Shih-Meng Hsu

Using Borehole Prospecting Technologies to Determine the Correlation between Fracture Properties and Hydraulic Conductivity: A Case Study in Taiwan

This study conducted different borehole prospecting techniques for hydrogeological investigations of fractured rock at three active landslide sites in southern Taiwan. Borehole televiewer logging, flowmeter logging, and packer hydraulic tests were performed to quantify various hydrogeological parameters including fracture width, fracture angle, flow velocity and hydraulic conductivity. The dependence of hydraulic conductivity on fracture properties was evaluated by correlation analysis. While a high positive correlation was found between hydraulic conductivity and fracture width (r  =  0.89), and flow velocity (r  =  0.87), no correlation with fracture angle was observed. In addition, it is worthwhile to note that the product of fracture width and flow velocity showed a strong correlation with hydraulic conductivity (r  =  0.97). The regression analysis also revealed that a power law relationship, with a coefficient of determination of 0.83, exists between the hydraulic conductivity and the product of fracture width and flow velocity. The rationality of this relationship was carefully verified by using another group of geophysical borehole measurements. The results demonstrated that it is capable of predicting the hydraulic conductivity of fractured rock based on borehole televiewer and flowmeter logging results.

Rock Mass Hydraulic Conductivity Estimated by Two Empirical Models

Undertaking engineering tasks such as tunnel construction, dam construction, mine development, the abstraction of petroleum, and slope stabilization require the estimation of hydraulic conductivity for fractured rock mass. The understanding of hydraulic properties of fractured rock mass, which involves the fluid flow behaviour in fractured consolidated media, is a critical step in support of these tasks. To obtain hydraulic properties of fractured rock mass, double packer systems can be adopted (NRC 1996). They can be used to determine the hydraulic conductivity in a portion of borehole using two inflatable packers. Although this type of test can directly measure the hydraulic parameter, costs of the testing are fairly high. Several studies (Snow, 1970; Louis, 1974; Carlsson & Olsson, 1977; Burgess, 1977; Black, 1987; Wei et al., 1995;) have proposed the estimation of rock mass hydraulic conductivity using different empirical equations, which were based on the concept that rock mass permeability decreases with depth, as shown in Table 1. These empirical equations provide a great feature for characterizing rock mass hydraulic properties quickly and easily. However, the applicability of these equations is very limited because depth is not the only significant variable on the prediction of rock mass permeability. Hydraulic properties of rock mass may vary with geostatic stress, lithology and fracture properties, including fracture aperture and frequency, fracture length, fracture orientation and angle, fracture interconnectivity, filling materials, and fracture plane features (Lee & Farmer, 1993; Sahimi, 1995; Foyo et al., 2005; Hamm et al., 2007). Thus, a more applicable empirical equation for estimating hydraulic conductivity of rock mass possibly must include the aforementioned factors. This chapter proposes two empirical models to estimate hydraulic conductivity of fractured rock mass. The first empirical model was based on the rock mass classification concept. The study developed a new rock mass classification scheme for estimating hydraulic conductivity of fractured rocks. The new rock mass classification system called as “HCsystem” based on the following four parameters: rock quality designation (RQD), depth index (DI), gouge content designation (GCD), and lithology permeability index (LPI). HCvalues can be calculated from borehole image data and rock core data. The second empirical model was simply based on results of borehole televiewer logging, flowmeter logging and packer hydraulic tests. Three borehole prospecting techniques for fractured rock mass hydrogeologic investigation were performed to explore various hydrogeologic characteristics, such as fracture width, fracture angle, flow velocity and hydraulic

Two-dimensional fracture mechanics analysis using a single-domain boundary element method

This work calculates the stress intensity factors (SIFs) at the crack tips, predicts the crack initiation angles, and simulates the crack propagation path in the two-dimensional cracked anisotropic materials using the single-domain boundary element method (BEM) combined with maximum circumferential stress criterion. The BEM formulation, based on the relative displacements of the crack tip, is used to determine the mixed-mode SIFs and simulate the crack propagation behavior. Numerical examples of the application of the formulation for different crack inclination angles, crack lengths, degree of material anisotropy, and crack types are presented. Furthermore, the propagation path in Cracked Straight Through Brazilian Disc (CSTBD) specimen is numerically predicted and the results of numerical and experimental data compared with the actual laboratory observations. Good agreement is found between the two approaches. The proposed BEM formulation is therefore suitable to simulate the process of crack propagation. Additionally, the anisotropic rock slope failure initiated by the tensile crack can also be analyzed by the proposed crack propagation simulation technique.

Numerical modeling of unsaturated layered soil for rainfall-induced shallow landslides

AbstractIn this paper, a pioneer study on numerical modeling of rainfall-induced shallow landslides in unsaturated layered soil using the variably saturated flow equation is presented. To model the shallow landslides, the infinite slope stability analysis coupled with the hydrological model with the consideration of the fluctuation of time-dependent pore water pressure and Gardner equation for soil water characteristic curve was developed. A linearization process for the nonlinear Richards equation to deal with groundwater flow in unsaturated layered soil is derived using the Gardner model. To solve one-dimensional flow in the unsaturated zone of layered soil profiles, flux conservation and the continuity of pressure potential at the interface between two consecutive layers are considered in the numerical discretization of the finite difference method. The validity of the proposed model is established in three numerical problems by comparing the results with the analytical and other numerical solutions. A...

