Applicability of Needle Penetration Test on Soft Rocks

Transcription

1 Applicability of Needle Penetration Test on Soft Rocks Zhigang Li PhD student, Faculty of Engineering, China University of Geosciences(Wuhan) Guangli Xu * Professor, Faculty of Engineering, China University of Geosciences(Wuhan) *Corresponding author, xu1963@cug.edu.com Xin Zhao Senior Engineer, Geological Survey Center in Wuhan, China Geological Survey Yongpeng Fu Senior Engineer, Geological Survey Center in Wuhan, China Geological Survey Chang Su Senior Engineer, Hubei Institute of Hydrogeology and Engineering Geology ABSTRACT In order to verify the applicability of the needle penetration test (NPT), which was developed in Japan to indirectly calculate the strength of soft rocks, three representative soft rocks, including chlorite schist, mudstone and sandstone, were selected to carry on the needle penetration test (NPT). And the same rocks were also tested by the method of uniaxial compressive test (UCT). The comparative results of these two methods show that the reliability of the NPT is closely related to the styles and arrangement assemblage feature of the rock-forming minerals, and the distribution of the structural plane. The method of NPT is the most applicable to the strength test of soft rocks with fine and uniform arrangement particles compared with rocks with large and uneven arrangement particles. But it s really inapplicable to the soft rocks which are fresh, low weathering and low strength caused by joint fissures. Therefore, NPT is reasonable and feasible in strength test of some certain types of soft rocks and it should be popularized and applied in the soft rock engineering. KEYWORDS: soft rocks; strength test; needle penetration test; needle penetration index; uniaxial compressive strength

2 Vol. 21 [2016], Bund INTRODUCTION Uniaxial compressive strength is a critical parameter for the rock mass characteristics description and classification. It s widely used in the geotechnical engineering design and calculation. According to the recommended method in the international society of rock mechanics (ISRM), high quality standard sample specimens are needed in the uniaxial compressive strength test. It takes a long time in the preparation of standard sample specimen and the heavy test equipment is also needed in this process. So it s really expensive and difficult to perform the uniaxial compression test. What s more, the fixed size of the standard specimen based on the specification and the recommended methods is often unavailable. Especially for soft rocks and those rocks have been weathered seriously, whose diagenesis are not completely finished, it seems impossible to get the strength index by using uniaxial compression test [1]. The point load strength test is one of the widely used methods in testing the strength of rock. Both samples with regular and irregular shapes could be tested by this method. Finally, the uniaxial compressive strength and tensile strength could be calculated using point load strength index (IS (50) ) multiplied by the related factor k [2-5]. However, is difficult to obtain the accurate value of the factor k generally as it is always related to the type and the weathering degree of rock [6]. The Schmidt hammer is another portable instrument used to speculate the uniaxial compressive strength of rocks indirectly. Because it needs to exert high impact energy on the rock, it is not suitable to perform the strength test on soft rock [7]. However, it is widely used to test the strength of nondestructive concrete in China. Portable hardness tester is a kind of instrument often used to calculate the uniaxial compressive strength of metal in metallurgical engineering. There are many scholars abroad have tried to apply it to the rock material [8], but in recent years, there is little report on the application of this method. There is no relevant standards and recommended method by using portable hardness tester to calculate the uniaxial compressive strength of rocks now, either. These simple and economic testing methods mentioned above are mainly used to calculate the uniaxial compressive strength of rocks. For loose, broken and soft rock, however, it is still very difficult in processing the suitable samples for the test methods above. In addition, when we encounter to some reinforcement engineering or geological engineering, such as natural or man-made rock structures, sites or buildings, due to the protection and other reasons, it is not allowed to take samples from these ancient site to explore the strength properties [9-10]. In order to solve these problems above, the needle penetration test, a kind of simple, convenient and quick operation method used in nondestructive soft rock strength test, the needle penetration test, is invented by Japanese [11-12]. The needle injection strength index of soft rock can be measured through the needle penetration test, and then the uniaxial compressive strength could be calculated. However, the needle penetration test could not be applied to all types of soft rock due to the numerous causes and species of soft rock. Therefore, the applicability and reliability of the needle penetration test on different types of soft rock should be further researched. Both needle penetration test and uniaxial compression experiment was carried out on three different types of soft rock in this article. The applicability and reliability of

