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Through the analysis of the sliding failure form of soil mass under the bearing push-extend reamed of Push-extend Multi-under-reamed Pile, in the paper, the law of coulomb-Mohr is used to establish a stress function and the theory of the sliding line is used to establish Prandtl regional stress field, which determines the stress calculating mode of soil mass and provides a theoretical basis for a further study of this type of pile ultimate bearing capacity of soil mass.

The failure performance of the foundation is the slide of soil mass, when the soil mass is sliding, we can usually find the through sliding surface (or called slip line). So the strength of the soil mass is actually the relative sliding resistance between one part of the soil mass and the other, which is the shear strength, and essentially is the friction between soil mass and soil mass. So the shear strength of soil mass is in conformity with the law of friction. Because it is the friction of the same kind of material between different parts, in order to distinguish it from the friction between different materials, it is known as the internal friction, the corresponding coefficient called internal friction coefficient. According to the analysis of theory and the experimental research on the failure mechanism of pile soil of the Push-extend Multi-under-reamed Pile, the results show that the destruction of the soil mass under the plate also complies with the law above and belongs to the sliding failure, and that the ultimate bearing capacity of the soil mass can be solved by the theory of slip line. Therefore, this article uses the theory of slip line to determine conforms to the stress calculating mode of failure modes of soil under the Pushextend Multi-under-reamed Pile [

At present, the most successful failure theory is to use stress to define the failure, in soil mechanics, the soil mass shear failure is often based on the theory of coulomb—Mohr strength. This theory considers that if the shear stress of any section is equal to the shear strength of the material, then material appears failure. As shown in

Symbolic rules: The symbols of normal stress and shear stress at any section of symbols, according to the custom(the right hand spiral rule), when the direction of rotation is clockwise, normal stress to tensile stress is positive, Shear stress to tensile stress and the order of shear stress satisfies the corkscrew rule is positive.

Significance of symbol: As shown in

As shown in

We get:

That is:

Let

So

Let

According to the formula (3), we get:

According to the law of Coulomb, in the stress field, on the failure surface where each other are in a 900 − φ acute angle, shear stress point to the opposite way to the intersection edges. As shown in

Explode stress to the stress on failure surface, as shown in

So the stress field is:

If

According to Mohr circle, we get

In Mohr circle, the radius which is corresponded to the failure surface_{X}, σ_{Y}, τ_{X}_{y}.

As shown in

According to Mohr circle, we get:

In the same way:

If the known stresses on arbitrary surface are

Simplified as

At first, choose a simple damaged area which is statically admissible, according to failure form of soil mass

under the pile, choose Prandtl region. Take two sets of curves, one is logarithmic spiral line with inclination angle φ, the other is the polar rays, we can also get the 900-φ set of curves, as shown in

Coincide r axis and t axis which are in

Establish polar coordinate and take the infinitesimal body, as shown in

we get:

In the same way, we get:

Take formula (14) into equation, ignore the gravity,

Take tanφ × (16) + (15), we get:

Then we get:

Take (17) into (14), we get:

Because the failure form of the soil mass under the bearing push-extend reamed of Push-extend Multi-underreamed Pile belongs to the sliding failure, thus, the theory of the sliding line can be put in use to analysis the stress under the destruction of the soil mass. Through the analysis of the theory above, the law of CoulombMohr is used to establish a stress function, and according to the yield condition and equilibrium condition and so on, applying Prandtl regional stress field, determines the calculation model of the stress of the soil mass under the bearing push-extend Multi-under-reamed Pile.

Normal stress

On the basis of the formula above, combining plastic potential theory and virtual work principle can determine ultimate bearing capacity of the soil mass under the push-extend Multi-under-reamed Pile, then can determine the calculation model of the bearing capacity of single pile further.

This work was financially supported by National Natural Science Foundation of China (51278224).