Journal of Sensors

Volume 2015, Article ID 470905, 12 pages

http://dx.doi.org/10.1155/2015/470905

## Deformation Monitoring of Geomechanical Model Test and Its Application in Overall Stability Analysis of a High Arch Dam

^{1}State Key Laboratory of Hydraulics and Mountain River Engineering, College of Water Resource & Hydropower, Sichuan University, Chengdu 610065, China^{2}School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

Received 16 November 2014; Revised 6 January 2015; Accepted 20 January 2015

Academic Editor: Fei Dai

Copyright © 2015 Baoquan Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Geomechanical model testing is an important method for studying the overall stability of high arch dams. The main task of a geomechanical model test is deformation monitoring. Currently, many types of deformation instruments are used for deformation monitoring of dam models, which provide valuable information on the deformation characteristics of the prototype dams. However, further investigation is required for assessing the overall stability of high arch dams through analyzing deformation monitoring data. First, a relationship for assessing the stability of dams is established based on the comprehensive model test method. Second, a stability evaluation system is presented based on the deformation monitoring data, together with the relationships between the deformation and overloading coefficient. Finally, the comprehensive model test method is applied to study the overall stability of the Jinping-I high arch dam. A three-dimensional destructive test of the geomechanical model dam is conducted under reinforced foundation conditions. The deformation characteristics and failure mechanisms of the dam abutments and foundation were investigated. The test results indicate that the stability safety factors of the dam abutments and foundation range from 5.2 to 6.0. These research results provide an important scientific insight into the design, construction, and operation stages of this project.

#### 1. Introduction

Currently, the construction of high arch dams in China is undergoing vigorous development. Several high arch dams about 300 m high, which represent an advanced class of arch dams, are planned or under construction. For example, the Xiaowan arch dam (294.5 m high) was built on the Lancang River. The Jinping-I arch dam (305 m high) on the Yalong River is under construction. The Baihetan (289 m high) and Wudongde arch dams (265 m high) on the Jinsha River and the Songta arch dam (313 m high) on the Nujiang River are currently in the design phase. Most of them involve great dam heights and huge reservoir capacities. These large-scale arch dam projects are accompanied with large flood discharges, high earthquake intensities, and complicated geological conditions [1]. To guarantee the safety of these projects, it is important to study the stability of both arch dams and dam foundations, to select optimum foundation reinforcement schemes, and to evaluate the strengthening effects of the corresponding reinforcement measures. A lot of efforts have been made to monitor the field performance of dams [2], but the instrumentation costs are expensive and the interpretation of field monitoring results is complicated. Geomechanical model testing is an important approach that can solve these problems.

Geomechanical model testing is a method that can reasonably simulate the dam under investigation, by taking into account the geological structure of the dam abutment and its reinforcement measures using certain similarity principles [3, 4]. The primary purpose of this test is to obtain deformation characteristics and failure pattern of the prototype through overloading or strength reduction [5]. Using this method, the influences of the geological structure on the dam safety can be evaluated, which can provide a reference for designing foundation reinforcement schemes. In addition, by studying the deformation monitoring data and the failure mechanisms of dam abutments and foundations, the safety factor of the dam and foundation can be determined. Such monitoring results are very intuitive for dam designers and decision makers.

Deformation monitoring is very important for geomechanical model tests. However, as the deformation of small-scale models is much smaller than the prototype, the deformation instruments should have very high precision. They should also have small sizes and light weights, so that they can be easily installed in the model dams. Currently, different types of deformation instruments, including mechanical sensors, inductive sensors, resistance strain gage, and differential transformers, are available. All of these instruments have been frequently used for deformation monitoring of geomechanical models and can meet the high-accuracy requirements. Due to the development of measurement technologies, especially the rapid development of computers and automation technologies, the measurement methods and techniques in model tests developed rapidly. These sensors are more accurate and reliable than conventional ones. For example, the internal displacement transducer developed by Zhang et al. [6] can be used to monitor the relative deformation of the structural planes in the rock mass. The small two-way resistance displacement sensor developed by Huang and Chen [7] can replace resistance strain gauges in the model instrumentation system. The fiber optic sensing method is another important advance for geomechanical model tests [8–10]. The fiber optic sensors have many advantages over conventional sensors, such as small size, high accuracy, and inherent resistance to corrosion and electrical noise [11–14].

Although the development of deformation testing techniques can meet the requirements of geomechanical model tests, more works should be conducted to relate the deformation measurements gained to the overall stability of high arch dams. Zhou et al. [15, 16], Liu et al. [17], and Zhang et al. [18] of Tsinghua University established an analysis and evaluation system for the model tests of several high arch dams in China, such as the Xianghongdian, Qingshiling, and Jinshuitan dams. They summarized the test results of these dams and grouped a set of safety evaluation methods based on (initial cracking load), (nonlinear start load), and (limit fracture load) values. They also introduced the evaluation index into the engineering design specifications of China. Peng et al. [19] of Tongji University presented a systematic method for slope safety evaluation utilizing multisource monitoring information. However, a complete set of stability evaluation systems was not formed for the comprehensive test method of geomechanical models.

