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Figure 10. Stress paths of the monotonic triaxial tests in p’-q space. The critical state line can be obtained by plotting the results of the monotonic triaxial compression tests on the p’-q diagram. Figure 10 plots the results of the tests in the p’-q diagram for the samples with OCR values of 1, 2, and 4. The slope of the critical state line using the data presented in Figure 10 was estimated to be approximately 0.72. The slope of the critical state line was used for the determination of cyclic threshold stress in Section 4.2.

Figure 10 Stress paths of the monotonic triaxial tests in p’-q space. The critical state line can be obtained by plotting the results of the monotonic triaxial compression tests on the p’-q diagram. Figure 10 plots the results of the tests in the p’-q diagram for the samples with OCR values of 1, 2, and 4. The slope of the critical state line using the data presented in Figure 10 was estimated to be approximately 0.72. The slope of the critical state line was used for the determination of cyclic threshold stress in Section 4.2.

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Index properties of the soil samples were obtained and summarized in Table 1. As shown in Table 1, the Bangkok clay was classified as inorganic clay with high plasticity (CH) in accordance with the Unified Soil Classification System (USCS). The results of the index properties shown in Table 1 are consistent with the findings obtained by previous researchers who have studied Bangkok clay's index properties at the AIT site (Shibuya, 2003 and Suksan, 2007).
Index properties of the soil samples were obtained and summarized in Table 1. As shown in Table 1, the Bangkok clay was classified as inorganic clay with high plasticity (CH) in accordance with the Unified Soil Classification System (USCS). The results of the index properties shown in Table 1 are consistent with the findings obtained by previous researchers who have studied Bangkok clay's index properties at the AIT site (Shibuya, 2003 and Suksan, 2007).
Figure 1. GDS Enterprise Level Dynamic Triaxial Testing System at the GTE Laboratory.
Figure 1. GDS Enterprise Level Dynamic Triaxial Testing System at the GTE Laboratory.
Figure 2. Configuration for extension top-cap and vylastic sleeve (modified after Sean, 2014). The internal submersible load cell was used with a load capacity of 1 kN. The accuracy of loadcell is +0.1% of the full range output which allows a measurement error of 0.001 kN. The extension top cap was used to ensure sufficient contact between the specimen top cap and the internal submerged load cell. The configuration of the extension top cap is presented in Figure 2.
Figure 2. Configuration for extension top-cap and vylastic sleeve (modified after Sean, 2014). The internal submersible load cell was used with a load capacity of 1 kN. The accuracy of loadcell is +0.1% of the full range output which allows a measurement error of 0.001 kN. The extension top cap was used to ensure sufficient contact between the specimen top cap and the internal submerged load cell. The configuration of the extension top cap is presented in Figure 2.
Figure 3. Specimen saturation by back-pressure (modified after Sean, 2013).
Figure 3. Specimen saturation by back-pressure (modified after Sean, 2013).
Table 2. Summary of undrained compression triaxial tests for Bangkok clay. The confirmation of soil sample saturation was carried on by checking the Skempton B-value (refer to Eq. 2). The B-value was determined using Au and Aos, as illustrated in Figure 4. The minimum B-value of 0.95 was used for the determination of sample saturation.
Table 2. Summary of undrained compression triaxial tests for Bangkok clay. The confirmation of soil sample saturation was carried on by checking the Skempton B-value (refer to Eq. 2). The B-value was determined using Au and Aos, as illustrated in Figure 4. The minimum B-value of 0.95 was used for the determination of sample saturation.
Figure 4. Specimen saturation by back-pressure (modified after Sean, 2013). The complete consolidation is achieved when the excess pore pressure is fully dissipated (see Figure 5). The consolidation process of the Bangkok soft clay used in this study usually took 3 to 5 days to achieve a 95% degree of consolidation or higher.
Figure 4. Specimen saturation by back-pressure (modified after Sean, 2013). The complete consolidation is achieved when the excess pore pressure is fully dissipated (see Figure 5). The consolidation process of the Bangkok soft clay used in this study usually took 3 to 5 days to achieve a 95% degree of consolidation or higher.
Figure 6. Plot of pressures vs. time during the rebound phase. After the completion of the first phase of the consolidation, a lower effective confining pressure was applied in the test to allow the sample to swell under the rebound phase (refer to Figure 6). The sample was considered as over-consolidated at this phase. 3.9.4.4 Cyclic unarained compression In the cyclic undrained compression tests, the specimen was subject to axial loading and unloading under constant cell
Figure 6. Plot of pressures vs. time during the rebound phase. After the completion of the first phase of the consolidation, a lower effective confining pressure was applied in the test to allow the sample to swell under the rebound phase (refer to Figure 6). The sample was considered as over-consolidated at this phase. 3.9.4.4 Cyclic unarained compression In the cyclic undrained compression tests, the specimen was subject to axial loading and unloading under constant cell
Figure 7. Load cell measurements during cyclic compression triaxial tests. In this study, the Equilibrium State Line method referenced in Loh (2011) was used to identify the threshold stress. For the normally to slightly over-consolidated clays (i.e. OCR = 1 - 2). The threshold stress of each series was determined by the intersection between the Line of Cyclic Stress Equilibrium State (LCSES) and the Critical State Line (CSL), which can be obtained by monotonic compression test in the p’-q diagram.