3 Vol. 21 [2016], Bund needle penetration test on different types of soft rock are explored through the comparative analysis of these two methods. INTRODUCTION OF THE TEST Testing Device The needle penetration test uses a light portable device, called needle penetrometer that make a needle penetrate into a rock surface (Fig. 1). The device mainly consists of chuck (fix the penetration needle), presser (measure load force and protect the penetration needle), penetration scale (0 10 mm, graduation is 1 mm), load scale (0 100 N, graduation is 10 N), load indicator ring (indicate the load force), NPR UCS correlation chart, removable cap (the grip contains spare penetration needles), penetration needle as shown in Fig. 1 [13]. Operation Method Figure 1: Needle Penetrometer and its parts 1chuck, 2presser, 3penetration scale, 4load scale, 5load indicating ring, 6NPR UCS correlation chart, 7removable cap, and 8penetration needle The needle penetrometer is simple to operate and easy to use. It can be used both on exposures rock in the field and on specimens prepared from blocks or cores. It doesn t require any special preparation of the specimens [14-15]. The operation methods of needle penetrometer as follows: First of all, the penetration needle should be inserted. So the presser is removed from the penetration scale, the chuck is turned counterclockwise, the penetration needle is inserted and the chuck is turned clockwise for fastening and fixing the needle. Then the presser and the load indicator ring are manually adjusted to zero. After above steps, the test could be made. The test is performed by holding tightly the device s main body with one hand and presser with the other hand, and slowly pushing the penetrometer needle into the specimen surface. The needle is pushed into the rock until the maximum load force (100 N) or the maximum penetration depth (10 mm) is attained, and the load force and penetration depth are read from the load scale and penetration scale. Then the test stop and the needle is slowly pulled out [16].

4 Vol. 21 [2016], Bund The tests on the same surface of the soft rock need to be repeated 3 to 5 times. If the test results are too scattered, the number of tests should be appropriately increased. In order to avoid the interaction of different penetration points, the distance between any two penetration points should be more than 10 mm. In addition, some cracks may appear in the process of penetration and may form a radially fractures zone. It should be noted that when did the radially fractures zone form. If it forms in the process of penetration, the test results should be discarded. If it forms during the needle is pulled out, the results is effective. What s more, if the sample damages along the weak structural plane in the process of penetration, such as along the bedding or schistosity plane, the test results should also be discarded [16]. Test Data Processing The needle penetration test is intended for the determination of the needle penetration index (NPI) and inferring the uniaxial compress strength of rocks. The needle penetration index (NPI) can be calculated from the Eq. (1) and the uniaxial compress strength (UCS) can be estimated by the Eq. (2) [17-18]. NPI=F/D (1) where F is the load (N), D is the depth of penetration (mm) and the unit of NPI is N/mm. where δ is uniaxial compress strength (UCS), MPa. δ= npi^0.978 (2) ROCK SAMPLES AND TEST PROGRAM Rock Samples For the study of the applicability of the needle penetration test in testing the strength of the soft rock, three types of soft rocks, including the chlorite schist, strong weathered mudstone and strong weathered sandstone are selected to carry out the needle penetration test and the uniaxial compression experiment. The types and location of rock samples Chlorite schist was taken from the collapse of Zhushan water plant in Shiyan City, Hubei province. This type of rock belongs to intermediary weathered rock. The mudstone and sandstone were token from two deep foundation pits in construction in Wuhan City, Hubei province. These two types of rocks belong to severely weathered rock. These three kinds of samples were tested by X-ray powder diffraction experiments. Results show that chlorite schist is mainly composed of chlorite, and small amounts of illite, quartz, feldspar and calcite. Mudstone is mainly composed of clay minerals, it means montmorillonite and illite, and a small amount of chlorite, quartz and feldspar. Sandstone is

