Porosity increment and strength degradation of low-porosity sedimentary rocks under different loading conditions

doi:10.1016/j.ijrmms.2015.02.002

International Journal of Rock Mechanics and Mining Sciences

Volume 75, April 2015, Pages 216-223

https://doi.org/10.1016/j.ijrmms.2015.02.002 Get rights and content

Introduction

Rock is quasi-brittle material containing pores, flaws and other types of discontinuities. Unlike some visible discontinuities and flaws, the pores are randomly distributed in rocks and usually invisible by naked eye. The essentially random distribution of pores has close correlation to rock׳s mechanical and engineering properties such as unconfined compression strength (UCS), Young׳s modulus, shear strength and sonic velocity. Several experimental and numerical studies have been conducted upon the relationship between porosity and mechanical properties of rocks. Brace and Riley [1] selected 15 different rock samples with porosity ranging from nearly zero to 40%, and reported variation in static uniaxial deformation between the low-and high-porosity rocks. Dunn et al. [2] carried out extensive laboratory tests and proposed an equation y=a×x^b to quantify the strong dependence of fracture strength on porosity (where y equals the stress difference at failure and x represents porosity, parameter a varied with degree of loading and b is a dimensionless constant ranging between −0.8 and −1.0). Vernik et al. [3] derived an empirical formula for a set of sandstones linking the compressive and shear strength and incorporating porosity as a textural parameter using an image analysis technique. Al-Harthi et al. [4] systemically investigated the influence of the porosity of vesicular basalt on its engineering properties such as compressive strength, Poisson׳s ratio and sonic pulse velocity. In the tests, the compressive strength decreased rapidly with the increasing porosity up to a porosity of 20%, after which the decrease was lessened. A three dimensional Discrete Element Method (DEM) was employed by Martin et al. to investigate the dependence of strength, elasticity and friction angle on porosity and crack density [5]. The 3D DEM simulation results demonstrated that ratio of unconfined compressive strength to unconfined tensile strength (UCS/UTS) decreases as the porosity increases. Fakhimi and Gharahbagh [6] similarly used DEM to study the effect of porosity, pore size and distribution on the mechanical behaviour of rock and demonstrated drastic effects on UCS, UTS, elastic modulus and crack initiation. Baud et al. [7] proposed an analytical model in which UCS and initial level of damage (initial porosity) and/or crack density in a rock were related.

For porous media such as rocks, the pores inside the intact rock in its original condition could be regarded as an initial damage condition for the rock, which can approximately be indicated and quantified by initial porosity (IP) [8], [9]. Generally, the volume of micro-pores of rock materials will increase gradually with the increasing loading applied on rocks whereas the strength decreases, which indicates the phenomenon of rock strength degradation. This can be quantified by a strength degradation parameter (SDP) defined as the strength of a rock specimen after a degree of loading, σ_c from that of initial specimen σ_c and expressed as a percentage of original strength according to $SDP = \frac{σ_{c} - σ_{i}}{σ_{c}} \times 100 %$

In this study, the porosity changes associated with strength degradation were investigated for a series of low-porosity rocks under uniaxial loading. Low-field Nuclear Magnetic Resonance (NMR) was used to measure porosity.

Section snippets

Test apparatus

A servo-controlled, Instronl 342 material testing machine with a maximum loading capability of 2000 KN was used for the compression tests. The Nuclear Magnetic Resonance System (NMR) was used to measure the signal decay of hydrogen atoms in the fully-saturated pore space of rock. Measured transverse relaxation time distribution (T₂), which depends on the size of water-saturated pores, can be converted to a pore size distribution [10], [11] as illustrated in Fig. 1. In Fig. 1, the shaded area

Initial porosity of specimens

The initial T₂ pore size distributions of all specimens are presented in Fig. 2. For the majority of specimens, the signals are at the highest point around 5000 µs, which means that the number of pores with similar size is the largest. Table 2 shows initial porosity of specimens.

Porosity increment of specimens under different loading conditions

The measured UCS of the group A samples were 76.4, 87.9 and 77.6 MPa; the 2nd stage tests for the other groups of samples were therefore terminated at 8, 16, 32, 48 and 64 MPa respectively as discussed earlier. Fig. 3

Conclusions

In this study, a non-destructive methodology of quantifying the strength degradation of a low-porosity sedimentary rock was presented. Porosity was identified initially using the NMR technique to understand the initial damage level of rocks. A servo-controlled material testing machine was employed for the compression tests. Specimens were loaded to approximately 10%, 20%, 40%, 60% and 80% of the UCS. Porosity increments of these partially loaded specimens were then identified by NMR. This is

Acknowledgements

The authors wish to acknowledge Prof. Steve Hencher of the University of Leeds for helping authors with language editing as well as some technical suggestions. The first author is also grateful to Xiaomeng Xue for his support in specimen preparation. This research is financially support by the Postgraduate Research and Innovation Foundation Program of Central South University (Grant no. 2011ssxt274).

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Technical NotePorosity increment and strength degradation of low-porosity sedimentary rocks under different loading conditions

Introduction

Section snippets

Test apparatus

Initial porosity of specimens

Porosity increment of specimens under different loading conditions

Conclusions

Acknowledgements

First page preview

Int J Rock Mech Min Sci

Int J Rock Mech Min Sci

Eng Geol

Int J Rock Mech Min Sci

Porosity dependence and mechanism of brittle fracture in sandstone

J Geophys Res

The impact of porosity and crack density on the elasticity, strength and friction of cohesive granular materials: insight from DEM modelling

Int J Rock Mech Min Sci

Technical Note
Porosity increment and strength degradation of low-porosity sedimentary rocks under different loading conditions