Many shaft collapses are related to the deterioration and failure of the masonry shaft lining materials. In modern mine shaft, concrete is widely used to provide support. To analyse shafts stability, the properties of the lining need to be well defined. The behaviour of masonry and concrete can be considerably affected by long-term exposure to harsh mine water. This paper presents a study which focuses on the weathering effects of mine water on lining materials (brick, mortar, and concrete). To reproduce the weathering process, samples were placed into solutions of potable water, artificial mine water, and a more aggressive mine-water solution for just less than one year. Four phases of laboratory tests were conducted throughout the time period to assess the degradation of mechanical properties of the lining materials. Particular attention is given to the degradation of material strength and stiffness. Results indicate that the harsh acidic mine water has pronounced detrimental effects on the strength and stiffness of mortar. The weathering process is shown to have the most significant effect on the stiffness of concrete and mortar. It is also shown that the use of mass loss as an index for evaluation of mechanical properties may not be appropriate.
Mine shafts allow movement of materials and personnel from surface to subsurface locations and are integral components of effective mine operations. The stability of a shaft, especially within the superficial soil layer near the ground surface, relies heavily on the shaft lining. During the operational life of a mine, shaft stability is essential in order to ensure economic viability of mining operations and to safeguard against injury and loss of life. Consequently, considerable effort is devoted to maintain the integrity of the shaft while the mine is active. Once mining activities cease, however, there is little financial incentive for maintaining shaft stability, and measures that have been adopted to provide long-term support to shafts have tended to be minimal and often proven to be inadequate. There is considerable variability in the nature of treatment works undertaken to ensure shaft stability in the long term. These include partial or complete infilling of the shaft using materials which may not be ideally suited for the intended purpose (usually left-over spoil found on-site), and capping systems built from wood which, with the passage of time, pose an increasing threat of sudden failure due to material degradation. In many cases, the shaft lining is the only structure left in place to support the surrounding soil. As the structural integrity of the lining decreases over time, the stability of the shaft structure can reach a critical state nearing collapse.
In UK, there are over 100,000 recorded mine shafts, with many more unrecorded shafts in existence [
Analysis of mine shaft stability relies on a good estimation of the material properties of the lining and the surrounding soil/rock, as well as an understanding of the interactions that occur between the shaft and the surrounding materials. A variety of research has been done to study the behaviour of shaft linings. Simple analytical methods for analysing mine shafts provide an efficient method for assessing stability. For example, Rama Mohana Rao [
An understanding of the mechanical properties of the shaft lining material is essential for any stability analysis. Use of design material properties is suitable for consideration of the shaft in the short term; however, the long-term evaluation of shaft stability is also important. There has been relatively little work done to study the deterioration of the mechanical properties of shaft lining materials over time within harsh mine shaft environments.
The majority of historic mine shafts are supported by masonry structures, typically using stone blocks and brickwork. This form of construction is no longer routinely used and concrete linings are generally preferred. The lining material of masonry and concrete can be significantly weathered due to factors such as water, climatic changes, salt, and acid attack. A variety of research has been conducted to understand the weathering process on brick, mortar, and concrete. These materials can be affected by soluble salts (e.g., [
The focus of this paper is the study of the degradation of the mechanical properties of shaft lining materials (brick, mortar, and concrete). The data presented provides some quantitative measurements of the deterioration of shaft materials’ strength and stiffness over time when subjected to a variety of weathering environments. The data is useful for input in long-term shaft stability calculations and numerical models. The paper is divided into three main sections. Section
The aim of this study was to determine harsh mine water and time effects on shaft lining materials. Brick, mortar, and concrete were tested. The bricks used were Mellowed Red Sovereign Stock supplied by Wienerberger Ltd., UK [
Test material: (a) brick, (b) mortar, and (c) concrete.
In order to replicate the environmental conditions that cause degradation of shaft lining materials, a number of samples were immersed for just less than one year in one of three prepared solutions: potable water, a representative mine water, and a more aggressive version of the mine water. Laboratory tests were performed at different time intervals to evaluate the weathering effects on the mechanical parameters of the test materials. The aggressive solution was used to model some extreme cases [
The expected pH and concentration of the main chemical components of the mine water and aggressive solutions are shown in Table
Concentration of the main chemical components for the immersion baths.
Mg (mg/L) | Na (mg/L) | Cl (mg/L) | SO4 (mg/L) | pH | |
---|---|---|---|---|---|
Mine water (measured [ | 31 | 15.7 | 13 | 360 | 6.0 |
Mine water (used in this study) | 40 | 14.5 | 12 | 353 | 5.2 |
Aggressive solution | 400 | 724 | 600 | 8182 | 1.3 |
Mg: magnesium; Na: sodium; Cl: chlorine; SO4: sulfate.