5 Vol. 21 [2016], Bund mainly composed of quartz firstly, feldspar secondly, and a small amount of clay mineral and calcite. As the standard specimen are needed in uniaxial compression test, an intact sample with relatively high strength are taken from the site so that to guarantee the standard size in processing specimen. Preparation of Specimens Specimen was prepared in accordance with the uniaxial compression test requirements in specification [19]. Chlorite schist schistosity extremely development, showing obvious anisotropy [20]. Each set of specimens were processed in two directions, the normal direction of the schistosity plane and the tangent direction of the schistosity plane. Both mudstone and sandstone are isotropic bodies, each processing 1 set of specimens, including four specimens for each set. Because of the cracks in mudstone and its low intensity of mudstone, it s impossible to process the cylindrical sample in standard. So square column samples were processed, whose size was mm. It s really difficult in processing samples of these three kinds of rocks. The success rate in processing specimens was lower than 50%. The standard specimens are shown in Fig. 2. Figure 2: Rock Samples 1chlorite schist(horizontal foliation)2chlorite schist(vertically foliation)3mudstone 4sandstone Test Program Chlorite schist shows obvious anisotropy, strength tests were performed in two directions, the normal direction and tangent direction of the schistosity plane. Two sets of samples had been prepared in two directions. Mudstone and sandstone can be regarded as isotropic body, so just one direction strength test need to process for each type of rock. In order to increase the reliability of the contrast test, needle penetration test was processed in each specimen firstly. And then carried out the uniaxial compression test on them. This could ensure the same lithology of rock specimens in comparing uniaxial compression test and needle penetration test, avoiding experiment error caused by the differences of specimen lithologic. It's necessary to note that needle penetration test is a nondestructive testing because little rock damage was induced by the fine injection needle. So needle penetration test made negligible effect on rock's integrity and

6 Vol. 21 [2016], Bund strength. Needle penetration test was performed on the upper surface or lower surface of the specimen. The injection direction is vertical with the surface of specimen. Each sample was tested three times, if the test result had a large discreteness, increase the testing times. In order to prevent the boundary effect and the interaction of the injection holes, the distance from boundary to needle penetration point was greater than 1 cm, and the distance between the two injection points must also be greater than 1 cm. If the needle penetration test make great cracking destruction on sample produce, then the sample had to be abandoned instead of doing the contrast test. The sketch of needle penetration test point is shown in Fig. 3, and the picture of needle penetration test is shown in figure 4. Figure 3: Schematic illustration of PLT position Figure 4: NPT on a chlorite schist sample in the laboratory The strength of the intact rock is usually obtained by the method of needle penetration test, as shown in Fig. 5. In general, the position where the needle injected locates in a small piece of intact rock mass among broken rock, so the strength measured by the needle penetration test is representatively the strength of intact rock. In order to ensure the accuration of the comparison results, the samples used for uniaxial compression test must be intact. After the uniaxial compression test, if the sample fracture surface was all along or partly along the fissures, the contrast test was considered failed, another sample need to be supplied and tested once more.

7 Vol. 21 [2016], Bund Figure 5: Schematic illustration of NPT on the intact rock RESULTS AND ANALYSIS Chlorite Schist Chlorite schist is fresh and less weathered. Both the needle penetration tests in the normal and tangent direction of schistosity plane did not appear to be invalid. The cracking surface in uniaxial compression test was fresh and was not in the schistosity of the sample. It turns out that fracture surface did not affect the uniaxial compressive strength test. The results of these two test groups in two directions are shown in Table 1 and Table 2. Specimen number Table 1: Results of NPT and UCT on chlorite schist sample Needle penetration test load (N) Penetration depth (mm) NPI (N/mm) Inferred UCS (MPa) Mean of Inferred UCS (MPa) Measurement UCS (MPa) P P P P Mean Standard deviation

