CC BY
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ВЕСТНИК ИНЖЕНЕРНОЙ ШКОЛЫ ДВФУ. 2013. № 4 (17)
GEOMECHANICS / ГЕОМЕХАНИКА
УДК 622.833
Meifeng Cai, Hua Peng
MEIFENG CAI, Professor, University of Science and
Technology Beijing, China.
HUA PENG, Professor, Institute of Geomechanics, Chinese
Academy of Geological Sciences, Beijing, China.
E-mail: [email protected]
IN-SITU STRESS MEASUREMENT AND ITS ENGINEERING APPLICATION
The information on the in-situ stress state is an essential prerequisite for design and construction of all engineering projects which involve excavations in rock because the entire process of excavation in rock is controlled by in-situ stress. The guiding rules of in-situ stress for design and construction of rock engineering in five aspects are introduced. Stress relief by overcoring technique and hydraulic fracturing technique are two mainly used stress measurement techniques in the world. Both advantages and disadvantages of the two techniques are briefly assessed. To make them suitable to be used at great depth and to increase their measuring reliability, accuracy and practical usability, a series of improving techniques have been developed. Suggestions for further development of in-situ stress measurement are provided.
Key words: in-situ stress, measurement, engineering application, techniques, new development.
Измерение напряжения в массиве горных пород и его применение в инженерном деле. Мейфенг Цай - профессор (Пекинский научно-технический университет, Пекин, КНР), Хуа Пенг - профессор Института геомеханики (Китайская академия наук о Земле, Пекин, КНР).
Данные напряженного состояния массива горных пород - необходимое условие для проектирования и строительства всех инженерных объектов, которые связаны с выемкой горных пород (весь процесс выемки зависит от напряжений в породном массиве). Вводятся пять основных положений для определения напряжений в нетронутом массиве горных пород при проектировании и строительстве горных инженерных объектов. Разгрузка от напряжений способом обуривания керна и метод гидравлического разрыва пласта - два главных метода измерения напряжения, используемые в мировой практике. Даны оценки
© Meifeng Cai, Hua Peng, 2013
преимуществ и недостатков этих методов. Разработан ряд усовершенствованных методик, пригодных для практического применения на больших глубинах и повышающих точность и надежность измерений. Представлены предложения по дальнейшему развитию измерений напряжения в массиве горных пород.
Ключевые слова: напряжение в массиве горных пород, измерение, инженерное приложение, методы, новая разработка.
Introduction
The knowledge of in-situ stress state is indispensable for design and construction of all engineering projects which involve excavation in rock, such as mining engineering, geotechnical engineering, hydro-electrical engineering, petroleum engineering, railway and road engineering, etc. The original stress in rock is a kind of natural stress existing in the Earth's crust. Before excavation, the stress in the Earth' crust is in equilibrium status. The excavation in rock breaks the equilibrium and makes the stress released. Just the released stress is the basic force to cause deformation and damage of the rock engineering [1, 3, 16]. In this way, the whole process of excavation in rock engineering is controlled by in-situ stress. The traditional design method of rock engineering is empirical analogy method. Along with the scale and depth of the excavation became larger and larger, the empirical analogy method lost its application value. To ensure reliability, stability and safety of the rock engineering, it is necessary to make quantitative calculation of the design. As any mechanical calculation needs mechanical boundary condition, in-situ stress state is the real mechanical boundary condition of the quantitative calculation for rock engineering design.
Moreover, the occurrence of dynamic damage hazards, such as rock burst and mining seismicity caused by excavation of rock engineering is getting more and more frequent and serious as the excavation depth of rock engineering is deeper and deeper. In-situ stress measurements show that the magnitude of in-situ stress is almost linearly increased with depth. High in-situ stress in the deep position of the rock engineering is the direct reason to cause dynamic damage hazards along with rock engineering excavation. In order to discover mechanisms of dynamic damage hazards and to determine prediction and prevention methods for the dynamic damage hazards, understanding of in-situ stress state, including its changing regularity with depth in the rock engineering area is necessary [2].
The earliest in-situ stress measurement for practical engineering use was conducted by Hast in the Scandinavian Peninsula. Through the measurement, it was found that the magnitude of horizontal stress was larger than the magnitude of vertical stress in the earth's crust. Further in-situ measurement and theoretical study revealed that formation of in-situ stress is related firstly to the Earth's tectonic movement and secondly to the Earth's gravity. Although the current in-situ stress state is controlled by the lasted tectonic movement, the all historical tectonic movements have influence on current in-situ stress state. Too many tectonic movements in the history caused the in-situ stress state is very complicated and changeable. Therefore, the in-situ stress state can not be
determined by mathematical or numerical calculation or modeling. The only way to get information of the in-situ stress state is carrying out real stress measurement in the engineering area.
