Influence of rock brittleness on TBM penetration rate in Singapore granite

Brittleness is one of the most important rock properties, which affects the rock fragmentation process induced by TBM cutters, and then the TBM penetration rate. This paper presents the different measurements of rock brittleness. The ratio of uniaxial compressive strength to Brazilian tensile strength was adopted to quantify the rock brittleness. By the laboratory tests, the brittleness indices of more than 100 samples cored in Bukit Timah granite along the tunnel alignments of T05 and T06 tunnels of DTSS in Singapore were obtained. The brittleness index varies in a large range from less than 10 to more than 25. The tendency shows that the brittleness index decreases with the increase of the weathering grade of granite. A series of models were then set up to simulate the effect of rock brittleness on rock fragmentation process using UDEC. With the decrease of the brittleness index, the crushed zone decreases and the number and length of the main cracks outside the crushed zone also decrease. It is obvious that with the increase of the rock brittleness index the cutter indentation process gets easier. Through the statistical analysis of correlation between rock mass properties and the corresponding TBM performance in tunnel projects, it was noted that TBM penetration rate increases with increasing rock brittleness, which is consistency with the numerical simulation results.     

Results of in situ stress measurements and their application to mining design at five Chinese metal mines

This paper reports on the measuring techniques and results of in situ stress measurements at five Chinese metal mines. The measurements illustrate the features of the in situ stress state at the five mines which are located in different regions in China. Application of the stress measurement results to mining design in the five mines is also presented.     

Construction and utilization of rock caverns in Singapore Part A: The Bukit Timah granite bedrock resource

The Bukit Timah granite underlaying central and northern Singapore has been studied for the feasibility of hosting large unsupported rock caverns. Detailed geological investigation, comprising mapping, seismic refraction and electrical resistivity surveys, deep vertical and inclined drilling, conventional and geophysical borehole logging and in-situ hydraulic testing (at a selected location), as well as laboratory testing, have been performed. The results indicate that the granite is of extremely good quality and suitable for the construction of large unsupported caverns. Therefore, the Bukit Timah granite should be considered as a valuable resource for cavern and underground space development in Singapore.     

Pressure tunnels in non-uniform in situ stress conditions

The bearing capacity of prestressed concrete-lined pressure tunnels is governed by the in situ stress of rock mass, which generally has different magnitudes in the vertical and horizontal direction. Two cases were distinguished, based on whether the vertical stress is greater than the horizontal stress or not.By means of a finite element model (FEM), the resulting distribution of stresses, strains and deformations due to tunnelling processes was revealed. The bearing capacity of pressure tunnels was determined based on the superposition principle. The results obtained demonstrate the significance of horizontal-to-vertical stress coefficients in the bearing capacity of pressure tunnels prestressed by grouting. Favourite locations where crack openings in the final lining may occur are identified, which is useful for taking measures regarding the tunnel tightness and stability.     

Hydraulic fracturing stress measurements in Hong Kong

Since 1990, a total of 149 hydrofrac stress measurements to about 200 m depth were conducted in 18 boreholes as part of several geotechnical site investigation programs in the Hong Kong area. The in situ tests were carried out by using the wireline hydrofrac technique to move the straddle packer tool within the 76 mm or 101 mm diameter boreholes. Although the tests were performed both in fractured and unfractured crystalline rocks and the boreholes are located in areas of pronounced topographic relief, the results yield a consistent orientation of the maximum horizontal stress of N 108°±28°. Above 150 m depth, the vertical stress S_v due to the weight of the overburden with given rock density is the minimum principal stress, while the few deeper data available suggest that the minimum horizontal principal stress is the least principal stress. The derived stress magnitudes can be reasonably summarized by the following normalized stress-ratios:

where z is the depth in meters and S_h and S_H are the minimum and maximum horizontal principal stresses. Due to the considerable scatter of the stress data at shallow depth above 100 m, it is suggested that further detailed in situ stress measurements be undertaken in areas where large-scale underground excavations are planned.     