Four phases of laboratory tests were performed throughout the weathering process (Phase 0 baseline test, followed by Phases 1 to 3 at 16-week intervals), as illustrated in Table
Lab test programme for determining weathering effects on the test material.
Phase 0 | Phase 1 | Phase 2 | Phase 3 | |
---|---|---|---|---|
Baseline | 16 weeks | 32 weeks | 48 weeks | |
Air | UCS/triaxial | |||
Potable water | UCS/triaxial | UCS/triaxial | ||
Mine water | UCS/triaxial | UCS/triaxial | UCS/triaxial | |
Aggressive solution | UCS/triaxial | UCS/triaxial | UCS/triaxial |
Mass loss was calculated at each phase for each material. Mass loss is the most traditional parameter to measure the degree of deterioration on experimentally weathered material samples. In this study, all the cylinder samples were prepared, cured (for mortar and concrete), oven-dried at 50°C for 24 hours, marked, and weighed before placement within the immersion baths. At each phase, the selected samples were rinsed with tap water to remove loose reaction products and placed into an oven at 50°C for 24 hours before weighing. For each sample, the mass loss at each phase was calculated as
Uniaxial compressive strength (UCS) and triaxial tests were conducted at each phase to determine strength and stiffness parameters and thereby quantitatively assess the effect of weathering on the mechanical properties of the samples. The UCS test measures the stiffness (Young’s modulus,
Triaxial tests were conducted using a Hoek cell [
The experimental results are presented and discussed in this section. Due to the practicality of preparing a large number of samples, those in potable water were not tested at Phase 2 for all three materials. The mortar samples within the aggressive solution suffered considerable degradation after 48 weeks and could not be tested. The strength of these samples was effectively reduced to zero. Therefore, no experimental data is given for mortar in the aggressive solution at Phase 3.
Figure
Mass loss of (a) brick, (b) mortar, and (c) concrete.
Figures
The stress-strain relationships of the brick, mortar, and concrete samples from the UCS tests are shown in Figures
UCS of brick in (a) potable water, (b) mine water, and (c) aggressive solution.
UCS of mortar in (a) potable water, (b) mine water, and (c) aggressive solution.
UCS of concrete in (a) potable water, (b) mine water, and (c) aggressive solution.
Figure
The UCS test data on concrete samples in Figure
The data in Figures
Brick: compressive strength, tangent modulus, and secant modulus.
Solutions | |||
---|---|---|---|
Phase | Potable water | Mine water | Aggressive solution |
Compressive strength (MPa) | |||
Phase 0 (0 weeks) | 11.66 | 11.66 | 11.66 |
Phase 1 (16 weeks) | 9.16 | 12.38 | 12.18 |
Phase 2 (32 weeks) | — | 14.26 | 11.05 |
Phase 3 (48 weeks) | 12.08 | 15.52 | 13.54 |
Tangent modulus (GPa) | |||
Phase 0 (0 weeks) | 3.06 | 3.06 | 3.06 |
Phase 1 (16 weeks) | 2.72 | 2.38 | 2.93 |
Phase 2 (32 weeks) | — | 4.82 | 3.47 |
Phase 3 (48 weeks) | 3.51 | 6.16 | 3.15 |
Secant modulus (GPa) | |||
Phase 0 (0 weeks) | 2.28 | 2.28 | 2.28 |
Phase 1 (16 weeks) | 1.81 | 2.03 | 2.36 |
Phase 2 (32 weeks) | — | 3.77 | 2.42 |
Phase 3 (48 weeks) | 2.92 | 4.91 | 2.45 |
Mortar: compressive strength, tangent modulus, and secant modulus.
Solutions | |||
---|---|---|---|
Phase | Potable water | Mine water | Aggressive solution |
Compressive strength (MPa) | |||
Phase 0 (0 weeks) | 10.19 | 10.19 | 10.19 |
Phase 1 (16 weeks) | 8.48 | 9.59 | 8.89 |
Phase 2 (32 weeks) | — | 12.42 | 7.17 |
Phase 3 (48 weeks) | 11.51 | 11.31 | — |
Tangent modulus (GPa) | |||
Phase 0 (0 weeks) | 4.85 | 4.85 | 4.85 |
Phase 1 (16 weeks) | 3.52 | 4.88 | 1.80 |
Phase 2 (32 weeks) | — | 4.73 | 1.00 |
Phase 3 (48 weeks) | 3.20 | 2.55 | — |
Secant modulus (GPa) | |||
Phase 0 (0 weeks) | 2.96 | 2.96 | 2.96 |
Phase 1 (16 weeks) | 2.45 | 3.31 | 1.33 |
Phase 2 (32 weeks) | — | 3.06 | 0.73 |
Phase 3 (48 weeks) | 1.61 | 1.74 | — |
Concrete: compressive strength, tangent modulus, and secant modulus.