Guiding rules of in-situ stress for design and construction of rock engineering
Mining, geotechnical and other similar rock engineering, such as hydropower, railway and road engineering projects, all involve excavation and construction in rock. Rock stress is a kind of natural stresses, which exists in the surrounding rock of the engineering before and during the whole process of excavation and operation of the engineering. It is the basic force to cause deformation and failure of rock engineering. Therefore, it is necessary to understand in-situ stress state for guiding design, construction and operation of rock engineering, especially in the following 5 aspects [6, 8].
Overall layout of engineering structures
In primary stage of engineering design, clear understanding of in-situ stress condition is a prerequisite for design of the overall layout of the engineering. In the design, the important engineering facilities, such as main chambers, shafts, tunnels and transport roadways should keep away from the high stress areas. Orientation of the above-mentioned excavation structures is best to
coincide with the direction of the major principal stress, i.e. Ci as shown in Fig.l. Because in such
layout, there are two smaller principal stresses in the vertical section of the excavations, which is favorable for stability of the excavation structures.
To make best overall layout of the engineering structures, in-situ stress measurement before the engineering design is necessary. Such as for mining engineering, in-situ stress measurement should be made in the early exploration stage. In that stage, there are some exploration boreholes can be used to make the measurement.
Selection of the optimal shape of underground tunnels and roadways
According to the elastic theory, to minimize stress concentration around the tunnels and roadways, their ideal sectional shape is an ellipse, in which the ratio of horizontal to vertical axes is best equal to the ratio of horizontal to vertical principal stresses in the section, as shown in Fig. 2. In such condition, the boundary of the tunnels and roadways will be in an even-compressed stress
Fig. 1. Best orientation of the tunnel
state, which means that the values of tangential compressive stress at every point of the boundary are equal. It is very favorable for stability of the tunnels, roadways and rock excavations.
Fig. 2. Best sectional shape of the tunnel
Jinchuan nickel mine is the largest nickel mine in China, whose nickel production occupies 90% of the nickel production of the whole country. This mine began construction in the mid-1960s, but until the mid-1970s, it could not be put into production. The main problem was that the transport tunnels and mining roadways could not be stabilized because big deformation or destabilization often appeared in a few months after completion of excavation. In-situ stress measurement was carried out for a few years in the late 1970s. The measuring results showed that the horizontal stress was remarkably larger than the vertical stress in the mine. The sectional shape of the original tunnels and roadways was shown in Fig. 3, with height greater than width. Based on the stress measurement results, the sectional shape of the tunnels and roadways was changed, as shown in Fig. 3. The changed section has a width larger than its height. After that, with improved shotcrete-rock bolts support, the stability problem of the tunnels and roadways was solved quickly and subsequently the mine was put into production in a short time [7].
Fig. 3. The original and changed shapes of the roadways in the Jinchuan mine Optimization of excavation sequences and steps of the engineering
Excavation in rock engineering is usually a complicated procedure. Different excavation sequences and steps will cause different mechanical effect, i.e. different stability status of the engineering. Because the structural shapes and excavation steps of the most rock engineering projects are very complicated, it is impossible to make engineering design, including selection of excavation methods and arrangement of excavation steps by quantitative theoretical calculation. Therefore, the traditional excavation design mainly relied on the man's experience and was less reasonable and reliable. The rapid development of computing technology with modern computers and numerical modeling methods provide efficient and powerful tools for quantitative calculation and optimal design
Original shape
Changed shape
of mining, geotechnical and other similar rock engineering excavations. All the calculation and design should be performed in a condition of known stress state in the engineering area.
Optimal design of high and steep slope in deep-concave rock engineering
Traditional method for slope design is 'limit equilibrium analysis' method. This method is based on the gravity equilibrium principle, which neglects the influence of horizontal tectonic stresses, rock mass characters, faults, etc. However, just these influential factors play key roles to control stability of the slope in deep-concave rock engineering.
Traditional assumption believes that the tectonic stress in hillside area has been thoroughly released. Stress measurement with both overcoring and hydraulic fracturing techniques was completed in Ekou iron mine, which is located in Shanxi Province of North-western China in 1992. In the mine, the final vertical height of the slope is 720 m and the excavation depth is 440 m. It was the first time to carry out systematical in-situ stress measurement in deep-concave open-pit mines in China. The measuring results showed that the horizontal stress was quite larger than the vertical stress, which denied the traditional assumption. For reliable and optimal design of the slopes in deep-concave open-pit mines, the function of the horizontal tectonic stress should be considered [15].