Radon in soil gas — Exhalation tests and in situ measurements

Radon in soil can move into buildings resulting in high radon daughter concentrations. The foundation of a dwelling should be adapted to the radon “risk” which is determined by the radon concentration and the air permeability of the soil. Different measuring procedures are discussed in this paper, both in situ measurements of radon content and laboratory tests on radon exhalation from different types of soils at different water contests.     

An in situ thermo–hydraulic experiment in a saturated granite II: analysis and parameter estimation

This paper is Part II of a series of two papers concerned with an in situ Thermo–Hydraulic Experiment carried out at the Underground Research Laboratory of Atomic Energy of Canada Limited. The Thermo–Hydraulic Experiment was designed to determine the thermoporoelastic parameters that control the magnitude of the pore pressure induced by thermal loading. The experimental set-up involved a heater installed in a sub-horizontal borehole drilled from an underground gallery, and piezometers and thermistors located at different distances from the heater in auxiliary boreholes drilled from an adjacent gallery. Several water injection and heater tests were conducted during this experiment. Part II focuses on the interpretation of the experimental data to estimate some thermoporoelastic constants of the Lac du Bonnet Granite. The interpretation is based on matching the pore pressure and temperature data with theoretical predictions based on singular thermoporoelastic solutions. To estimate the parameters, the distances from the center of the test zone to all piezometers and thermistors have to be known. However, sufficiently accurate distances for back-analyses of the experiment could not be obtained due to measurement uncertainties. A two-step scheme is described in this paper to minimize the uncertainty in the distances. In a first step, parameters lumping distance and material constants were estimated by matching a theoretical prediction to experimental data. In a second step, the material constants were extracted from the lumped coefficients using a least square technique involving the tentative distances deduced from the borehole survey.     

高温三维应力下鲁灰花岗岩蠕变本构关系的研究

采用中国矿业大学的20 MN高温高压岩体三轴试验机进行了高温三维应力下大尺寸Φ200 mm×400 mm鲁灰花岗岩蠕变特性的实验研究,温度最高达到了600℃,轴向应力最高达到了175 MPa。通过对实验数据的分析,发现高温三维应力条件下静水应力引发鲁灰花岗岩发生蠕变变形,提出了考虑静水应力作用时稳态蠕变率的本构方程,得到了鲁灰花岗岩蠕变本构方程的参数A1,A2,?Q,m,n。研究还发现静水应力下的蠕变和差应力下的蠕变曲线特征相同,同样可以划分为初始蠕变阶段、稳态蠕变阶段和加速蠕变阶段三个阶段,鲁灰花岗岩的蠕变变形是温度,差应力和静水应力的函数,温度、差应力和静水应力的升高都会加速鲁灰花岗岩的蠕变变形。更多还原     

温度和应力循环作用下花岗岩力学特性变化规律试验研究

采用多功能岩石高温三轴试验机,对花岗岩试件进行温度上限为100℃～600℃、应力上限分别为各温度下70%和85%单轴抗压强度的温度和应力循环试验,揭示温度和应力循环过程中花岗岩力学特性的变化规律,研究表明:(1)随循环次数增加,花岗岩弹性模量逐渐增大,每次循环加载的上限应变总体呈减小趋势,与应力上限为70%单轴抗压强度相比,应力上限为85%单轴抗压强度时上限应变的降低程度更大;(2)除600℃外,试件经温度和应力循环作用后的单轴抗压强度都大于对应实时温度下的强度值,其中循环温度上限为300℃时,其强度值增幅最大,在循环应力上限为70%与85%抗压强度条件下,增幅分别达到57.1%和50.9%;(3)经温度和应力循环后,花岗岩试件的强度产生明显变化,而峰值应变与实时温度下的峰值应变相差不大,说明从变形条件研究岩石的稳定性比强度条件研究岩石的稳定性更符合试验规律。研究结果对受温度和应力循环作用的深部岩石工程的稳定性研究有一定的借鉴价值。     

