Brittleness index of lightly cemented soil in ring shear tests

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The brittleness index exhibits the strain-softening behaviour of soil and is often used to access the shear mechanism in slope failure. It has been widely investigated for diffirent materials, especially for brittle materials.

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Nội dung Text: Brittleness index of lightly cemented soil in ring shear tests

Brittleness index of lightly cemented soil in ring shear tests > NGUYEN THANH DUONG1; NGUYEN VAN HAI2 1 Hanoi University of Mining and Geology Email: nguyenthanhduong@humg.edu.vn 2 Mien Trung University of Civil Engineering Email: nguyenvanhai@muce.edu.vn ABSTRACT: contractive behavior; IB = 0 indicates the peak strength, f = residual strength, r (no strength loss), there is no strain-softening The brittleness index exhibits the strain-softening behaviour of and the soil shows a highly dilative behavior. The rate of strength soil and is often used to access the shear mechanism in slope loss can be an indicator of progressive failure or flow slide. Bishop et al. (1971) showed that there were three factors afftecting the failure. It has been widely investigated for diffirent materials, brittleness of a soil, including: Dilatancy at failure, reorientation, especially for brittle materials. However, the brittleness index of and cementation bond. Taylor (1948), Skempton and Bishop (1950) lightly cemented soils has not been investigated yet. In this study, indicated that the difference between peak and residual strength of sand was directly related to the rate of dilatancy at failure. the brittleness index of lightly cemented clay simulated by adding Accordingly, the IB values increased as the rate of dilatancy a small content of cement to clay will be investigated in ring shear increased and decreased as the effective normal stress increased (Bishop et al., 1971). For remoulded samples, since the test. A series of ring shear tests were conducted at different cementation bond was destroyed and there was no dilatancy effective normal stress levels from 98 kPa to 392 kPa and shear behavior, the brittleness of remoulded clay was closely related to rates from 0.02 mm/min to 20 mm/min. The test results showed the reorientation of the platy clay particles. For undisturbed samples with natural cementation bond (weathered and that the brittleness index decreases with the increasing of unweathered blue London clay, brown London clay) , the IB is effective normal stress, cement content, and shear displacement rather high with the value of above 0.7 (Bishop et al., 1971). rates. However, the brittleness index was almost unchanged when the cement content was higher than 2%. Keywords: Brittleness index, cemented soil, cement content, ring shear test 1. INTRODUCTION The main factors affecting the potential damage caused by landslide are the volume of landslide mass, run-out, velocity and acceleration. The run-out, velocity and acceleration of landslide moving are primarily controlled by the loss of soil strength after peak (strain-softening behavior). The highest strength loss often Figure 1. Defition of f and r in ring shear tests observed in first time failure in materials showing a brittle behavior. Some typical materials exhibit this phenomenon are The brittleness index of soil proposed by Bishop et al. (1971) hard soil, soft rock, overconsolidated and cemented clay (Yerro et determined from ring shear tests was known as drained brittleness al., 2016). To evaluate the strain-softening behavior of soil, the index. Similarly to drained brittleness, the undrained brittleness brittleness index (IB) is often used and it is a primary factor of the index based on undrained trixial tests was aslo investigated. In slope response. The values of IB can be determined based on the triaxial test, the brittleness index can be estimated as IB= (qpeak- following equation (Bishop et al., 1971):  � qpost-peak)/qpeak (Highter and Tobin, 1980; Pineda et al., 2016). The I � �1 � (1) qpost-peak is defined as the deviatoric stress at shear strain higher  IB values range from 0 to 1, where IB = 1 indicates the residual than 15%. In their research, Highter and Tobin (1980) found that strength, r =0 (100% strength loss) and the soil shows a highly the undrained brittleness index of mine tailings depent on the ISSN 2734-9888 10.2021 279