The shiplock in the Three Gorge project of China is a key engineering structure for the project. It is located at the left side of the Yangtze River with a length of 1 600 m. Both sides of the shiplock are high and steep slopes with the maximum vertical height of 170 m. The shiplock walls at two sides are absolutely vertical with height of 50-70 m. So the shiplock of the Three Gorge project is the typical deep-concave rock engineering with high and steep slopes. To ensure permanent stability of the shiplock, stability analysis and optimum design of the high and steep slopes are necessary and critical. To provide reliable information for the analysis and design, in-situ stress measurement with the largest scale in China has been carried out since 1984. The measuring results show that the stress state in the measuring region is dominated by horizontal tectonic stress with lateral factor of 1.08-1.82, which provides solid basis for scientific design and construction and for safe operation of the shiplock.
Prediction of rock burst and other dynamic disasters induced by deep excavation
Rock burst and other dynamic disasters induced by deep excavations are closely related to rock stress state. They are all dynamic process of energy accumulation and release during excavation. It is the only way for making "time-space-strength" prediction of dynamic disasters induced by deep excavations that to quantitatively calculate magnitude and distribution of underground energy accumulation as well as its evolutionary process based on clear understanding of in-situ stress state [13, 14]. Because quantitative calculation with numerical modeling methods can provide detailed information on magnitude and distribution of stress and strain in the
surrounding rock. From the stress and strain, the magnitude and distribution of the energy accumulation around the excavation face can be obtained by calculation with Equation (1).
n y
E = ^-fa1e1+(72£2+(J3e3JxVe', (1)
were Ci, 02 and <33 are three principal stresses in the element; £1, £2 and £3 are three principal strains in the element; Ve is volume of the element; n is the total number of the elements in the rock mass
n
surrounding the working face of the excavation. Then, ^ yj is the total volume of surrounding rock
1=1
mass in which the energy is accumulated.
Based on the knowledge of earthquake theory, from the accumulated energy E, the magnitude of earthquake corresponding this accumulated energy can be calculated with Equation (2).
LgE = 4.8 + 1.5M, (2)
where E is the total energy released by the earthquake with unit of J; M is the magnitude of the earthquake.
A strong earthquake took place in Wenchuan of China in May 12, 2008. The recorded energy released by Wenchuan earthquake waves is 10167 J, from which the value of M equals 7.9333 according to Equation (2). So, magnitude 8.0 of this earthquake was finally announced by Earthquake Bureau of China [5].
Prediction of seismic magnitude of rock burst caused by deep mining excavation
in Sanshandao gold mine
Level position -780 m~-825 m -825 m—-870 m -870 m—-915 m -915 m—-960 m -960 m—-1005 m -1005 m—-1050 m
Maximum unit energy/(J/m3) 4 9.9585X104 1.0918X105 1.2078X105 1.3689X105 1.5825 X105 2.3784 x105
Accumulated energy/J _ 3.9742 x109 9 4.3964 x109 4.9828 x 109 9 5.7603 x 109 9 8.6574 x109
Seismic magnitude - 3.1 3.2 3.2 3.3 3.4
The dynamic hazards caused by excavation, such as rock bursts are man-made earthquakes which have the similar inducing mechanisms. Therefore, Equation (2) can be used to predict seismic magnitude of rock burst according to the accumulated energy in rock mass surrounding the working face of the excavation [4]. Using this method, prediction of the seismic magnitude of rock burst caused by mining excavation in deep position of Sanshandao gold mine is made. The mine is located in the coast of Yellow sea within Shandong Province of China, whose average surface level is 0 m. The prediction results are shown in Table. In the Table, 'level position' means the position along depth of the mining level where mining excavation is made; 'maximum unit energy' means the maximum value of energy induced by mining excavation in a unit volume of the rock mass surrounding the excavation working surface; 'accumulated energy' means the total energy induced by mining excavation in a range 10 m distant to the excavation volume.
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Techniques for in-situ stress measurement
Stress relief by overcoring technique and hydraulic fracturing technique are two most commonly used in-situ stress measurement techniques in the world. Besides them, there are also several other techniques, such as acoustic emission technique, differential strain curve analysis, sonic wave anisotropy measurement and borehole wall breakout measurement techniques which are used in different engineering fields to supplement the two main techniques. They are all relatively simple and easily to be performed in normal conditions. Their measuring accuracy is lower than the two main techniques and suitable for primary estimation of the in-situ stress state.