在地应力测量中准确求解最大、最小水平应力问题 的探讨

目前,在地应力测量和应用中,人们常常用近于水平的两个主应力或其在水平面上的投影来代替或估计最大和最小水平应力,这在一般情况下误差不大,但是当应力结构比较特殊时这种代替或近似将带来较大的误差,甚至给工程实践带来危害。以投影近似为例按 3 种三维应力状态分别讨论了方位误差随应力形因子 <> R 与主应力轴倾角的变化和量值误差随 <> R 、主应力轴倾角和最大、最小主应力量值之差的变化。并用原地应力测量资料求得了方位和量值误差。理论分析和实例都证明了用近于水平的两个主应力的投影代替两个水平应力的误差可能很大。建议在研究与水平应力相关的问题或与水平方向相关的物理参量时采用本文的计算方法求出准确值。     

Assessment of indoor HONO formation mechanisms based on in situ measurements and modeling

The photolysis of HONO has been found to be the oxidation driver through OH formation in the indoor air measurement campaign SURFin, an extensive campaign carried out in July 2012 in a classroom in Marseille. In this study, the INCA‐Indoor model is used to evaluate different HONO formation mechanisms that have been used previously in indoor air quality models. In order to avoid biases in the results due to the uncertainty in rate constants, those parameters were adjusted to fit one representative day of the SURFin campaign. Then, the mechanisms have been tested with the optimized parameters against other experiments carried out during the SURFin campaign. Based on the observations and these findings, we propose a new mechanism incorporating sorption of NO₂ onto surfaces with possible saturation of these surfaces. This mechanism is able to better reproduce the experimental profiles over a large range of conditions.     

The in situ stress states associated with core discing estimated by analysis of principal tensile stress

Core discing occurs due to tensile stress induced by boring within or below a core stub when the minimum principal stress is nearly in the same direction as the core axis. To determine the effects of the core length on the magnitude and direction of tensile principal stress, a finite element analysis was carried out for an HQ core of different lengths for 77 in situ stress conditions. According to the minimum value and the mean inclination relative to the core axis for ‘the maximum semi-axial tensile stresses’, 30 in situ stress conditions were identified as being stress conditions under which core discing is likely to occur, and conditions necessary for in situ stress were proposed. The critical tensile stress, which is the tensile stress that can produce a tensile fracture that propagates throughout a cross-section, was analyzed for these stress conditions and a new criterion for core discing, which can be applied to a core of any length, was proposed. The stress conditions estimated by the criterion were consistent with previous experimental results for a long core and for thin discs. According to the criterion, the relationship between the core length and the in situ stress necessary for core discing was examined. Our analysis showed that the stress field can be divided into three regions and that core discing of short length mostly occurs at great depth. The average relationship between the core length and the disc thickness was determined by assuming that the position of a fracture is given by the mean position of ‘the maximum semi-axial tensile stresses’. Our theoretical estimates reproduced previous experimental results regarding the effects of stress magnitude on the thickness of the disc. Thus, the present proposed criterion can be used to estimate the stress condition for core discing with a given disc thickness.     