PHÁT TRIỂN X ÂY DỰNG BỀN VỮNG TRONG ĐIỀU KIỆN BIẾN ĐỔI KHÍ HẬU KHU VỰC ĐỒNG BẰNG SÔNG CỬU LONG initial void ratio and consolidation pressure, especially the initial ring shear test with an outer diameter of 10 cm, inner diameter of 6 void ratio. Accordingly, the undrained brittleness index of mine cm, and a height of 2 cm. tailing materials decreased as the initial compaction ratio increased. It was also found that when the intial compaction ratio was lower than 70-75%, the undrained brittleness index was higher than 0.5. The index for all types of tailings decreased quickly to zero when the intial compaction ratio increased from about 75 to 80%. This showed that the possibility of a flow slide in mine tailings could be prevented if the initial compaction ratio was above 80%. Regarding consolidation pressure, the undrained brittleness index decreased with the increasing of consolidation pressure. With regard to the void ratio, the research of Sadrekarimi and Olson (2011) showed that the brittleness index of some types of sand increased as the void ratio increased. These authors also conducted ring shear tests on sands with a very large shear displacement of above 20m. At this large shear displacment, the sand particle in shear zone will be damaged, rearranged, and reoriented. They found that the brittleness index of original, undamaged sand was greater then that of damaged sand. This Figure 2. Large consolidation tank (Duong and Hao, 2020) indicated that the residual shear strength of sand will be decreased Bishop type ring shear apparatus was employed in this study at a very large shear displacement due to the damage of sand in (Figure 3). The details of this apparatus were reported in previous shear zone. Pineda et al. (2016) found that the undrained shear studies (Suzuki et al., 2017; Duong et al., 2018; Duong and Suzuki, strength of Ballina clay reduced about 50% after reaching the peak 2020; Thanh et al., 2020). In ring shear apparatus, the specimens which indicated the fragile post-peak behavior and could be were sheared at different normal stress levels from 98 kPa to 392 associated to progressive failure. In drained triaxial test, the kPa and different shear rates from 0.02 to 20 mm/min. research results of Fatahi et al. (2015) showed that the brittleness index of treated municipal solid wastes increased as the content of fly ash-quicklime increased up to 20% and it was almost constant when the content of fly ash-quicklime exceeded 20%. Additionally, the brittleness index of treated soil decreased with the increasing of effective confining pressure from 100 to 300 kPa. Based on the simulation of landslide travelling, Yerro et al. (2016) presented that the run-out and velocity of landslides in brittle materials increased as the brittleness index increased. Additionally, there was a strong correlation between run-out and brittleness index of soil. In general, since the brittleness index is a key factor for evaluation of slope failure mechanism, it has been widely investigated for diffirent materials, especially for brittle materials. In fact, many cases of landslide occurred in naturally cemented soils such as shale, sandstone, and mudstone which often show brittle behavior (Suzuki et al., 2017). However, the brittleness index of lightly cemented soils has not been investigated and clarified yet. In this study, the brittleness index of lightly cemented clay will be investigated in ring shear test. A small content of cement was Figure 3. Bishop-type ring shear apparatus mixed with kaolin clay to simulate the natural cementation in soil. 3. TEST RESULT AND DISCUSSION 2. SAMPLE PREPARATION AND TEST METHOD The test results of ring shear test conducted on cemented and A commercial kaolin clay type in Japan and Ordinary Portland non-cemented clay at different normal stress levels and shear rates Cement (OPC) was used in this study. Kaolin clay has the specific are listed in Table 1. The results showed in this Table have been density (s) of 2.618 g/cm3, liquid limit (WL) of 62%, plasticity index reported in