Pressuremeter test in permafrost on the Qinghai-Tibet plateau

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1 Pressuremeter test in permafrost on the Qinghai-Tibet plateau Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN W.B. Yu, Y.L. Zhu, Y.M. Lai, J.M. Zhang, X.F. Zhang, H.P. Li & S.J. Zhang State Key Laboratory of Frozen Soil Engineering, CAREERI, CAS, Lanzhou, China ABSTRACT: In-situ preboring pressuremeter tests were conducted on the Qinghai-Tibet Plateau to investigate the mechanical behavior of permafrost along the Qinghai-Tibet Highway. The in-situ pressuremeter test data are analyzed in this paper. Three mechanical characteristic parameters of permafrost are obtained in the test (the initial horizontal pressure P 0, the critical plastic load P e and the limit pressure P f ), and then the pressuremeter modulus E and the shear modulus G are calculated. The relationships between the pressuremeter test parameters and their influencing factors (soil temperature and water content) are discussed. Results show that the short-term strength parameters of the frozen soils vary linearly with water content and soil temperature. 1 INTRODUCTION The design and performance of foundations requires knowledge of the in-situ behavior of soils. However, such in-situ tests have hardly ever been performed in China in permafrost regions. It is well known that China is the country with the third biggest area of permafrost, which occupies about 21.5% of its territory. A lot of engineering construction is carried out on permafrost or seasonally frozen ground. Especially with the West Development Plan of China being performed, many key projects, such as the Qinghai-Tibet Railway, Natural Gas Transportation from the West to the East, Water Transportation from the South to the North etc., are either being constructed or are going to be carried out. Therefore, the study about the in-situ borehole pressuremeter test in permafrost regions seems to be very relevant. Wu et al. (1983) adopted the embedded method to investigate the in-situ bearing capacity of permafrost and developed a formula expressing the relationship between the long-term strength of frozen soil and the soil temperature. Zhu et al. (1983) used the same method to research the creep behavior of ground ice on the Qinghai-Tibet Plateau and quoted the creep equation for ground ice. Zhang & Zhu (2000) carried out in-situ sonic tests to investigate the sonic behavior of seasonally frozen ground in the same regions. Ladanyi (1972) and Ladanyi & Melouki (1993) had done some field research on the creep behavior of frozen soils and ice by means of a borehole dilatometer and pressuremeter. Yu et al. (2000) carried out tests in permafrost regions using the pressuremeter and obtained qualitative relationships between the mechanical parameters and the soil temperature. The pressuremeter is a convenient tool for investigating the properties of soils in the field. The test results deduced from the pressuremeter curves can be used to determine the allowable bearing capacity of a foundation, and to predict settlements. Therefore, it was widely used in the engineering investigation. In this research project, the authors adopted the TEXAM monocell pressuremeter, which was imported from Canada, to test the pressure and displacement relationship of permafrost on the Qinghai- Tibet Plateau. The procedure for analyzing the test results and the formula showing the relationships between the pressuremeter mechanical parameters, such as pressuremeter modulus E, shear modulus G, critical plastic load P e, and water content and soil temperature are presented in this paper. 2 TEST CONDITIONS AT THE TEST SITES In this research, a series of tests were conducted at five sites along the Qinghai-Tibet Highway. They are Tuotuo River, Qingshui River, Fenghuo Shan Site, Kunlun Pass and Hoho Xili. The type, structure and physical characteristics of the frozen soils tested at these sites are different. The types of soil tested include sandy gravel, fine sand, coarse sand and clay. The ground temperature at the testing sites varied from about 0.15 C to about 3.5 C. The permafrost tables at these sites varied from about 1.5 m to 2.0 m, and water content varied from about 17.8% to about 65.2%. 3 TEST EQUIPMENT AND TEST METHOD 3.1 Test equipment The test instrument used in this research is the TEXAM pressuremeter system. It utilizes a monocell hydraulically inflated probe. A mechanical actuator is used to displace a piston with a cylinder filled with 1277