In this paper, a relationship for assessing the stability of dam safety is established based on the comprehensive model test method. A stability evaluation system is presented based on the deformation monitoring results. Two inflection points are proposed to indicate the stability condition of high arch dams. Finally, the comprehensive model test method was applied to study the overall stability of the Jinping-I high arch dam. A three-dimensional (3D) destructive test of a geomechanical model dam was conducted under reinforced foundation conditions. The deformation characteristics and failure mechanisms of the dam abutments and foundation were investigated in detail.

#### 2. Comprehensive Test Method for Geomechanical Models

##### 2.1. Principles of the Comprehensive Test Method

For geomechanical models, three test methods are widely used, including the overloading method, the strength reduction method, and the comprehensive method. The overloading method mainly considers the upstream overloading effect on the stability of the dam abutments. The strength reduction method focuses on the effects of the decreasing mechanical strengths of the rocks and weak structural planes in the abutments and foundation on the dam stability. The comprehensive method is a combination of the overloading and strength reduction methods, through which a variety of factors can be investigated within one model [20]. For the overloading method, multistage loadings are generally accomplished by using jacks installed on the upstream surface of the model dam, through which the water overloading is applied. The overloading method is extensively applied in geomechanical model tests because of its convenience. The comprehensive method can consider more factors simultaneously, but overloading and strength reduction must be conducted in one model, which results in a certain degree of difficulty. Here, we proposed a comprehensive test method for geomechanical models, which takes advantages of special temperature analogous materials [21–23].

The safety factor of the comprehensive test method was assessed by using the basic concepts of degree of safety (or the point safety factor) and the principles of the overloading method and the strength reduction method. The point safety factor can be expressed as follows: where is the point safety factor, is the design water pressure on the upstream dam surface (sliding force), is the shear friction coefficient, is the cohesion, is the normal force of the sliding surface, is the sliding surface area, is the normal stress on the surface of the integral infinitesimal, is the integral infinitesimal area, and is the shear strength at the surface of the integral infinitesimal.

Equation 1 can be rewritten as

Equation 2 shows that in the overloading method the material mechanical parameters of and and the shear strength will remain constant. Thus, we can increase the design water pressure until the model dam fails. The coefficient of the overload that corresponds to the failure of the model dam is called the overloading safety factor .

Equation 2 can be rewritten as in the following format:

Equation 3 represents the concept of the strength reduction method. This method is under constant load conditions, but the mechanical parameters and of the rock mass and structural plane of the model gradually decrease in the test, until the model dam fails. The lowest coefficient of the mechanical parameters of the model materials in the model tests is called the strength reduction safety factor .

If the in 3 is decomposed into two parts, that is, the strength reduction coefficient and the overloading coefficient , the following relationship can be obtained:

Equation 4 shows the concept of the comprehensive test method, from which the comprehensive safety factor can be represented as follows:

According to 1 to 5, the comprehensive method is advantageous because it considers not only the overloading effect of the dam but also the influence of decreasing strength of rock mass and structural planes. Thus, this method integrates the merits of the overloading method and the strength reduction method.

##### 2.2. Safety Factors of the Comprehensive Test Method

As shown in 5, the safety factor in the comprehensive test method is a product of the strength reduction coefficient and the overloading coefficient . The strength reduction coefficient is the reduction factor of the mechanical parameters and , which mainly consider the softening effect of rock masses and structural planes under the actions of reservoir water. This situation occurs under the normal operation conditions of a dam. In the dam stability analysis, the degree of strength reduction for the structural planes is often assigned a value of 15%~30% according to engineering experiences [24]. Thus, the strength reduction coefficient is between 1.15 and 1.3 in the comprehensive model tests.

The overloading coefficient represents multiples of the design water pressure , which can be withstood by the dam structure. The situation when the dam is subjected to an unusually large load is considered [25]. Obtaining the overloading coefficient in the comprehensive test method is very important. The deformation of the dam body and dam foundation directly reflects the dam stability and potential damage. Therefore, in a geomechanical model test, the monitoring results of the deformation of the dam and dam abutments should be analyzed in detail. In this study, the deformation catastrophe theory [26] is used to propose characteristic points of the deformation curves obtained from model tests, which provides a basis for evaluating the overloading coefficient. A typical curve between the dam deformation and the overloading coefficient of a dam model is shown in Figure 1.