Suzuki et al. (2017). Based on peak and residual (Ip) of 21.8, clay fraction (CF) of 35.3%. Kaolin clay in the form of dry strengths, the brittleness index was calculated following the powder was mixed with distilled water to about two time its liquid formula 1 and presented in Table 1. As shown in this table, the IB limit, then a small content of OPC was added and mixed again. In values of kaolin clay, 2% cemented clay, and 4% cemented clay this study, to simulate the natural cementation in some soft rock decease from 0.66 to 0.55, from 0.47 to 0.37 and from 0.47 to 0.36 such as claystone, mudstone, the OPC with 2% and 4% of the dry with the increasing normal stress from 98 kPa to 392 kPa, weight of the clay was added. The slurry samples were then respectively. The relationship between brittlenss index (IB) and poured and consolidated in a consolidation tank under a pressure effective normal stress levels is shown in Figure 2. In general, the of 98 kPa (Figure 2). The procedure of sample preparation has been dependence of IB on the effective normal stress observed for kaolin reported in previous study (Duong et al., 2018; Duong and Suzuki, clay and cemented clay in this study is similar to the results 2020; Suzuki et al., 2017; Thanh et al., 2020). The preconsolidated observed for different soils from previous studies (Bishop et al., samples were cut and trim to produce the annular specimen for 1971; Fatahi et al., 2015; Highter and Tobin, 1980). As shown in 280 10.2021 ISSN 2734-9888
Table 1. Test results of ring shear test Samples Normal stress (kPa) Shear rates (mm/min) Peak strength, p Residual strength, Brittleness index (kPa) r (kPa) (IB) 98 0.2 53.1 18.1 0.66 196 0.2 102.5 42.4 0.59 392 0.2 184.6 82.6 0.55 Kaolin (0% 98 0.02 98.5 31.8 0.68 cement) 98 0.1 97.8 33.1 0.66 98 1 99.0 44.1 0.55 98 10 80.4 51.8 0.36 98 0.2 79.6 42.0 0.47 196 0.2 123.9 63.7 0.48 392 0.2 215.2 135.1 0.37 2% cement 98 0.04 121.3 61.4 0.49 98 2 116.5 64.5 0.45 98 6 130.5 79.4 0.39 98 10 124.1 83.3 0.33 98 0.2 106.1 56.6 0.47 196 0.2 145.6 78.1 0.46 392 0.2 228.6 145.5 0.36 4% cement 98 0.04 149.9 82 0.45 98 1 156.1 87 0.44 98 6 138.9 77.5 0.44 98 10 144.4 80.6 0.44 98 20 148.5 84.7 0.43 Figure 4, for kaolin clay, the IB values considerably decrease as the normal stress increases from 98 to 196 kPa and slightly decreases when the normal stress exceeding 196 kPa. By contrast, the IB values for cemented kaolin clay (2% and 4% cement) are almost unchanged when the normal stress levels increase from 98 to 196 kPa and significantly decreases with the stress exceeding 196 kPa. This phenomenon can be attributed to the increase of stiffness of cemented clay when cement is added. This indicates that the cement content affects the effect of effective normal stress levels on the brittleness index. Regarding cement content, the relationship between IB and cement content at different effective normal stress levels is plotted in Figure 5. As shown, the IB values decrease significantly when the cement content increases from 0 to 2% and are almost unchanged Figure 5. Relationship between brittleness index and cement content when the cement contents increase from 2% to 4%. This indicates that the residual strength increases substantially as the cement content increases from 0 to 2%. Figure 4. Relationship between brittleness index and effective normal stress Figure 6. Variation of brittleness index of various soils with effective normal stress ISSN 2734-9888 10.2021 281
PHÁT TRIỂN X ÂY DỰNG BỀN VỮNG TRONG ĐIỀU KIỆN BIẾN ĐỔI KHÍ HẬU KHU VỰC ĐỒNG BẰNG SÔNG CỬU LONG The variation of brittleness index with effective normal stress of rates. It decreases as the shear rates increases from 0.02 samples in this study and of some soil types obtained from Bishop mm/min to 20 mm/min. et al. (1971) is presented in Figure 6. As shown in this figure, the Acknowledgment variation of brittleness index of kaolin sample (0% cement) with This research is funded by Vietnam National Foundation for increasing effective normal stress is similar in tendencey with that Science and Technology Development (NAFOSTED) under grant of Brown London clay, weathered and unweathered blue London number 105.08-2019.315. clay. Accordingly, the IB values significantly