Integrating Multiple Subsurface Exploration Technologies in Slope Hydrogeologic Investigation: A Case Study in Taiwan

This study aims at presenting an integration of different downhole prospecting techniques for hydrogeologic investigation in an active landslide area. A series of subsurface exploration technologies were conducted, including borehole image scanning, electric logging, groundwater velocity measurements, and double packer testing. Both acoustic and optical borehole loggings as well as electric logging were applied to identify lithology, water bearing capacity and fracturing of the formation around the boring. Subsequently, borehole flow logs were used to indicate the distribution of permeability and hydraulic connectivity of fractures along the borehole. Based on the above prospecting results, test sections of hydraulic tests can be arranged. Finally, hydraulic packer tests were carried out to further characterize the hydrogeologic system of the site and quantitatively determine the hydraulic properties of major hydrogeologic units. Integrating multiple downhole prospecting techniques on slope hydrogeology investigation not only provides hydraulic properties for a study area, but also brings information to establish a hydrogeologic conceptual model and process the model simulation.

Characteristic of multispectral images and well yields of hydrogeological units in fracture bedrock, Taiwan

To continue the groundwater monitoring networks in the plain areas for further demand of clean water resources, the groundwater exploration of mountainous regions becomes a crucial issue in Taiwan. In this study, 75 boreholes were drilled for conducting investigation by the integration of subsurface exploration technologies, including drilling, core inspection, geophysical logging, and hydraulic testing. Besides, 48 groundwater monitoring wells were constructed with pumping testing. For further understanding the characteristic of potential groundwater areas, analysis of hydrogeological unit (HGU) and Landsat imagery layer stacking were considered. Results show that the trend of potential well yields in each rock type unit is related with features extracted from Landsat images.

Fractured-bedrock aquifer studies based on a descriptive statistics of well-logging data: A case study from the Dajia River basin, Taiwan

A simple descriptive-statistical approach is proposed for evaluating the transmissivity of fractured-bedrock under field conditions. Based on standard well logging and identification of structural discontinuities from borehole televiewer, the approach consists of four steps: (1) determination of discontinuity properties, namely the frequency of discontinuities and the ratio of open fractures along the borehole; (2) ranking the three well logs and the discontinuity properties into tertiles (high-to-low); (3) performing statistical analysis (F-test) for the three well logs to assess the significance of open fractures on the water-transmitting potential and then assigned a weighting factor; and (4) a semi-quantitative indicator, RGVFO, is obtained by multiplying all predictive indexes and weighting factors. It is a multi-criteria evaluation procedure that elucidates the quantitative and qualitative importance of all selected geophysical indexes. A high cross-correlation between transmissivity to RGVFO value is found, which indicates that by a combined consideration of well logging and structural discontinuities, an appropriate estimate of bedrock water-transmitting potential can be derived.

Modelling of Crack Propagation in Anisotropic Material Using Single-Domain Boundary Element Method

This paper evaluates the stress intensity factors (SIFs) at the crack tips, predicts the crack initiation angles and simulates the crack propagation path in the two-dimensional cracked anisotropic materials using the single-domain boundary element method (SDBEM) combined with maximum circumferential stress criterion. Numerical examples of the application of the formulation for different crack inclination angles, crack lengths, degree of material anisotropy, and crack types are presented. Furthermore, the propagation path in Cracked Straight Through Brazilian Disc (CSTBD) specimen is numerically predicted and the results of numerical and experimental data compared with the actual laboratory observations. Good agreement is found between the two approaches. The proposed BEM formulation is therefore suitable to simulate the process of crack propagation. Additionally, the anisotropic rock slope failure initiated by the tensile crack can also be analyzed by the proposed crack propagation simulation technique.

Identification of conductible fractures at the upper- and mid- stream of the Jhuoshuei River Watershed (Taiwan)

The movement and storage of ground water in the mountainous region has a significant impact on the dynamics of surface water flow. An adequate identification of the conductible fracture in the aquifer has thus received growing interest over the past decades. This paper summarizes the major findings from the first year of a hydrogeological investigation program initiated by the Central Geological Survey, Ministry of Economic Affairs (MOEA) of Taiwan since 2010, with a special focus on exploring in detail the fracture permeability. During the on-site investigation, geophysical logging was applied to delineate the lithostratigraphic characteristics of bedrock aquifers. The hydraulic conductivity of 67 observation segments was estimated by the constant head injection method. From the information gathered in this study, the hydraulic conductivities of the identified fractured medium above a depth of 40m are more than one order higher than that of the matrix. The occurrence of ground water in a fracture network, however, is found to be not solely governed by lithological composition, but more possibly by fracture porosity and spacing. A simple linear relationship was found by plotting the hydraulic conductivity against the product of total porosity and cubic aperture ratio (fracture spacing/sealed-off interval between the packers).

Coupled stability analyses of rainfall induced landslide: a case study in Taiwan piedmont area.

This study presents a 2D time-dependent infiltration-seepage-stability coupled hydrogeological model to simulate the seepage and slope stability of an active landslide site in northern Taiwan. The conceptual model and corresponding hydraulic and mechanical parameters applied in the model were based on a series of in-situ investigations and laboratory experimental results. A seepage analysis was conducted, and the model was calibrated and verified from the field monitoring data in order to minimize the difference between the computed pore pressures and the observed piezometric levels. A stability analysis coupled with the results of seepage analysis was performed to determine the slip potential of the landslide site under the rainfall brought up by two typhoon events. Various designed rainfall with different patterns, intensity and accumulated amount was then introduced in the model. The relationship between different rainfall characteristics and stability of the landslide site was identified and provided a good indication for the risk management under various rainfall conditions.