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Laboratory investigation of hydraulic properties of deformable rock samples subjected to different loading paths

  • Qian Yin
  • Hongwen Jing
  • Guowei Ma
  • Haijian Su
  • Richeng LiuEmail author
Paper
  • 42 Downloads

Abstract

This study experimentally investigated the internal fracture geometries and hydraulic properties of deformable rock samples subjected to various loading paths. Three loading paths, uniaxial compression, conventional triaxial compression, and triaxial prepeak unloading, were administered to granite samples. A high-precision X-ray microfocus-computed-tomography scanning system was adopted to explore the internal failure modes of the deformable samples, and a series of flow tests with different hydraulic pressures and confining stresses were then performed. The results show that the samples after uniaxial compression exhibit a typical splitting failure mode; however, for conventional triaxial compression and prepeak unloading, the samples generally show tensile-shear failure modes. The relationship between the flow rate and pressure gradient of the sample after uniaxial compression can be best described using the Forchheimer law. Both the linear and nonlinear coefficients in the Forchheimer law increase with increasing confining stress. As the confining stress increases from 4 to 20 MPa, the critical hydraulic gradient that quantifies the onset of nonlinear flow increases from 5.33 to 56.74, but the transmissivity decreases due to the fracture closure. For water flow through the samples after conventional triaxial compression and prepeak unloading, two representative types of nonlinear flow behaviors induced by inertial effects and fracture dilation were observed. Different loading paths lead to different failure mechanisms and thus different fluid-flow responses, and the samples subjected to prepeak unloading with a high confining pressure possess a more significant flow capacity.

Keywords

Laboratory experiments/measurements Loading paths Rock fracture Groundwater flow Equivalent permeability 

Etude en laboratoire des propriétés hydrauliques d’échantillons de roche déformables soumis à différents modes de chargement

Résumé

La présente étude a exploré par voie expérimentale la géométrie de la fracturation interne et les propriétés hydrauliques d’échantillons de roche déformables soumis à divers modes de chargement. Trois modes de chargement, la compression uniaxiale, la compression triaxale conventionnelle et le déchargement triaxial avant-pic, ont été appliqués à des échantillons de granite. Un système de balayage aux rayons X haute définition par tomographie numérisée microfocus, a été adopté pour explorer les modes de défaillance interne des échantillons déformables et une série de tests d’écoulement sous différentes pressions hydrauliques et contraintes de confinement a été réalisée. Les résultats montrent que les échantillons soumis à une compression uniaxiale offrent un mode de défaillance cassante typique; cependant, pour une compression triaxiale conventionnelle et un non déchargement avant pic, les échantillons montrent généralement des modes de défaillance de type traction-cisaillement. La relation entre le débit et le gradient de pression de l’échantillon sous compression uniaxiale peut être décrite par la loi de Forcheimer. Les coefficients linéaires et non-linéaires de la loi de Forcheimer croissent tous deux quand la contrainte de confinement croît. Quand la contrainte de confinement croît de 4 à 20 MPa, le gradient hydraulique critique qui quantifie le début de l’écoulement non linéaire croît de 5.33 à 56.74, mais la transmissivité décroît du fait de la fermeture de la fracture. Pour un écoulement d’eau à travers les échantillons après une compression triaxiale conventionnelle et un non déchargement avant pic, deux types significatifs de comportements d’écoulement non linéaires, induits par des effets inertiels et une dilatation de la fracture, ont été observés. Les différents modes de chargement conduisent à des mécanismes de défaillance différents et donc à des réponses de l’écoulement du fluide différentes et les échantillons soumis à un non déchargement avant pic, avec une pression de confinement élevée, possèdent une capacité d’écoulement plus importante.

Investigación en laboratorio de las propiedades hidráulicas de muestras de roca deformable sometidas a diferentes vías de carga

Resumen

Este estudio investigó experimentalmente las geometrías internas de fractura y las propiedades hidráulicas de muestras de roca deformable sometidas a diversas vías de carga. A las muestras de granito se les administraron tres vías de carga, compresión uniaxial, compresión triaxial convencional y descarga triaxial previa al máximo. Se adoptó un sistema de tomografía computarizada por microfoco de rayos X de alta precisión para explorar los modos de falla interna de las muestras deformables, y luego se realizaron una serie de pruebas de flujo con diferentes presiones hidráulicas y tensiones de confinamiento. Los resultados muestran que las muestras después de la compresión uniaxial muestran un modo típico de fallo de división; sin embargo, para la compresión triaxial convencional y la descarga antes del máximo, las muestras muestran generalmente modos de fallo por tracción-cizallamiento. La relación entre el caudal y el gradiente de presión de la muestra después de la compresión uniaxial puede describirse mejor utilizando la ley de Forchheimer. Tanto los coeficientes lineales como los no lineales de la ley Forchheimer aumentan con el aumento de la tensión de confinamiento. A medida que la tensión de confinamiento aumenta de 4 a 20 MPa, el gradiente hidráulico crítico que cuantifica el inicio del flujo no lineal aumenta de 5.33 a 56.74, pero la transmisividad disminuye debido al cierre de la fractura. Para el flujo de agua a través de las muestras después de la compresión triaxial convencional y la descarga antes del máximo, se observaron dos tipos representativos de comportamientos de flujo no lineal inducidos por los efectos inerciales y la dilatación de la fractura. Diferentes vías de carga conducen a diferentes mecanismos de fallo y, por lo tanto, a diferentes respuestas de flujo del fluido, y las muestras sometidas a una descarga antes del máximo con una presión de confinamiento elevada poseen una capacidad de flujo más significativa.