2 Figure 1. Schematic drawing of the test equipment. of the drilling tool is D 1, the diameter of the deflated probe is D 2 and the initial diameter of the borehole is D 3. Generally, the tolerances on the diameter are: D 2 D D 2 and 1.03 D 2 D D 2. The test procedure is as follows: Step 1: Prepare the testing hole carefully according to the boring standard. Step 2: Put the probe into the borehole and adjusting it to the proper situation. Step 3: Record the initial volume and the pressure, then increase the volume by 40ml each step. After the volume is increased, it is necessary to wait for 30 s and 60 s, and to write down the volume and the corresponding pressure, respectively. Step 3 was repeated until the pressure descended quickly when the volume increment was finished. the inflation fluid (a kind of antifreeze solution of 50% water and 50% ethylene glycol by volume). The maximum volume of the cylinder is 1732ml and the pressure gauges have two measurement ranges (2.5MPa, 10MPa). The temperature of fluid and probe is accordant to the environment air temperature and it is about 5 C before the start of the test in this research. A schematic drawing of the test equipment is shown in Figure Saturating the system After the system has been connected correctly, the antifreeze solution was injected into the system, and the saturating operation was carried out to exclude the air out of the system and to improve the test precision. In this research, a solution of 50% water and 50% ethylene glycol by volume was used, it allows the instrument to work under low temperature. 3.3 Calibrating the system Because of the expansion of the system itself, calibration of the system is necessary, otherwise the system error will be very big. This operation includes volume and pressure calibration, from which the calibrated equations of volume and pressure can be obtained. 3.4 Preparing the borehole and performing the test The preparation of a quality borehole is the single most important step in obtaining a satisfactory pressuremeter test. Two conditions are required to achieve a quality borehole, so that the diameter of the borehole must be within a certain tolerance. The diameter 4 DETERMINING THE MECHANICAL PARAMETERS 4.1 Correcting the readings of pressure and volume Because of the membrane restraint of the probe and the expansion of the equipment itself, the volume and pressure readings on the gauges do not represent the real values from the soil mass surrounding the borehole. Therefore, the readings of pressure and volume must be corrected to obtain a better estimate of the real values acting on the borehole wall. The corrected pressure is calculated by: P P r P l P i (1) where P denotes the corrected pressure, P r, P l and P i are the reading values of pressure and the static liquid pressure and the membrane restraint pressure, respectively. Furthermore, the static liquid pressure P l is calculated from the following formula: P l 10(h Z)r i (2) where h is the height of pressuremeter, Z is the distance from the middle point of probe to the ground, and r l denotes the liquid density, where the liquid is mixed by volume as 50% water and 50% ethylene glycol. P i is determined from the results of calibrating the probe for pressure. The corrected volume is calculated by: V V r V i (3) where V r and V i denote the actual reading and the volume loss, respectively. V i is determined from the results of calibrating the probe for volume. The corrected values of P and V were used to draw the P V curve (Fig. 2). Three mechanical characteristic 1278

3 values of frozen soils (P 0, P e and P f ) can be deduced from the curve. P 0 is the initial horizontal pressure of the frozen soil, at nominally zero radial deformation, P e stands for the critical plastic pressure at which soil begins to yield and experience the plastic phase and P f is the (ultimate) limit pressure. 4.2 Determining the characteristic values P (MPa) frozen sandy gravel frozen clay V (ml) The typical testing curve is similar to the static load test curve. It has three parts: the initial pressurizing phase, where the slope is gradual, then the elastic phase which is also called the straight line phase, and has a much steeper slope. The last one is the plastic phase where the soil begins to yield and fail gradually. The following is the procedure for obtaining the three characteristic parameters. The straight-line section of the P V curve should be extended to intersect the horizontal co-ordinate, denoted by V 0. A line should then be drawn parallel to the vertical co-ordinate through V 0 to intersect the curve and the pressure corresponding to this intersect is P 0. The point where the