Stress relief by overcoring technique
Advantages and problems of the stress relief by overcoring technique. Stress relief by overcoring technique has been used for in-situ stress measurement for a long time since 1950s'. According to a rough estimate, about 70-80% of in-situ stress measurement data in the world were obtained by this technique. Stress relief by overcoring technique is especially suitable for in-situ stress measurement in mining and geotechnical engineering because in such engineering projects, there are many entrances, such as shafts, tunnels, inclines and roadways which can be used as accessing way to the position where the stress measurement is conducted. Therefore, a big cost can be saved for driving a special thoroughfare to the measuring position [12].
There were two main problems which influence reliability and accuracy of overcoring stress measurement: (1) most of the overcoring stress measurement devices use resistance strain gauge as the sensing elements and the sensed strain values induced by stress relief during overcoring are interpreted to in-situ stress state based on the known relationship between stress and strain of the rock. The sensed strain values are recorded by a Whetstone bridge type indicator. Because the resistance strain gage is susceptive to temperature changes, the additional strain induced by temperature change will cause big error of the measured stress values. So, correct temperature compensation is critical for validity and accuracy of the measurement. (2) traditional method to calculate rock stress from the measured strain values is based on the elastic theory which supposes that the rock is linearly elastic, continuous and isotropic. However, field rock mass has some extent of non-linearity, discontinuity and anisotropy. The difference between the practical and assumed rock conditions will cause error in the calculated stress values. To solve the two problems, following techniques were developed by the authors [10].
Full temperature compensation technique. Traditional temperature compensation method uses dummy gauge as compensation element, which is not effective for devices which are bonded to rock during the measurement, such as the hollow inclusion strain gauge. Because according to the principle of Whetstone bridge, for effective compensation, the sensed strain values by dummy gage and by the working gage during temperature change should be the same. However, the working strain gage is bonded to rock, its deformation behavior is mainly controlled by the rock. In order to sense the same
strain value, the dummy gage must at the same position and also bonded to rock as the working gage, which is not allowed because the dummy gage should be stress free. To solve this problem, a full temperature compensation technique has been developed by the author which consists following 4 main points [10]:
(1) Resistance elements in the Wheatstone bridge type indicator are all of low temperature coefficient except the working strain gauge connected from the measuring position, which ensures no considerable additional strain induced by temperature change is recorded by the indicator.
(2) Temperature changes at the measuring point are continuously recorded by a thermistor during ovecoring.
(3) After completion of the overcoring test, the overcore with the working strain gages still inside it is put in a temperature controllable oven to make a temperature calibration test, from which the thermal strain rate, i.e. strain value induced by temperature change of 1°C, for each strain gauge is obtained.
(4) The lead wire of the strain gauge can also induce remarkable additional strain due to temperature change. To solve this problem, the same length and same type of lead wire coming from the same measuring point as the working strain gauge is connected to a neighboring arm of the strain gage in the bridge, which neglects the thermal effect of lead wire of the strain gage.
From the calibrated thermal strain rate and recorded temperature change during overcoring, the additional thermal strain values for every strain gauges can be determined and then eliminated from the total measured strain values to get the correct strain values for stress calculation.
Techniques for modifying the effect of nonlinearity, anisotropy and discontinuity of the rock mass on calculation of in-situ stress state. To solve the second problem mentioned above, some techniques have been developed for interpretation of the measured strain caused by stress relief to original stress state considering nonlinearity, discontinuity and anisotropy of the practical rock mass.
(1) To interpret the measured strains to in-situ stress state needs the value of deformation modulus of the rock. For nonlinear elastic rock, the value of the deformation modulus depends on stress level. To ensure correct interpretation, the value of deformation modulus should exactly correspond its stress level. Because the values of stress and deformation modulus are both undetermined, an iteration program is used for the interpretation. Before calculation, the nonlinear relationship between deformation modulus and stress value should be determined and inserted into the iteration program.
(2) For orthotropic and transverse isotropic rocks, a method to determine anisotropic parameters with biaxial loading test of overcore has been put forward, which makes it possible to calculate rock stress from the measured strain based on the constitutive equations of these rocks.
(3) The results of the biaxial loading test of overcore can also be used to determine anisotropic factor of strain value measured by each strain gauge. If the rock is ideally isotropic, the strain values measured by strain gauges at the same direction should be equal under biaxial loading.
