed first piles. The full-casing hole protection method ensures tight interlocking between piles, and the pile concrete solidifies into one body, forming a good overall continuous structure.

(10) The reinforcement ratio is relatively low. This is because bored secant piles can adopt an arrangement of alternating reinforced concrete piles and plain concrete piles;

(11) The impermeability is stronger. This is because bored secant piles are constructed continuously, with no construction joints between piles, whereas the construction joints at the panel joints of underground diaphragm walls are often weak points for seepage prevention;

(12) Construction is flexible. This is because bored secant piles can turn and change alignment as needed during construction, making them more suitable for constructing foundation pits with variable planar geometries or various arc shapes.

Process principle diagram of secant pile construction:

Forms of reinforcement cages in single pile construction:

Key construction points:

Determination of interlocking thickness and verticality requirements for bored secant piles

The interlocking thickness d between adjacent piles is selected according to pile length. The shorter the pile, the smaller the interlocking thickness (but it should not be less than 100 mm); the longer the pile, the larger the interlocking thickness.

Calculate according to the following formula: d - 2(kl + q) ≥ 50 mm

Where:

l — pile length;

k — verticality of the pile;

q — allowable positioning error at the hole mouth;

d — design interlocking thickness of the bored secant pile;

The meaning of the above formula is to ensure that the minimum design interlocking thickness at the pile bottom is not less than 50 mm.

Taking a pile length of 30 m (i.e., 3000 mm) and an interlocking thickness of 20 cm (i.e., 200 mm) as an example:

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