curve deviates from a straight line is P e. Finally, the pressure corresponding to the asymptote of the plastic phase is P f. 4.3 Determining pressuremeter modulus and shear modulus According to the plane elasticity theory and the Lamé radial expansion equation of an infinite elastic medium, the valid equations are: G V 1 [ P/ V] (4) E 2G(1 ) (5) V 1 V p V m (6) from Equations 1 3, we can obtain: E 2(1 )(V p V m ) P/ V (7) where P and V are the change in pressure and volume, respectively, in the straight line section of the curve, is the Poisson s ratio (a value of 0.3 is used in this research), V p is the initial volume of probe, with a value 870ml, and V m is the volume denoted by the middle point of the straight line phase. The pressuremeter modulus and the shear modulus are calculated from Equations 5 and 7, respectively. 5 TEST RESULTS Typical P V curves for different types of frozen soil are shown in Figure 2. Figure 2. Pressuremeter pressure-volume (P V) curves. Table 1. Measured and calculated values of the five mechanical parameters for frozen soil at different temperatures. T P 0 P e P f E G Soil type C MPa MPa MPa MPa MPa clay sandy gravel * T is temperature, and the water content of the clay is 42.6%, and of the sandy gravel is 25.7%. Table 2. Measured and calculated values of the five mechanical parameters for frozen soil at different water contents. W P 0 P e P f E G Soil type % MPa MPa MPa MPa MPa clay sandy gravel * W is the water content of frozen soil, and the temperature of the clay is 1.0 C and of the sandy gravel is 2.5 C. Pressuremeter curves for the two types of frozen soil are shown in Figure 2, and indicate that the shortterm pressuremeter test is successful. From these curves, it is easy to determine the initial horizontal pressure P 0, the critical plastic load P e and the ultimate load P f, which are the three mechanical characteristic parameters of frozen soil. 1279

4 The shear modulus G of the frozen soils tested can be obtained from the P V curves, and then the elastic modulus E can be calculated in terms of the assumed values of the Poisson s ratio. In order to analyze the relationships between the mechanical parameters and soil temperature and water content, respectively, the tested and calculated results were classified according to similar test conditions, as shown in Tables DISCUSSION Table 1 contains the test results for soils at similar water contents and dry densities, but the temperature of each test is different. From this table, it is possible to determine the relationships between the mechanical parameters of each soil and the soil temperature. The value of each mechanical parameter increases with decreasing soil temperature. In Table 2, mechanical parameters are obtained from soils at similar soil temperatures, but with varying water content and dry density. Because all the test soils are nearly saturated or saturated, the dry density changes with the change of water content, and the relationship between the dry density and the pressuremeter test mechanical parameters is not analyzed here. It is shown that the calculated mechanical characteristic parameters increase with increasing water content. The reason is that as the water content increases, the cementation between the solid mineral grains and the ice enhances the stiffness, and the strength of the frozen soil mass increases correspondingly. Tables 1, 2 show that: G values of all short-term tests vary between about 35MPa and 222MPa, with an average value of 109MPa, P 0 values of all tests vary between about 0.2MPa and 0.88MPa, with an average value of 0.52MPa, P e values change from 0.5MPa to 1.6MPa, with an average value of 1.02MPa, P f values of these tests change from 0.85MPa to 3.3MPa, and their mean value is 1.78MPa. Pressuremeter deformation moduli of all these short-term tests, as expressed by E, are seen to be much higher, going up from 90MPa to 578MPa, with an average value of 284MPa. The average value of each mechanical parameter is shown in Tables 1, 2 to be different for each type of frozen soil. In detail, the average value of each mechanical characteristic parameter for clay is lower than for the sandy gravel. The P e values for the clay vary from 0.5 MPa to 1.39MPa, with an average value of 0.93MPa. The P e values for the sandy gravel vary between about 0.85MPa and 1.6MPa, and the average value for them is 1.24MPa. The data were analyzed by two indeterminate regression methods. Just two of the mechanical parameters P e and G could be selected for analysis, and linear relationships are given. For frozen clay, they are: P e W 0.38T (8) G