Structural Parameter Expression Method for Loess Strength Index and Soil Pressure Calculation for Loess Shield Tunnels

Abstract:

In order to obtain the reasonable surrounding rock pressure with the consideration of the characteristics of loess strata to guide the tunnel structure design， by comparing the field test data of surrounding rock pressure of shield tunnel and the new Austrian tunneling method （NATM） tunnel， and in combination with the construction practice of loess shield tunnel， the rationality of calculating the surrounding rock load of loess shield tunnel by deformation surrounding rock pressure is verified. The calculation method of deformation surrounding rock pressure of deep-buried loess shield tunnel is constructed by introducing the strength parameters of loess stratum through Fenner formula. The stress-strain relationship and stress structural parameters of deep buried undisturbed loess are studied by triaxial shear tests， and the calculation method of loess strength index under stress structural parameters is then constructed. The calculation framework of structural parameter field of surrounding rock of deep-buried loess shield tunnel is established in combination with the stress-strain variation law of surrounding rock under the excavation and support. The calculation method of surrounding rock pressure for deep-buried loess shield tunnel is finally given based on the loess strength criterion modified by structural parameters. It is found that the undisturbed loess has an obvious stress peak and strain softening compared with the remolded loess. There is a good fitting relationship between loess stress structural parameters and cohesion， while the correlation with internal friction angle is low. Under the circumstances that only the change of surrounding rock pressure caused by the structural evolution from the loess stratum immersion is considered， when the water content increases from 1 % to 25 %， the radius of plastic circle increases from 4.6 m to 5 m， and the surrounding rock pressure increases from 587 kPa to 622 kPa.

Key words:loess shield tunnel;surrounding rock pressure;triaxial shear test;Fenner formula;structural parameters