Numerical modelling of in situ stress calculation using borehole slotter test

Accurate determination of in situ stress magnitude and orientation is an essential element of the design process of all underground openings. A stress calculation method was proposed for the new stress measurement technique using the borehole slotter device. Two major objectives were the focus of this research work. The first goal was to simulate the slotter test process numerically and delve into the mechanisms involved in this test. A precise 3D numerical model of a typical slotter test condition was constructed using the FLAC3D code. The effects of variations in rock mass deformation modulus on the strain/stress relieve, and thus borehole slotter test results, were investigated numerically. The second objective of the work was to employ 3D modelling in the interpretation of slotter field tests conducted at Bakhtiari dam site. These tests were aimed at determining the stress magnitude and orientation to be used in the design of underground chambers and tunnels associated with Bakhtiari dam. The stress regimes measured in field were applied as boundary condition on the constructed 3D model and a backward analysis was conducted. For each case the actual strain field measured was compared against strain field calculated numerically. Accordingly, the boundary condition (stress magnitude and orientation) associated with the model results that provided the best fit to the measured data was determined as the governing stress regime. A good agreement was achieved between numerical and field test results. The obtained numerical results provided valuable insights in selecting the governing in situ stress condition from a set of recorded field data.     

Determination of three dimensional in situ stress from core discing based on analysis of principal tensile stress

To determine all of the components of in situ stress from core discing, both the directions and magnitudes of the principal in situ stresses must be determined for a disc of a given thickness. In this study, we analyzed the direction and magnitude of tensile stress below an HQ core stub for 11 core lengths using stress conditions under which core discing is likely to occur. First, based on an analysis of the direction of tensile stress below the core stub, we propose a method for determining directions of in situ stress from the height distribution at the periphery of the end surface of a disc. This method can be used with a disc of any thickness. Next, based on an analysis of the magnitude of tensile stress in the central part of a core, we propose a linear criterion for core discing, which can be applied to a core of any length. This criterion was in good agreement with an empirical formula obtained previously in laboratory experiments. By combining information on the direction of in situ stress and the linear criterion for core discing, we propose a method for determining all of the components of in situ stress from core discing under the assumption that vertical stress is given by the overburden stress. Finally, these methods were applied to discs obtained from a field where hydraulic fracturing was performed to measure horizontal stresses. The results showed that the azimuths of the principal stresses estimated from core discing were consistent with those of the principal horizontal stresses determined by hydraulic fracturing and that while the magnitudes of the principal horizontal stresses estimated from core discing showed a large scatter, they were similar to those determined by hydraulic fracturing.     

Study on the influence of confining stress on TBM performance in granite rock by linear cutting test

Rock stress problems induced by overburden or anisotropic stresses are significant to the TBM tunneling. In this paper, the effect of different confining stressed conditions on TBM performance are investigated by using full-scale cutting tests with large intact granite specimens (1000 mm × 1000 mm × 600 mm). In these tests, the effects of confining stresses on the normal force, rolling force, the cutting coefficient and specific energy are analyzed. It is found that the confining stress has significant impact on the normal force and rolling force. Specifically, for the same cutting spacing and penetration depth, the normal force increases with increasing confining stress due to enhancement of the rock resistance strength; meanwhile the rolling force decreases gradually with increasing confining stress. The stress deviation between two confining directions affects the optimum penetration that corresponds to small specific energy. The results provide better understanding of the effect of confining stress on the TBM performance and also recommend some guidelines for TBM tunneling under stressed geological condition.     

Analysis of stress measurements using a numerical model methodology

A method is described that enables the boundary conditions of numerical models to be calibrated to individual or groups of stress measurements. The method was developed to interpret stress measurements made in mines where it is not possible to locate the measurement points far enough away from excavations to obtain a direct measurement of the pre-mining stress field. It can also be used to analyze measurements that are influenced by surface topography. The stress field at any point is assumed to be comprised of gravitational and tectonic components. The tectonic component is assumed to act entirely in the horizontal plane in the far-field and at the model boundary. Unit normal and shear tractions are applied to the model boundaries and the response is computed at the location of the measurement points in the model. An optimization procedure is used to compute the proportions of each unit response tensor that is required, in addition to the gravitational stress, to reproduce the measured stress at the measurement point in the model. Scaling of the measured stress tensor can be included in the optimization to account for incorrect rock modulus scaling of the measured strains. The method is demonstrated using a synthetic set of stresses from a numerical model in which the measurements are influenced by a nearby excavation and topography.