decrease with the increasing of effective normal stress to about 100 kPa. At above REFERENCES 100 kPa, the values of IB are almost independent of effective Bishop, A.W., Green, G.E., Garga, V.K., Andresen, A., Brown, J.D., A new ring normal stress. Differently from soil samples, the IB values of shear apparatus and its application to the measurement of residual strength. cemented soil samples (2% cement, 4% cement) are almost Geotechnique 21, 273–328, 1971. independent of effective normal stress up to 200 kPa. The Duong, N.T., Suzuki, M., Rate Effect on the Residual Interface Strength Between difference here can be attributed to the stifness and the formation two Different Soil Layers, in: Geotechnics for Sustainable Infrastructure of cementation bond when adding cement to the soil samples. Development. Springer, pp. 985–992, 2020. Duong, N.T, Hao, D. V., Consolidation Characteristics of Artificially Structured 0.80 Kaolin-Bentonite Mixtures with Different Pore Fluids. Advances in Civil Engineering, 0.70 2020 Fatahi, Behnam, Khabbaz, H., Fatahi, Behzad., Improving Geotechnical Brittleness index (IB) 0.60 Properties of Closed Landfills for Redevelopment Using Chemical Stabilization 0.50 Techniques. Ground Improvement Case Histories: Chemical, Electrokinetic, Thermal and Bioengineering 239, 2015. 0.40 Highter, W.H., Tobin, R.F., Flow slides and the undrained brittleness index of 0.30 some mine tailings. Engineering Geology 16, 71–82, 1980. 0% cement Pineda, J.A., Kelly, R.B., Suwal, L., Bates, L., Sloan, S.W., Geotechnical 0.20 2% cement characterization of Ballina clay. Geotechnical and Geophysical Site Characterisation 5 – Lehane, Acosta-Martínez & Kelly (Eds), 2016. 0.10 4% cement Sadrekarimi, A., Olson, S.M., Yield strength ratios, critical strength ratios, and 0.00 brittleness of sandy soils from laboratory tests. Canadian Geotechnical Journal 48, 0.01 0.1 1 10 100 493–510, 2011. Skempton, A.W., Bishop, A.W., The measurement of the shear strength of soils. Shear displacement rate (log), mm/min Geotechnique 2, 90–108, 1950. Figure 7. Relationship between brittleness index and shear displacement rates Suzuki, M., Van Hai, N., Yamamoto, T., Ring shear characteristics of discontinuous plane. Soils and Foundations 57, 1–22, 2017. In this study, the effect of shear displacement rates on Taylor, D.W., Fundamentals of soil mechanics. LWW, 1948. brittleness index of soil in ring shear test was also investigated. Thanh, D.N., Thi, N.N., Van, H.N., Chau, L.N., Tien, P.V., Suzuki, M., The relationship between brittleness index and shear Characteristics of shear strength at the interface between two soil layers in ring displacement rates is presented in Figure 7. As indicated, the shear apparatus. Journal of Materials and Engineering Structures «JMES» 7, 575– brittleness index of studied samples generally decreases as the 581, 2020. shear displacement rates increases. The highest decrease of Yerro, A., Alonso, E.E., Pinyol, N.M., Run-out of landslides in brittle soils. brittleness index with increasing shear rates is observed for Computers and Geotechnics 80, 427–439, 2016. kaolin clay. The decrease tendency of brittleness index here can be an indicator that the residual strength significantly increases with the increase of shear rates. 4. CONCLUSIONS The brittleness index of non-cemented and lightly cemented clay at different effective normal stress levels and shear rates in ring shear tests has been investigated in this study. Based on the analysis of test results, some main conclusions can be drawn as follows: The brittleness index of kaolin and cemented clay decreases as the effective normal stress levels increases. However, the effect of normal stress on the brittleness index may depend on the cement content. The research results also indicates that the brittleness index depends on the cement content. Accordingly, the brittleness index decreases as the cement content increases, especially from 0 to 2%. With the cement content above 2%, the brittleness index is almost unchanged. Besides the effective normal stress and cement content, this study also shows that the brittleness index obtained from the results of ring shear tests depends on the shear displacement 282 10.2021 ISSN 2734-9888