8 Vol. 21 [2016], Bund Table 2: Results of NPT and UCT on chlorite schist sample ( 平行片理方向 ) Specimen Needle penetration test Measurement number Penetration NPI Inferred UCS Mean of Inferred UCS (MPa) load (N) depth (mm) (N/mm) (MPa) UCS (MPa) P P P P Mean Standard deviation According to the uniaxial compression test, the strengths in the normal direction and tangent direction of the schistosity plane of the chlorite schist are MPa and MPa, respectively. According to the classification standard of hardness degree of engineering rock in "GB " [21], as well as the discreteness of test data, Chlorite schist belongs to soft rock. Its strength is in measurement range of the needle penetration test. For the strength in the normal direction of schistosity plane, the results from these two kinds of test methods appeared to be much different. On the contrary, the results of the strength in the tangent direction of schistosity plane appeared to be little different. The reliable of needle penetration test is researched through the following analysis. The strength of each sample in the normal direction resulted from needle penetration test is far more than the corresponding value resulted from uniaxial compression test. And most of the penetrations of the injected needle were not more than 1 mm. These results have broken through the measurement limit of the needle penetration instrument. Certainly these tests exceeded the optimum scope of the instrument. The injected needle was broken frequently in this process. Therefore, this group of needle penetration test data in the normal direction is not reliable. The strength of each sample in tangent direction resulted from needle penetration test is almost the same with the corresponding value resulted from uniaxial compression test. And most of the penetrations of the injected needle ranged from 1 to 1.6 mm. Though these values had not broken through the limit of the needle penetration instrument, they were indeed exceeded the optimum scope. When the needle penetrated in the tangent direction of fault, the results were affected by the faults [12]. Therefore, this group of needle penetration test data in the tangent direction is still not reliable.

9 Vol. 21 [2016], Bund Based on the analysis above, the needle penetration test is not applicable to the little weathered but fault developed chlorite schist. Mudstone Faults existed in most of the mudstone samples. Due to the good cementation, rock samples were not cracked in the preparation processing. The needle penetration test is successful on these samples. In the uniaxial compression test, most samples with fault were destroyed along the fault surface, causing extremely low measured strength. So many samples were added to take the uniaxial compression test. Eventually results of four specimens without fault are chosen. The contrast results of these two methods are shown in Table 3. Table 3: Results of NPT and UCT on mudstone sample Needle penetration test Specimen Mean of Measurement number Penetration NPI Inferred UCS load (N) Inferred UCS UCS (MPa) depth (mm) (N/mm) (MPa) (MPa) N N N N Mean Standard According to the uniaxial compression test, the strength of the mudstone is 4.32 MPa. It belongs to extremely soft rock. According to the needle penetration test, the strength of the mudstone is 5.00 MPa, It belongs to soft rock. The results due to these two methods are basically the same. Because mudstone is mainly composed of clay mineral with very fine particles, and mineral is arranged relatively uniform. As the mudstone is strong weathered, the injection needle penetrates easily. These measured values are all distributes in the optimal scope of the needle penetration instrument. The surface of the standard specimen is smooth. The property of mudstone is uniform. The data obtained have little discreteness and high precision. What s more, the selected sample is an intact block, so the values from these two kinds of test are almost the same. As a result, the applicability of the needle penetration test in mudstone intensity is really good.

10 Vol. 21 [2016], Bund Sandstone Sandstone is deeply weathered with really low intensity. The needle penetration test went well and no sample damaged. The failure surface of samples was fresh in uniaxial compression test. There was no fault affecting the uniaxial compressive strength. The contrast test is successful and the results are shown in Table 4. Table 4: Results of NPT and UCT on sandstone sample Needle penetration test Specimen Measurement load Penetration NPI Inferred UCS Mean of Inferred number UCS (MPa) (N) depth (mm) (N/mm) (MPa) UCS (MPa) S S S S Mean Standard According to the uniaxial compression test, the strength of the sandstone is 4.42 MPa. It belongs to extremely soft rock. According to the needle penetration test, the strength of the sandstone is 6.08 MPa, It belongs to soft rock. It didn't appear to be much different from these two methods. Sandstone is mainly composed of quartz, feldspar and clay mineral. The particles of quartz and feldspar are large, forming the rock skeleton. The skeleton gap is full of clay mineral. The mineral arranges relatively uniform overall. If the injected needle encounter the clay mineral in skeleton gap in the process of needle penetration, it penetrates easily. If the injected needle encounter big particle of quartz, the penetration becomes difficult. So the test result of needle penetration test on sandstone are of high discreteness. But it is nice to see that each measured values was in the optimal scope of the needle penetration instrument. To sum up, the applicability of the needle penetration test on sandstone is just the average. Comprehensive Analysis of the Contrast Test The strengths of four groups of rock specimens measured by needle penetration test and uniaxial compression test are listed from Table 1 to Table 4. The mean value and standard deviation of the results are also obtained. The reasons for the difference between these two methods are further

