Assessment of cyclic liquefaction of silt based on two simplified procedures from piezocone tests

Transcription

1 3 rd International Symposium on Cone Penetration Testing, Las Vegas, Nevada, USA Assessment of cyclic liquefaction of silt based on two simplified procedures from piezocone tests H.F. Zou, S.Y. Liu, G.J. Cai & G.Y. Du Southeast University, Nanjing, China A.J. Puppala The University of Texas at Arlington, Arlington, Texas, USA ABSTRACT: Simplified procedures have been widely accepted and used in practice for assessing cyclic liquefaction resistance of sands and silts. This paper presents an experience, centered on the use of simplified procedures based on piezocone tests, to evaluate the cyclic liquefaction potential for a new business district in Suqian, Jiangsu province, China. Improvement with cross-shaped vibration wing and resonance theory was conducted on the site. Piezocone tests were performed on the soils both before and after resonance improvement. Two CPT-based simplified procedures were used to evaluate the cyclic liquefaction potential of silt both before and after improvement. This paper confirmed the applicability of piezocone tests for assessment of cyclic liquefaction of silt. 1 INTRODUCTION The simplified procedure for evaluating liquefaction potential that has become the most widely used method throughout the world. This method was originally proposed by Professor Seed and his coworkers (Seed and Idriss, 1971) from empirical evaluations of field observations and field and laboratory test data using the standard penetration test (SPT). Later, simplified procedures based on cone penetration test (CPT) were developed and soon drew great interest (Youd et al. 1997, 2001). The CPT-based simplified procedures can give more consistent, repeatable and continuous results than the SPT (Liu et al. 2011; Cai et al. 2012; Juang et al. 2003). As a recent development, CPT, with built-in pressure transducers capable of measuring pore water pressure simultaneously with tip resistance (qc) and sleeve friction resistance (fs), is commonly referred as piezocone penetration test, PCPT or CPTu (Lunne et al. 1997; Cai et al. 2010, 2011; Liu et al., 2008). In the framework of the simplified procedure, the calculation or estimation of two variables is required for evaluation of liquefaction potential of soils: (1) the seismic demand on a soil layer, expressed in terms of cyclic stress ratio (CSR); and (2) the capacity of the soil to resist liquefaction, expressed in terms of cyclic resistance ratio (CRR) (Youd et al., 2001). The first SPT-based simplified procedure defined the condition for triggering liquefaction by a boundary curve that separates liquefied cases from nonliquefied cases in a plot of CSR versus overburden stress-adjusted SPT blow counts. The first CPT- 873

2 based methods were essentially a conversion from the SPT-based methods, using empirical SPT CPT correlations (Robertson and Campanella, 1985; Seed and de Alba, 1986). Olsen, (1988, 1997) proposed a classification chart-based technique, whereas Robertson and Wride (1997, 1998) presented a soil behavior type index (Ic)-based technique. The Robertson and Wride (1998) method was recommended by the 1998 NCEER Workshop for its ease of application (Youd et al., 2001). Over the last two decades, the expanded database has promoted the development of various other correlations (e.g., Juang et al., 2003, 2006, 2008; Idriss and Boulanger, 2006; Moss et al., 2006; Robertson, 2009; Ku and Juang, 2012). The Robertson and Wride (1998) method has been updated several times. Robertson (2009) proposed a unified CPT-based method for both cohesive and cohesionless soils. This method consists of three segments: an equation for sand-like soils that is basically the same as the Robertson and Wride (1998) method, a semi-empirical equation for clay-like soils, and a transition between the two sets of equations. The is recognized as the most widely used CPT-based method in practice (Juang et al., 2013). However, Juang et al. (2008) also proposed a CPTubased model for evaluating liquefaction potential of both cohesive and cohesionless soils. The most important feature of the Juang et al. (2008) model is the recognition of the importance of pore pressure parameter (Bq) in its formulation. Ku and Juang (2012) demonstrated that the CPTu-based model per Juang et al. (2008) appeared to offer a more reasonable characterization of the cyclic softening resistance of fine-grained soils. The objective of this paper is to evaluate the performance of the and Juang et al. (2008) method for assessment of cyclic liquefaction potential of silt for the construction of a new business district in Suqian, Jiangsu province, China. Deep vibratory compaction equipment, with frequency-variable piling vibrator, was adopted for ground improvement by inserting a probe of crisscross section into the 15 m-thick silt deposit. The selected two simplified procedures were used to evaluate the cyclic liquefaction potential of silt both before and after improvement. 2 GEOLOGICAL CONDITIONS AND BASIC SOIL PROPERTIES Most soil deposits in Northern Jiangsu Province of China are composed of sand and silt. The soil profile at the site consists of a mean thickness of 8m of Yellow River alluvial silt fill on a natural deposit of clay. The groundwater table (GWT) was located about 3.0 m below the ground surface. The main physical properties of soils at the site are shown in Table 1. Figure 1 shows the particle size distribution and the basic properties of Suqian silt. Suqian silt is classified as ML in unified soil classification system (USCS). According to the Chinese code for seismic design of buildings, the design peak acceleration amax = 0.30g, where g is the gravitational acceleration. The design earthquake magnitude Mw is determined as Mw = 8. Table 1 The main physical properties of soils Soil layer G s w (%) e 0 W L (%) I P E s /MPa Fill Silt Clay Clay

