Analytical Study On The Influences Of The Solidification Of Grouting Materials On The In-situ Stress Measurement By Rheological Stress Recovery Method

The Rheological Stress Recovery(RSR)method is an emerging method applicable to deep fractured soft rocks and utilizes the rheological properties of rocks for ground stress measurement.The practicality of the method has been verified by engineering.The threedimensional ground stress at the measurement point is obtained by burying multi-directional pressure sensors in the surrounding rock.After the pressure sensor is pushed into the borehole,the borehole must be grouted,and the surrounding rock,the grouts and the sensor form a system that changes the mechanical field together,meanwhile the surrounding rock stress is transferred to the sensor through the grouting layer.As the slurry solidifies,its Poisson’s ratio,elastic modulus and shrinkage strain change with time,which affects the stress distribution in the surrounding rock and grouting layer,and thus the deformation nature of the grouting material determines the pressure perceived by the sensor and affects the in-situ stress measurement.In addition,as the borehole grouting does not have conditions to vibrate and maintain,the grouting material cannot be fully compacted,and the water loss leads to increased capillary force and tensile force,resulting in plastic pulling cracks of the grouting layer and detachment between different material interfaces.This paper adopts a research method combining theoretical analysis,indoor tests and numerical simulations to study the deformation properties and changes in the internal structure of the grouted body during solidification,analyze the interference of grout solidification on the sensor stress measurement in the borehole,propose and demonstrate effective improvement methods,and aim to improve the theory and technical application of the RSR method for in-situ stress measurement.The main research contents of the paper are as follows:(1)Analyze the necessity of the grouting in the in-situ stress measurement by RSR method,analyze the factors affecting the values of elastic modulus,Poisson’s ratio and shrinkage strain of the grouted layer through domestic and foreign experimental and theoretical research results,and obtain the range of parameter values.A three-dimensional model of the surrounding rock-grouting layer-sensor structure is established to study the influence of the grouting layer on the sensor stress perception when the above three parameters are taken at different values.(2)To study the property changes of the grouting material during solidification,Poisson’s ratio measurement test is carried out to obtain the functions used to describe Poisson’s ratio,elastic modulus and shrinkage strain.Numerical simulations were used to analyze the effect of the change in the properties of the grouting layer in the three-layer structural model on the in-situ stress measurements.It was found that the change in Poisson’s ratio caused the stress ellipsoid of the early measurements to be rounded;the change in the elastic modulus caused the stress ellipsoid to be smaller and flatter in shape;and the hydration shrinkage caused the sensor to measure more stress,but the sensor surface locally showed tensile stress as a result.The effect of the joint influence of multiple factors shows the influence characteristics of each single factor,but the influence of each factor cannot be separated by the summation relationship.(3)A numerical model was used to analyze the effects of grout interface detachment to form a debonded layer,plastic tension cracking to form cracks,and material inhomogeneity on the ground stress measurement.It is found that the normal stress on the sensing surface is almost zero when the surface is located below the debonded layer;when the crack intersects with the sensing surface,the normal stress on the sensing surface at the intersection is almost zero;the material inhomogeneity makes the sensing surface in the lead hammer direction feel the force more poorly.(4)Three methods to improve the ground stress measurement by RSR method are proposed and demonstrated.One is to develop the data post-processing method to establish the relationship between the measured stress curve and the stress theoretical curve obtained from the model with stable properties to realize the recovery of early stress data.Second,the geometric form of the measurement instrument is optimized,and the perturbation of the stress field can be reduced by switching to a cylindrical sensor and making the sensor surface as flat as possible.Third,using prefabricated transfer columns and reducing the thickness of field grouting can effectively weaken the effect of solidification of grouting materials.