The ratio of the measured strain value of each strain gage to the averaged strain value at the same direction is defined as 'anisotropic factor' of strain value measured by each strain gage, which is used to modify the measured strain value during overcoring to get correct strain value for in-situ stress calculation.
(4) To evaluate performance of rock stress measurement devices in various rock conditions with different distribution and extents of anisotropy and discontinuity through systematical laboratory modeling tests. Based on the modeling test results, the in-situ stress measurement result is modified according to the real rock condition.
(5) Using numerical modeling and iteration methods to modify the effect of nonlinearity, anisotropy and discontinuity on the measured strain values, which makes the calculated results of rock stress close to the real situation step by step.
Application of the full temperature compensation technique and techniques for modifying the effect of anisotropy, non-homogeneity and discontinuity have remarkably increased the reliability and accuracy of the overcoring stress measurement [9, 11].
Hydraulic fracturing technique
Advantages and problems of the hydraulic fracturing technique. Hydraulic fracturing technique is an efficient technique for in situ stress measurement at deep position. It was mainly used for in situ stress measurement in hydro-electrical and water conservancy engineering, petroleum engineering, highway and railway engineering, etc., but less used in mining engineering. However, in the recent years it was already used in mining engineering for in-situ stress estimation at the exploration stage of the mine. It is an important progress for mine administrator to provide information on in-situ stress state before design and construction of the mine. At the exploration stage, there is no any entrance to underground positions for stress measurement with overcoring and other techniques, so hydraulic fracturing technique is the only economical and convenient technique for detecting the underground stress state of the mine because it can use the already drilled exploring boreholes to make the hydraulic fracturing tests, which saves a lot of cost for drilling special entrances if using overcoring and other techniques. Hydraulic fracturing technique has also been effectively used in deep-concave open-pit mines due to the same reason, i.e. lack of entrance to underground position, as mentioned above. The traditional design of open-pit mines is based on limit equilibrium analysis which neglects the influence of in-situ stress on stability of the mine, but for design of deep-concave open-pit mines, overall mechanical analysis with numerical modeling or other techniques is necessary and information on in-situ stress state is indispensible.
The hydraulic fracturing technique has 2 basic disadvantages: (1) Basically, it is a 2D measuring technique. In order to get 3D in-situ stress state, measurement at three boreholes which are cross the same measuring point and not parallel each other is necessary. These conditions are difficult to be satisfied. (2) It is assumed that the fracture in the rock borehole wall is initiated at the minimum tangential stress position. If there are original fractures, the assumption may be not valid.
Although the hydraulic fracturing technique is said to be efficient for in-situ stress measurement at great depth, the measuring depth for in-situ stress measurement using this technique is limited. The traditional hydraulic fracturing equipment used at great depth has 3 bottleneck problems.
(1) The pressure-enduring ability of the sealing packers and pressurizing system of the hydraulic fracturing equipment is not enough. Because at depth over 1000 m, the pressure supplied by the pressurized water should be high enough to make the borehole wall fractured. However, the high enough pressurized water will also make the sealing packers and water pipelines damaged or lose function.
(2) Along with increase of the measuring depth, the pressure of ground water is increased. In many engineering fields, such as in many coal mines, the more than several hundreds meters thick soil layers make the borehole at measuring position filling with slurry. It will strongly influence sufficient pressure relief and removal of the sealing packers after completion of the fracturing test.
(3) The traditional hydraulic fracturing equipment uses double-loop system. Because the borehole is more than 1000 m deep and will cross thick soil layers, the hydraulic fracturing equipment will suffer removal difficulty due to stuck on the borehole wall during transferring to the lower or upper positions.
Improvement of the hydraulic fracturing technique for using at great depth. To increase pressure-enduring ability, removal flexibility and measuring accuracy of the traditional hydraulic fracturing technique with equipment, following 5 techniques were developed by the author and used in Wanfu coal mine which is located in the alluvium of Yellow River with 700 meters thick overburden soil layer within Shandong Province of China [4].
(1) A new type of sealing packer with special structure was developed, whose pressure-enduring capacity is 70 MPa which is enough for hydraulic fracturing test at 2000 m depth.
(2) A single-loop hydraulic fracturing system was developed, in which a push-pull switch with high strength is used for transforming the pressurizing lines to the sealing packers or to the sealed section of the borehole.
(3) An automatic valve for low-pressure relief of the sealing packer was developed, which automatically makes pressure relief of the sealing packers after completion of the fracturing test.