Solutions | |||
---|---|---|---|
Phase | Potable water | Mine water | Aggressive solution |
Compressive strength (MPa) | |||
Phase 0 (0 weeks) | 33.67 | 33.67 | 33.67 |
Phase 1 (16 weeks) | 32.13 | 33.05 | 34.91 |
Phase 2 (32 weeks) | — | 32.13 | 26.31 |
Phase 3 (48 weeks) | 36.87 | 30.83 | 20.48 |
Tangent modulus (GPa) | |||
Phase 0 (0 weeks) | 18.67 | 18.67 | 18.67 |
Phase 1 (16 weeks) | 17.81 | 17.17 | 16.16 |
Phase 2 (32 weeks) | — | 18.31 | 11.12 |
Phase 3 (48 weeks) | 18.94 | 16.16 | 6.56 |
Secant modulus (GPa) | |||
Phase 0 (0 weeks) | 13.90 | 13.90 | 13.90 |
Phase 1 (16 weeks) | 13.77 | 12.60 | 11.86 |
Phase 2 (32 weeks) | — | 13.53 | 8.28 |
Phase 3 (48 weeks) | 13.26 | 11.73 | 3.95 |
For the brick UCS test data (Table
For the mortar samples, it can be seen that the compressive strength of mortar in potable and mine water showed little variation and increased slightly from Phase 1 to 3 due to the effect of curing. It is also noted that the tangent and secant modulus data from the mine water do not show a variation any greater than the data from the potable water. The aggressive solution had a pronounced effect on tangent modulus, secant modulus, and compressive strength of the mortar (Table
For concrete samples, potable and mine water had little effect on compressive strength and stiffness; a small variation is observed from Table
Typical plots of maximum axial stress against confining pressure from the triaxial tests on brick, mortar, and concrete (Phase 0) are shown in Figure
Maximum axial stress against confining stress for triaxial tests on (a) brick, (b) mortar, and (c) concrete at Phase 0.
Shear strength envelope of brick in (a) potable water, (b) mine water, and (c) aggressive solution.
Shear strength envelope of mortar in (a) potable water, (b) mine water, and (c) aggressive solution.
Shear strength envelope of concrete in (a) potable water, (b) mine water, and (c) aggressive solution.
According to the data in Figures
For mortar (Figure
For concrete in the aggressive solution (Figure
According to the results presented in this study, it was found that the decrease of compressive and shear strength has different trends compared to the mass loss data of the three materials under acid attack. Therefore, it may be inappropriate to evaluate the strength degradation of the materials from the mass loss data. This observation agrees with other findings that the variation of mass loss does not necessarily reflect the variation of mechanical properties [
A programme of weathering tests was undertaken to examine the effect of harsh environmental conditions that are characteristic of flooded mine shafts on the mechanical properties of shaft lining materials, namely, brick, mortar, and concrete. Three different immersion solutions were used: potable water, representative mine water, and a more aggressive solution. Four phases of laboratory tests were conducted to assess the degradation of the mechanical properties of the materials. According to the test results, the following conclusions can be drawn: For the samples in the aggressive solution, the data showed that, in terms of mass loss, all three materials were sensitive to acid attack. A similar trend of variation of mass loss with time was observed for all material samples in the aggressive solution. The weathering process had a pronounced effect on the behaviour of the mortar and concrete. Based on UCS tests, variable degrees of weathering resulted in a bilinear trend of mortar stiffness during loading and an almost linear decrease of Young’s modulus of the concrete during the weathering process. A considerable decrease in the compressive strength of mortar and concrete samples was measured. The test results showed that Young’s modulus was found to be more sensitive to acid attack compared to compressive strength for both materials. This suggests that large deformation may be observed within shaft linings before significant collapse occurs. According to the triaxial results, the shear strength of the three materials was shown to be sensitive to the attack of the aggressive solution. After 48-week immersion in the aggressive solution, the shear strength of brick was reduced by more than 50% at the lowest confining stress. Mortar and concrete showed different behaviour under the aggressive solution attack in the triaxial tests due to different effects of confining pressure on the materials. A sudden drop of shear strength of mortar was found, whereas the shear strength of concrete was shown to reduce gradually with time.
The authors declare that they have no competing interests.
The authors would like to acknowledge the financial support provided by the European Commission Research Programme of the Research Fund for Coal and Steel (RFCS). The work described in this paper was undertaken as part of the “Mine Shafts: Improving Security and New Tools for the Evaluation of Risks” (MISSTER) project, Grant Agreement RFCR-CT-2010-00014.