Anti-slide pile design

1. Design conditions of anti-slide piles

The basic conditions for using anti-slide piles are as follows: ① The landslide has obvious sliding surface, and the non-fluid plastic body above the sliding surface can be stabilized by piles. (2) Below the sliding surface is a relatively complete rock or dense soil layer, which can provide sufficient anchoring force. In addition, it should also have more reasonable economic conditions and more convenient construction conditions.

2. Anti-slide pile design steps

(1) Geological survey

Through geological investigation, the cause, nature, scope and thickness of landslide are grasped, and its state and development trend are analyzed.

(2) Calculate the landslide thrust and its distribution in the pile.

The sliding body part with the same sliding direction and sliding speed within the landslide range is taken as a calculation unit, and one or several geological vertical sections along the main sliding direction are selected as representatives to calculate the sliding force, and the force on each pile is the landslide thrust within the pile spacing range. Various slicing methods, such as transfer coefficient method, can be used in concrete calculation.

The distribution of landslide thrust in pile body is complex, which is related to factors such as landslide type and stratum conditions. In the design and calculation, if the sliding soil layer is cohesive soil, stone-sandwiched soil and other strata with large cohesion, it can be simplified to rectangular distribution; If it is non-cohesive soil such as sand and gravel, triangular distribution can be used; Between the two, it can be set to a trapezoidal distribution.

(3) According to the topography, geological conditions and construction conditions, determine the position and layout scope of the pile.

Anti-slide piles should generally be arranged in the lower part of the landslide, because the lower sliding surface is slow and the sliding force is small. Piles are generally arranged in a row, and the arrangement direction is vertical or nearly vertical to the sliding direction; For large-scale, complex or longitudinally long landslides with large sliding force, two or three rows can be arranged; When the sliding force is particularly large, the staggered arrangement of clubs can be adopted.

(4) Determination of pile parameters

According to the magnitude of landslide thrust, topography and stratum properties, the pile length, anchorage depth, pile section size and pile spacing are drawn up. Proper spacing between piles should ensure that soil will not be squeezed out between piles. Therefore, when the sliding body is complete and compact or the sliding force is small, the pile spacing should be large, otherwise it should be small, and the common pile spacing is 6 ~ 10m. In addition, it can also be determined according to the shear strength of the pile.

Pile sections are mostly rectangular and circular. When a rectangle is used, the front side is generally shorter, the side side is longer, and the side length is generally 2 ~ 4m.

The anchorage depth of the pile should ensure that it can provide sufficient resistance. In practical design, it is required that the lateral pressure transmitted by the anti-slide pile to the stratum below the sliding surface is not greater than the lateral allowable compressive strength of the stratum, but the anchoring force will not increase significantly if the anchoring length is too large.

(2) Types of anti-slide piles

There are mainly the following types of anti-slide piles.

1) According to the pile materials, it is divided into: wooden pile, steel pipe pile, reinforced concrete pile, etc.

2) According to the cross-sectional shape of pile body, it can be divided into circular pile, pipe pile, square pile and rectangular pile.

3) According to the pile-forming technology, it is divided into bored piles and dug piles;

4) According to the stress state of piles, they are divided into fully buried piles, cantilever piles and buried piles;

5) According to the stiffness of pile body, it is divided into rigid pile and elastic pile;

6) According to the pile combination form, it can be divided into single pile, bent pile and rigid frame pile.

7) According to the constraint conditions of pile head, it can be divided into ordinary pile and anchor pile.

The basic form of commonly used anti-slide piles is shown in Figure 2-2 1. Fully buried piles (Figure 2-2 1A) and cantilever piles (Figure 2-2 1B) are widely used; Embedded piles (Figure 2-2 1C) are generally used in the case of large landslide thickness, which can save costs; Pile cap (Figure 2-2 1d) connects two rows of piles at the pile head with caps, so that the piles and the soil between piles bear the same force; Rigid piles (Figure 2-2 1e, F, G) can effectively exert the * * * interaction between two piles and reduce the buried depth of the piles; Anchor cable pile (Figure 2-2 1h) is to add several bundles of anchor cables to the pile head or the upper part of the pile and fix them in the stable stratum below the sliding surface, which can increase the lateral fulcrum and resistance, reduce the bending moment and shear force of the pile, and thus reduce the section and buried depth.