(4) Two sets of pressure monitors are used to detect water pressure in the pipeline, which ensure the measuring accuracy of water pressure supplied to the sealed section of the borehole.
(5) The pressure-loading and unloading process is automatically program-controlled, which eliminates the influence of manual control in the traditional hydraulic fracturing system on the measuring results.
With the improved equipment and techniques, in-situ stress measurement at depth of more than 1000 m has been successful completed in Wanfu coal mine. The maximum depth of the measuring point was 1105 m, which created a new record of measuring depth for in-situ stress measurement using standard hydraulic fracturing technique in China in 2004.
Further improved techniques and spot test results in Xinjiang oil field. After the measurement in Wanfu coal mine, the hydraulic fracturing equipment and technique were further improved [3]. A series of newly invented or developed equipment were used. The diameter of the
sealing packers was increased from (J>130 to cf>230 and made of specially developed high strength
material. Several sealing packers can be serially connected to increase their sealing pressure values. An ultrasonic scanning probe is used to detect fractures in the borehole wall which can determine less than 0.1 mm wide fractures with their direction. The high-pressurizing system with a specially developed pump, the automatic controlling with data collection system and the sealing packer are integrally connected, which is sent to the measuring point in the borehole with a armoured cable, as shown in Fig. 4. The cable is also used as remote controlling signal line to the automatic controlling with data collection system in the measuring point.
Fig. 4. New type of hydraulic fracturing stress measurement system
With the further improved hydraulic fracturing equipment and technique, on-the-spot experiment was successfully completed in an exploration borehole in Xinjiang oil field in 2010. The maximum measuring depth has reached 2800 m, which changed the record of measuring depth with hydraulic fracturing technique again in China.
Conclusions and suggestions
(1) The knowledge of in-situ stress state is indispensable for design and construction of all engineering projects which involve excavations in rock, such as mining engineering, geotechnical engineering, hydro-electrical engineering, railway and road engineering and petroleum engineering, etc. It should be emphasized that for reliable design and construction of rock engineering, information on in situ stress state must be provided before design and construction of the engineering, which means that it is necessary to carry out primary in situ stress measurement in the early exploration stage of the engineering. Progress and development of in-situ stress measurement since the end of 1950s' in the world have made the scientific level of rock engineering design and construction much improved and enhanced.
(2) Stress relief by overcoring technique and hydraulic fracturing technique are two mainly used techniques for in-situ stress measurement in various rock engineering fields in the world. Stress relief by overcoring technique has been used for in-situ stress measurement for a long time and relatively matured with little bit higher measuring accuracy than other techniques. It is especially suitable for detailed in-situ stress measurement in mining and other similar engineering at operation stage in which there are many entrances to the position where the stress measurement is conducted. Hydraulic fracturing technique is an efficient technique for in situ stress measurement at deep position. It is suitable for stress measurement at the early exploration stage of underground mining and other similar engineering as well as surface excavation like open-pit mining engineering where is no any entrance to underground positions for stress measurement with overcoring and other techniques. For important engineering projects, both overcoring and hydraulic fracturing techniques are used to ensure the reliability of the measuring results by mutual comparison and checking.
(3) During the last 20 years, new ideas and new techniques of in-situ stress measurement were developed based on the demand of various engineering projects which were towards to larger scale, deeper position and more complicated geological and environmental conditions. Especially, a series improving techniques and equipment have been developed for the two main stress measurement techniques. Full temperature compensation technique, techniques for modifying the effect of nonlinearity, anisotropy and discontinuity of the rock on the measuring results, wireless automatic data transforming and collection techniques, and so on have been successfully used for increasing the measuring reliability, accuracy and practical usability of the overcoring technique. Application of new type of sealing packers with high pressurizing strength, automatic valve for low-pressure relief of the sealing packers, ultrasonic scanning probe for precise detecting of fractures on the borehole wall, high-pressurizing system with a specially developed pump and automatic controlling with data collection system has significantly increased the pressure-enduring ability, removal flexibility, measuring accuracy, automatic and remote control level of the measurement for the traditional hydraulic fracturing technique.
(4) Current commonly used techniques for in-situ stress measurement, including overcoring and hydraulic fracturing techniques are all "point" measuring techniques. Based on the measured results, using fitting and back analysis methods to build an in-situ stress model for an engineering area is useful. In recent years, nonlinear plural regression, neural network, numerical modeling, fuzzy comprehensive appraisal, genetic algorithm and particle swarm optimization methods were widely used for the analysis.
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