Figure 7. Load cell measurements during cyclic compression triaxial tests. In this study, the Equilibrium State Line method referenced in Loh (2011) was used to identify the threshold stress. For the normally to slightly over-consolidated clays (i.e. OCR = 1 - 2). The threshold stress of each series was determined by the intersection between the Line of Cyclic Stress Equilibrium State (LCSES) and the Critical State Line (CSL), which can be obtained by monotonic compression test in the p’-q diagram.
Figure 9. Test results of the monotonic undrained compression. Figure 8. Stress paths of a dynamic triaxial test in p’-q diagram.
Figure 9. Test results of the monotonic undrained compression. Figure 8. Stress paths of a dynamic triaxial test in p’-q diagram.
the soil specimen. Figure 9 shows the results of the monotonic compression tests for three samples with OCR values of 1, 2, and 4. As shown in Figure 9, the maximum deviator stress values of 166.74, 128.09, and 118.55 kPa were obtained for the samples with OCR values of 1, 2, and 4, respectively.
the soil specimen. Figure 9 shows the results of the monotonic compression tests for three samples with OCR values of 1, 2, and 4. As shown in Figure 9, the maximum deviator stress values of 166.74, 128.09, and 118.55 kPa were obtained for the samples with OCR values of 1, 2, and 4, respectively.
The developments of total axial strain and excess pore water pressure for the samples of the NC series with both 2 Hz and 4 Hz loading frequencies are presented in Figure 11. Figure 11 indicates that, for the cases with the deviator stresses, ou, of 74 and 120 kPa, the non-failure condition was observed for the number of cycles of up to 10,000 cycles. The resilient state was observed for these tests. When the soil samples were subjected to deviator stress of 153.0 kPa, the axial strain and excess pore pressure rapidly developed. These phenomena were described in the literature as “Cyclic Softening” when the development of significant strain or strength loss is observed in fine-grained soils exhibiting clay-like behavior (Eric, 2016). As shown in Figure 11, the failure was observed at approximately 400 and 2000 loading cycles for 2 Hz and 4 Hz load frequencies, respectively.
The developments of total axial strain and excess pore water pressure for the samples of the NC series with both 2 Hz and 4 Hz loading frequencies are presented in Figure 11. Figure 11 indicates that, for the cases with the deviator stresses, ou, of 74 and 120 kPa, the non-failure condition was observed for the number of cycles of up to 10,000 cycles. The resilient state was observed for these tests. When the soil samples were subjected to deviator stress of 153.0 kPa, the axial strain and excess pore pressure rapidly developed. These phenomena were described in the literature as “Cyclic Softening” when the development of significant strain or strength loss is observed in fine-grained soils exhibiting clay-like behavior (Eric, 2016). As shown in Figure 11, the failure was observed at approximately 400 and 2000 loading cycles for 2 Hz and 4 Hz load frequencies, respectively.
total axial strain remains fairly constant after a certain number of loading. The intersect of the LCSES line, and the critical state line can be utilized to determine the threshold stress of the soil. As indicated in Figure 12, the threshold stress values of the NC samples with 2 and 4 Hz load frequencies were determined to be 134 and 140 kPa, respectively.
total axial strain remains fairly constant after a certain number of loading. The intersect of the LCSES line, and the critical state line can be utilized to determine the threshold stress of the soil. As indicated in Figure 12, the threshold stress values of the NC samples with 2 and 4 Hz load frequencies were determined to be 134 and 140 kPa, respectively.
Figure 13. Developments of total peak strain vs. time for various CSR values (a) frequency = 4 Hz and OCR = 2 and (b) frequency = 4 Hz and OCR =4. 5 CONCLUSIONS This paper presented the results of the threshold stress evaluation for the Bangkok soft clay under cyclic loading. The following conclusions were drawn from the findings of the study:
Figure 13. Developments of total peak strain vs. time for various CSR values (a) frequency = 4 Hz and OCR = 2 and (b) frequency = 4 Hz and OCR =4. 5 CONCLUSIONS This paper presented the results of the threshold stress evaluation for the Bangkok soft clay under cyclic loading. The following conclusions were drawn from the findings of the study:
monotonic loading conditions. For the entire samples of the NC and OC series, the CSR values range from 0.67 to 0.84. This finding is consistent with the common range of the threshold stress observed for high plasticity soft clay (Lefebvre, 1988 and Loh, 2011). The OC soil samples with OCR = 2 and OCR = 4 have CSR values of 0.67 to 0.71, whereas the NC soil samples have CSR values of 0.80 to 0.84. It was observed that the threshold stress tends to decrease when the over-consolidation ratio increases. Table 3. Cyclic threshold stresses of Bangkok soft clay.
monotonic loading conditions. For the entire samples of the NC and OC series, the CSR values range from 0.67 to 0.84. This finding is consistent with the common range of the threshold stress observed for high plasticity soft clay (Lefebvre, 1988 and Loh, 2011). The OC soil samples with OCR = 2 and OCR = 4 have CSR values of 0.67 to 0.71, whereas the NC soil samples have CSR values of 0.80 to 0.84. It was observed that the threshold stress tends to decrease when the over-consolidation ratio increases. Table 3. Cyclic threshold stresses of Bangkok soft clay.
Related topics:
High-speed trainCyclic LoadingSoft ClayExcess Pore Pressure
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