Figure 2-2 1 Basic forms of common anti-slide piles

In practical work, the specific pile type should be selected according to the type, scale and geological conditions of landslide, as well as the geotechnical conditions, construction conditions and construction period of sliding bed.

(3) Calculation width of anti-slide pile

Referring to the design of bridge pile foundation, when the design width of anti-slide pile section is b or the diameter is d and the total is greater than 0.6m, calculate the width (Bp):

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④ Determine the length of the pile.

The length of the anti-slide pile consists of the upper part and the lower part of the sliding surface. The length above the sliding surface should be checked to ensure that the sliding body will not slip out of the pile top. When checking the top, it should be considered that the reduction of groundwater drainage section after pile formation may increase the groundwater level after pile formation. In practical engineering, the length of many piles exceeds the actual needs, resulting in waste, which leads to the emergence of embedded anti-slide piles. Superelevation and pile length indicate that the design of pile length is unreasonable. The length below the buried sliding surface should not exceed the allowable elastic resistance of the soil, and whether the sliding surface develops downwards should be considered to ensure the stability of the pile. The buried depth of cantilever pile below the sliding surface is generally 1/3 ~ 1/2 of the pile length, depending on the anchorage conditions.

(5) Calculation model of anti-slide pile

1. cantilever pile method and foundation coefficient method model

The existing calculation methods generally regard the soil layer as an elastic foundation, which conforms to Winkler hypothesis, and calculate the anti-slide pile as an elastic foundation beam. According to the different treatment methods of the soil in front of the pile above the sliding surface, the calculation methods of the anti-sliding pile can be divided into two types: one is cantilever pile method, in which the landslide thrust above the sliding surface and the residual anti-sliding force of the soil in front of the pile (that is, the maximum resistance that the soil in front of the pile can provide in a stable state) are taken as design loads; if the residual anti-sliding force is greater than the passive earth pressure, the passive earth pressure is used to replace the residual anti-sliding force, and the lateral pressure, displacement and internal force of the anchorage section are calculated. This method is simple in calculation and widely used in practical work. The second method is the foundation coefficient method, which takes the landslide thrust on the pile above the sliding surface as a known load and the whole pile as an elastic foundation beam, as shown in Figure 2-21c. When this method is adopted, it is required that the resistance before the pile is not greater than the residual anti-sliding force and passive earth pressure, otherwise the residual anti-sliding force and passive earth pressure should be adopted.

2. Calculation model of elastic resistance of soil beside pile

Assume that the resistance of the stratum at Y under the surface to the pile is

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Where: σy is the resistance of the stratum at Y under the surface to the pile (kPa); Xy is the horizontal displacement (m) of the pile at Y under the surface; K is the foundation coefficient, or the elastic resistance coefficient; Bp is the calculated width of the pile (m).

The foundation coefficient is related to the depth, and its calculation formula is

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Where: y is the depth (m) from the embedded section to the sliding belt; Y0 is a constant (m) related to geotechnical classification; N is a constant that varies with rocks and soil; M is the proportional coefficient of foundation coefficient changing with depth; Other symbols have the same meanings as before.

When n = 0, k is constant and does not change with depth, and its corresponding calculation method is called "K" method, which is suitable for hard rock stratum and undisturbed hard clay. When n = 1 and Y0 = 0, k = my, which means that k is distributed in a triangle along the depth. The corresponding method is called "M" method, which is suitable for hard plastic-semi-hard sandy clay, etc.

3. Calculation model of rigid pile and elastic pile

When the stiffness of the pile is much greater than the constraint of soil on the pile, the deformation of the pile can be ignored when calculating the internal force of the pile, and the pile can be regarded as a rigid body, that is, a rigid pile. This simplification has little influence on the calculation results. On the contrary, the influence of pile deformation should be considered, that is, piles should be regarded as elastic piles.

