Abstract: In the layered soil foundation mainly composed of silty clay and silty clay, test piles with full cross-section pressure sensors installed at the end of piles are used to continuously and intensively collect pile tip resistances in the whole penetrating process with high density. throughout the entire process. According to the pile tip resistance curves of multiple penetration strokes, the change stage of the initial penetration of the pile tip resistance into the soil layer and the change stage of continuous penetration into the same soil layer are defined, which are divided into linear steep increase segments, nonlinear ramp segments, and platform segments, nonlinear slow-down section, linear steep drop section. When the pile end continuously penetrates through the interface of multiple soil layers, the change stage of the pile end resistance lacks a linear steep increase section compared with the initial penetration soil layer. The initial value of the steep drop of the previous stroke is close to the final value of the steep rise of the latter stroke, which can reflect the limit of the elastic compression of the soil, and is calculated by the average value of Qc, and is used as the pile foundation design. The pile tip resistance curve during the pile sinking process is similar to the static penetrating Qc curve. The maximum tip resistance of every layer in the layered soil is linear with the Qc value of the static penetrating probe and the average number of standard penetrations. Analysis of this situation found that the residual end-resistance after restoration was linearly related to the Qc value of cone penetration test(CPT) and the N value of standard penetration test(SPT). The fitting formula of elastic limit pile end resistance and ultimate pile tip resistance and residual tip resistance are put forward; and the fitting formula of ultimate pile tip resistance is more accurate than that of empirical parameter method, static cone penetration method and limit equilibrium theory.

Key words: layered foundation, pile tip resistance, residual pile tip resistance, static cone penetration