11 Vol. 21 [2016], Bund explored. And the applicability of the needle penetration test on different types of soft rock are discussed. Strength values from the needle penetration test are all higher than the corresponding values resulted from the uniaxial compression test in these contrast tests except the strength in the tangent direction of the chlorite schist schistosity plane. That s mainly because the strength of soft rock not only depends on its composition mineral, content and the permutation and combination of the mineral, but also greatly influenced by the fault in soft rock. The needle injection just affect a small zone of the sample surface in the needle penetration test. The testeresult is mainly determined by the mineral composition and distribution arrangement. The fault existing in the inner space of the rock has little effect on the needle penetration result. So the measured value from the needle penetration test represents the strength of intact rock without joints and fissures. However, the uniaxial compression test is performed on the complete rock sample, the existing joints and fissures in the rock will reduce the result more or less. And different rock specimen has different joints and fissures, resulting different influence on the test result. Therefore, the intensity value form uniaxial compression test is lower than the needle penetration test, the discreteness is also greater than the needle penetration test. Compare the standard deviation of different soft rocks resulted from the needle penetration test. The data discreteness of mudstone is smaller than the sandstone. This indicates that the data discreteness of the needle penetration test depends on the mineral particle size, the permutation and combination. The smaller mineral particle and more uniform arrangement, the smaller discreteness of the data and more reliable of the needle penetration test and vice versa. The standard deviation of mudstone and sandstone is far smaller than the standard deviation of chlorite schist (in the tangent and in the normal direction of schistosity). It shows that when the tested result is beyond the measuring range of needle penetration instrument, the discreteness of data increases. So, the needle penetration test is suitable for the seriously weathered soft rock with loose mineral. Then the test result is in the optimal scope of the instrument, the smaller particle and more uniform arrangement, the smaller the discreteness and the better the applicability of the test, and vice versa. However, for the less weathered but with fault developed soft rock, the applicability of this method is really poor. CONCLUSION (1) Through the contrast test on the chlorite schist, mudstone and sandstone by the methods of needle penetration test and the uniaxial compression test, the applicability of needle penetration test is poor on the chlorite schist, good on the mudstone, and just the average on sandstone. (2) The results of the needle penetration test depends on the mineral composition, the permutation and combination, and even the structure plane distribution of the soft rock. The applicability of this method is the most reliable when the mineral particles of the soft rock is small and uniform. When the mineral is large but arranged uniform, the applicability of this method is just in the average. But when the soft sock is fresh, less weathered and with low strength caused by developed fault, the reliability of this method is really poor.