不同荷载路径可变形岩样水力特征的室内研究

摘要

本研究通过实验研究了不同荷载路径可变形岩样的内部裂缝几何形状和水力特征。对花岗岩样品进行了三个荷载路径试验:单轴压缩,常规三轴压缩和三轴峰前卸荷。采用高精度X射线计算机断层成像扫描系统探讨了可变形样品的内部破坏模式,并进行了系列不同液压和约束应力的流动实验。结果表明,单轴压缩后的样品呈典型的劈裂破坏模式;然而,对于常规三轴压缩和峰前卸荷,样品通常显示拉伸-剪切破坏模式。使用Forchheimer定律可以很好描述单轴压缩后样品的流速和压力梯度之间的关系。 Forchheimer定律中的线性和非线性系数都随着约束应力的增加而增加。当约束应力从4 MPa增加到20 MPa,量化非线性流动开始的临界水力梯度从5.33增加到56.74,但由于裂缝闭合,导水系数减小。对于常规三轴压缩和峰前卸荷后通过岩样的水流,观察到惯性效应和裂缝扩张引起的两种典型的非线性流动行为。不同荷载路经导致不同的破坏机理并因此导致不同的流体流动响应,并且经受具有高围压的峰前卸荷的样品具有更显著的流动能力。

Investigação laboratorial das propriedades hidráulicas de amostras de rocha deformada submetidas a diferentes caminhos de carregamentos

Resumo

Este estudo investigou experimentalmente as geometrias internas e as propriedades hidráulicas de fraturas de amostras deformadas de rocha submetidas a vários caminhos de carregamento. Três caminhos de carregamento, compressão uniaxial, compressão triaxial convencional e descarregamento pré-pico triaxial, foram aplicados às amostras de granito. Um sistema de escaneamento de alta precisão de tomografia computadorizada de raios-X foi adotado para explorar os modos de falha interna das amostras deformadas, e foram realizados vários ensaios de escoamento com diferentes pressões hidráulicas e confinamentos de tensões. Os resultados mostram que, após a compressão uniaxial, as amostras exibiram um típico modo de falha de separação; no entanto, para a compressão triaxial convencional e para o descarregamento pré-pico, as amostras mostraram modos típicos de falha do tipo tensão-cisalhamento. A relação entre a taxa de escoamento e o gradiente de pressão das amostras, após a compressão uniaxial, pode ser melhor descrita usando a lei de Forchheimer. Ambos os coeficientes linear e não linear na lei de Forchheimer aumentam com o aumento do estresse de confinamento. Conforme o estresse de confinamento aumenta de 4 até 20 MPa, o gradiente hidráulico crítico, que quantifica o começo do escoamento não linear, aumenta de 5.33 até 56.74, mas a transmissividade diminui por causa do fechamento da fratura. Para o escoamento de água através das amostras, após a compressão triaxial convencional e para o descarregamento pré-pico, foram observados dois tipos representativos de comportamentos de escoamento não linear induzidos por efeitos inerciais e dilatação de fratura. Diferentes caminhos de carregamento ocasionam diferentes mecanismos de falha e assim diferentes respostas fluido-escoamento, e as amostras submetidas ao descarregamento pré-pico com elevada pressão de confinamento possuem uma capacidade de escoamento mais significativa.

Notes

Funding information

The financial support from the Fundamental Research Funds for the Central Universities, China (No. 2018XKQYMS07), is gratefully acknowledged.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  • Qian Yin
    • 1
  • Hongwen Jing
    • 1
  • Guowei Ma
    • 2
  • Haijian Su
    • 1
  • Richeng Liu
    • 1
    Email author
  1. 1.State Key Laboratory for Geomechanics and Deep Underground EngineeringChina University of Mining and TechnologyXuzhouChina
  2. 2.College of Architecture and Civil EngineeringBeijing University of TechnologyBeijingChina

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