W 38.47T (9) for frozen sandy gravel soil, they are: P e W 0.10T (10) G W 25.51T (11) where T is the soil temperature in C, W is the water content as a percentage and the unit of P e and G is MPa. In these formulae (Equations 8 11), the range of soil temperatures is from 0.15 C to 3.5 C and the range of water content values is from 17.8% to 65.2%. As with the P e and G values, the other parameters also show an approximately linear relationship to the soil temperature and water content. The pressuremeter is a convenient tool for performing in-situ tests. It has many advantages but also some shortcomings. According to the testing theory, which is based on an assumption that the expansion undergoes cylindrical plane strain, the pressure acts radially on the soil, which is also in the horizontal plane, so the results mainly denote the radial and tangential properties of frozen soil. So the anisotropy of frozen soil should be considered when analyzing the test results. The quality of the boreholes also influences the quality of the test results, but it could be minimized if the recommended way (Research Department and Technology Turner-Fairbank Highway Research Center 1989) of preparing boreholes is strictly followed and if proper training of the drilling crew takes place. From the testing point of view, one of the primary advantages of the pressuremeter is that the test can be performed in many kinds of soils. This ensures that the site investigation will lead to useful results. The second advantage of the pressuremeter test is that it represents an in-situ load test. A number of loading sequences can be duplicated within this in-situ load test: long-pressure steps for long-term loading, rapid inflation for impact loading, reloading and unloading cycles. The third advantage is that, to a certain extent, the quality of the test can be judged from the shape of the test curve and several important mechanical parameters can be deduced from it. It is worth noting that all the mechanical parameters obtained from these tests are short-term mechanical 1280

5 properties. Because the long-term strength properties are obviously different from the short-term properties, so long-term tests should be performed next time. Furthermore, comparisons to other in situ and laboratory tests should be made. 7 CONCLUSIONS 1. The test proves that the pressuremeter can be used to investigate the mechanical properties of soils in permafrost regions. 2. The in-situ mechanical characteristic parameters of frozen soil, obtained from these short-term pressuremeter tests, are approximately linear to the main influencing factors (soil temperature, water content), the values of the parameters obtained increase when soil temperature decreases and the water content increases. 3. In order to further the use of the pressuremeter in investigating the properties of frozen soil in cold regions engineering, comparisons including both field and laboratory tests should be made in future research. ACKNOWLEDGEMENTS This study was supported in part by the National Natural Science Foundation of China (Grant No and ) and by the Foundation of Hundred People Plan of Chinese Academy of Sciences (to Dr. Y.M. Lai) and by the Knowledge Creative Engineering of CAREERI CAS (Grant No. CACX ). REFERENCES Ladanyi, B In situ determination of undrained stress strain behavior of sensitive clays with the pressuremeter. Canadian Geotechnical Journal 9: Ladanyi, B. & Melouki, M Determination of creep properties of frozen soils by means of the borehole stress relation test. Canadian Geotechnical Journal 30: Research Department and Technology Turner-Fairbank Highway Research Center The pressuremeter test for highway application. FHWA-IP U.S. Department of Transportation Federal Highway Administration: Virginia. Wu, Z.W., Liu, Y.Z. & Xie, X.D Field experiments of bearing capacity of frozen soils. In G.P. Institute (ed.) Professional papers on permafrost studies of Qinghai- Xizang Plateau: Beijing: Science Press. Yu, W.B., Zhu, Y.L., Zhang, J.M. & He, P Studies on using a pressuremeter test to determine the mechanical properties of frozen soils. Journal of Glaciology and Geocryology 22(4): Zhang, J.M. & Zhu, Y.L In-situ sonic test of seasonally frozen ground in Lanzhou, China. In Jean- Francois Thius (ed.) Ground freezing 2000: Rotterdam: Balkema. Zhu, Y.L., Liu, Y.Z. & Xie, X.D Field creep test research on ground ice in Qinghai-Tibet Plateau. In G.P. Institute (ed.) Professional papers on permafrost studies of Qinghai-Xizang Plateau : Beijing: Science Press. 1281

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