12 Vol. 21 [2016], Bund (3) The needle penetration test is easy, economic and fast in measuring the strength index of soft rock, including the tensile strength of soft rock as well as other mechanical parameters, such as C φ values. This test method can be applied to soft rock survey engineering and properties testing of soft rock engineering. And a lot of test and theoretical calculation should be performed for more widely usage of this method. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support provided by the China Geological Survey Program (NO ). REFERENCES [1] Ngan-Tillard DJM, HK Engin, Verwaal W, et al. Evaluation of micro-structural damage caused by needle penetration testing Bull Eng Geol Environ, 2012, 71: [2] LI Xian-wei, FU Xue-min: A study on the point load test of irregular lump rocks Chinese Journal of Geotechnical Engineering, 1987, 9(1): (in Chinese) [3] Wang Xianghou, Li Mingyong, She Luofei, et al. Discussion and Research with Irregular Test Piece of the Point Load Measurement Rock Strength Chinese Journal of Rock and Soil Mechanics, 1995, 16(4): (in Chinese) [4] FU Zhi-liang, WANG Liang: Comparative experimental research on point load strength, uniaxial compressive strength and tensile strength for rocks in roof and floor of coal seam Chinese Journal Rock Mechanics and Engineering, 2013, 32(1): (in Chinese) [5] Guo Manli: Discussion on adaptability of point load test of rocks Rock and Soil Mechanics, 2003,24(3): , 494. (in Chinese) [6] Read JRL, Thornton PN, Regan WM: A rational approach to the point load test Proceedings of the Australian New Zealand Geomech Conference, 1980, vol. 1: [7] Aydin A: ISRM suggested method for determination of the Schmidt hammer rebound hardness: revised version International Journal of Rock Mechanics and Mining Science, 2009, 46: [8] Meulenkamp F, Alvarez GM: Application of neural networks for the prediction of the unconfined compressive strength (UCS) from Equotip hardness International Journal of Rock Mechanics and Mining Science, 1999, 36 (1): [9] Nian-chao Zhang, Nong Zhang, Hua-yang Wang, Bai-long Ma, Yuan-tian Sun, and Joan Esterle: Floor Failure Depth of the Roadway in Soft Rocks of Deep Coalmine: A Case Study The Electronic Journal of Geotechnical Engineering, 2016 (21.5) pp Available at ejge.com.

13 Vol. 21 [2016], Bund [10] Ulusay R, Erguler ZA: Needle penetration test: Evaluation of its performance and possible uses in predicting strength of weak and soft rocks Engineering Geology, 2012, : [11] JGS (Japanese Geotechnical Society): Japanese standards and explanations of geotechnical and geoenvironmental investigation methods. Tokyo: JGS Publication, 2012, pp (in Japanese) [12] JSCE (Japan Society of Civil Engineers): A suggested method for investigation and testing of soft rocks. Committee on Rock Mechanics of JSCE, (in Japanese) [13] Omer Aydan, Akira Sato, Masatoshi Yagi: The Inference of Geo-Mechanical Properties of Soft Rocks and their Degradation from Needle Penetration Tests Rock Mech Rock Eng, 2014, 47: [14] Aydan O: The inference of physico-mechanical properties of soft rocks and the evaluation of the effect of water content and weathering on their mechanical properties from needle penetration tests Symposium of ARMA, Chicago, 2012, vol. 1: [15] Aydan O, Seiki T, Ito T, et al. A comparative study on engineering properties of tuffs from Cappadocia of Turkey and Oya of Japan. In: Proceedings of the Symposium on Modern Applications of Engineering Geology, Turkish National Group of Engineering Geology, 2006, Denizli, pp [16] Resat Ulusay, Omer Aydan, Zeynal A. Erguler, et al. ISRM Suggested Method for the Needle Penetration Test Rock Mech Rock Eng, 2014, 47: [17] Maruto Corporation. Penetrometer for soft Rock: Model SH-70 Instruction Manual. Tokyo, Japan, (in Japanese) [18] Erguler ZA, Ulusay R: Estimation of uniaxial compressive strength of clay-bearing weak rocks using needle penetration response Rock Mechanics, Proceedings of 11th Congress on Int, Lisbon, 2007, vol 1, pp [19] China Electricity Council: GB/T Standard for testing method of engineering rock mass. Beijing: China Planning Press, (in Chinese) [20] LIU Sheng-li, CHEN Shan-xiong, YU Fei, et al. Anisotropic properties study of chlorite schist Rock and Soil Mechanics, 2012, 33(12): (in Chinese) [21] The Yangtze river academy of sciences: GB Standard for engineering classification of rock masses. Beijing: China Planning Press, (in Chinese) 2016 ejge

14 Vol. 21 [2016], Bund Editor s note. This paper may be referred to, in other articles, as: Zhigang Li, Guangli Xu, Xin Zhao, Yongpeng Fu, and Chang Su: Applicability of Needle Penetration Test on Soft Rocks Electronic Journal of Geotechnical Engineering, 2016 (21.23), pp Available at ejge.com.