Rigid pile has large cross section and short length, and its stiffness is infinite relative to surrounding rock and soil. The elastic pile has small cross section, long length and small relative stiffness. Generally, large-section excavated piles are mostly rigid piles.

When the pile is buried under the sliding surface, it is designed as a rigid pile; When, according to the elastic pile design.

K method:

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M method:

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In formulas (2-59) and (2-60), α is the deformation coefficient of the pile (1/m); M is the proportional coefficient (kPa/m2) of the variation of foundation coefficient with depth; Bp is the calculated width of the pile (m); EW is the elastic modulus of concrete (kpa); I is the moment of inertia of pile section, and d is the side length of rectangular pile along the landslide thrust direction (m); C is the lateral foundation coefficient of pile bottom (kPa/m).

4. Calculation model of column bottom support conditions

In general, the top end of anti-slide pile is freely supported, and the bottom end is divided into free support, hinged support and fixed support according to the degree of constraint, as shown in Figure 2-22.

Figure 2-22 Calculation Model of Cantilever Pile and Foundation Coefficient Method

(1) Free support

In the AB section below the sliding surface, when the stratum is soil or weak and broken rock, the pile bottom has obvious movement and rotation, which can be considered as free support.

(2) Retention bracket

When the rock layer at the bottom of the pile is complete, but the pile does not go deep into this layer, it can be considered as hinged support.

(3) Fixed bracket

The rock stratum at the bottom of the pile is hard and complete. When the pile is embedded deeply, it can be treated as a fixed end.

For cantilever pile method, the load on the pile above the sliding surface is known, so it is easy to get the deformation and internal force of the pile with this section. "M" method can be used to calculate the reaction force of soil around piles.

(6) Calculation of internal force of anti-slide pile

The internal force calculation of anti-slide pile can be divided into rigid pile and elastic pile, and can also be divided into cantilever pile and fully buried pile. Considering the sliding force above the sliding surface and the residual sliding force in front of the pile as external forces, the lateral stress and internal force of the pile can be easily calculated from the top down by general mechanical methods. For the elastic resistance of rock and soil below the sliding surface, rigid piles can be solved by angular displacement method or dimensionless method; Elastic piles can be solved by "M" method or dimensionless method.

(7) Checking calculation of shear and bending of anti-slide piles

The control effect of piles mainly depends on their own strength. According to the construction position of piles, they can be divided into shear stress piles and bending stress piles. In general, the allowable stress of pile is regarded as the limit value of pile control effect. In order to obtain the design safety factor, the required anti-sliding force (PR) of pile per unit width can be obtained according to the following formula:

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Where: Fs is the safety factor of bending stability; N is the normal component of sliding mass per unit width (kn/m); μ is the pore water pressure per unit width (kn/m); C is the cohesion of the soil in the sliding zone (kPa); L is the length of the sliding surface (m); PR is the required anti-sliding force of pile with unit width (kn/m); T is the sliding force per unit width (kn/m); Other symbols have the same meanings as before.

Because the position of the anti-slide pile is different from the geotechnical properties, the stress state of the pile is also different. Generally speaking, the pile at the depth of 2/3 from the sliding surface bears the maximum load, which is called bent pile. The pile shall satisfy the following formula:

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Where: σmax is the allowable stress intensity of steel pipe bending (t/m2); V is the axial force (t) generated by the pile; Mmax is the maximum bending moment generated by the pile (t m); AP is the cross-sectional area of pile reinforcement (including reinforcement material) (m2); ZP is the section coefficient of pile reinforcement (including reinforcement material).

When the strength of rock and soil is high and the studied pile is rigid, the pile on the sliding surface is subjected to shear force, so it is often designed as a shear pile, which should satisfy the following formula:

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Where: Smax is the maximum shear force generated on the pile (t); τp is the allowable shear stress strength of steel pipe (t/m2); Ap is the cross-sectional area of steel pipe (m2); τs is the allowable shear stress strength of reinforced material (t/m2); The same is true of the cross-sectional area (m2) of the reinforcing material.