3 100 Percent finer (%) m m m m m m m m E-3 1E-4 Partical size (mm) Figure 1. Particle size distribution of Suqian silt The piezocone penetration device used in this study is produced by Vertek- Hogentogler & Co. of USA. The equipment is a versatile piezocone system equipped with advanced digital cone penetrometers fitted with 60 cone and 10 cm 2 tip area which can provide measurement of cone tip resistance qt, sleeve friction fs, and penetration pore-water pressures (u2) with a porous filter 5 mm thick located at shoulder position u2. All CPTU tests were conducted at a penetration rate of 2 cm/sec, and the data was collected every 5 cm. The dimension of the vibro probe developed by Southeast University, China is 0.6 m in width and 15 m in length (Chen et al. 2013). In addition, circular openings of 0.1 m in diameter are spaced at 0.8 m along the probe, which aims to reduce probe impedance. These openings also provide better contact between the probe and the soil. At the same time it will transmit vibration to a larger area, which can further enhance the compaction effect. Furthermore, the lighter weight due to reduction in cross sectional area also gives rise to relatively larger displacement amplitude which is beneficial to soil densification. The weight of the vibrator and probe was about 7 tons that was guided by a leader mounted on a 50-ton crawler crane. Typical CPTu results before and after vibration compaction are depicted in Figure 2. The horizontal distance between the two CPTu was less than 5m. Thus, a reasonable comparison can be conducted to assess the applicability of vibro-compaction for silt improvement. The performance of Robertson (2009) and s for assessment of cyclic liquefaction potential of silt before and after ground improvement can then be evaluated. The hydrostatic pressure line based on the ground water table (GWT) at the site is also included in Figure 2. It is shown that both the qt and fs were increased significantly. The excess pore water pressure induced by the penetration of CPT in the silt before improvement was reduced to almost zero, or even negative values, due to the densification impact of vibrocompaction. Similar CPTu response (i.e. contractive to dilative pore pressures) in silts after ground improvement was also reported by Campanella et al. (1982) and Robertson (2012). 875

4 Soil Profile Fill Silt Depth (m) Cone tip resistance, q t (MPa) Sleeve friction, f s (kpa) Pore water pressure, u 2 (kpa) u 0 GWT Clay 10 Clay Before improvement After improvement Before improvement After improvement Figure 2. Typical results of piezocone tests before and after improvement at Suqian site Before improvement After improvement 3 ANALYSIS OF TEST RESULTS AND DISCUSSION 3.1 Analysis of results before compaction Figure 3 presents the comparison of evaluation of liquefaction potential based on the two methods before ground improvement. It is reasonably assumed that the soils are not susceptible to liquefaction above the ground water table, i.e. above 3m depth. Hence, FS > 1 above GWT. For the silt from 3m to 8m in depth (below the GWT), consistent and comparable profiles of liquefaction resistance were obtained for the Robertson (2009) and s. The normalized cone tip resistance in Figure 3 was defined separately. In the, the normalized cone tip resistance is determined according to Idriss and Boulanger (2004, 2006) using an exponent α in simple iterative procedure. Whereas, in the, the normalized cone tip resistance is calculated using the soil behavior type index, Ic. However, the stress-normalized resistances from the two methods were identical, as shown in Figure 3. The profiles of CRR along with the factor of safety were also consistent except at several depth intervals. Considerable variation was observed when the two methods were applied to the lower clay. The profile of CRR of the clay given by the was much larger than that by the Robertson (2009) method. The different mechanism of liquefaction may be attributed this variation. For the clay, the flow liquefaction is the major concern in engineering practice (Robertson 2009, 2010). However, sand is more susceptible to the cyclic liquefaction. 876

5 CSR 7.5 Normalized tip resistance CRR 7.5 Factor of safety Depth (m) 6 8 FS= Figure 3. Comparison of results of the two selected methods before improvement 3.2 Analysis of results after compaction Figure 4 gives the comparison of evaluation of liquefaction potential based on the Robertson (2009) and the s after ground improvement. Above the GWT, the normalized cone tip resistance and cyclic resistance ratio almost remained the same as that before compaction. Thus, the vibro-compaction seemed to be unsuitable to the soil in shallow depth and/or above the GWT. One reason is due to the lack of effective confining stress for the vibro-compaction. Other ground improvement technique, e.g. the dynamic or surface compaction, is needed to densify the soils in the shallow depth. The liquefaction resistance of silt increased significantly after ground improvement from 4m to 8m in depth. The factor of safety increased from the average value 0.6 before improvement to the average value 2.8 after improvement. It is shown that the vibro-compaction was effective to increase the liquefaction resistance of the Suqian silt at this site. After improvement, the possibility of cyclic liquefaction of silt decreased and could satisfy the allowable design requirements. For the silts from 4m to 8m, the profile of factor of safety given by Juang et al. (2008) is larger than that by Robertson (2009). Note that the soil behavior type indexes both in the and the are function of many factors, such as soil plasticity, fines content, mineralogy, soil sensitivity, and stress history, some of which might be changed by the vibro-compaction greatly. Thus, application of both the Robertson (2009) and s in heavily overconsolidated soils should be cautious because the soil behavior type indexes are often directly correlated to the fines content correction functions (Robertson, 2009). The probabilistic approaches, e.g. Juang et al. (2006) and Moss et al. (2006), appear to offer better evaluation of liquefaction potential to account for several geotechnical uncertainties in seismic analysis. But new approaches still need to be further developed to account for the influence of high overconsolidation ratio. For the lower clay, the vibro-compaction appears to reduce the liquefaction resistance of the soil. The profiles of factor of safety decreased obviously with FS < 1 at several depth intervals. Nevertheless, the cone tip resistance, which represents the failure strength of soil, has no megascopic change after the ground improvement, as shown in Figure 2. Several factors could be attributed to this irregularity. These include the loss of accuracy of sleeve friction in the saturated soft clay deposit (Lunne et al. 1997) and the potential loss of saturation of porous filter when the probe passed the dense silt (Campanella et al. 877

6 1982; Robertson 2012), as shown in Figure 2. The accuracy of the is primarily influenced by the loss of accuracy of sleeve friction in this case. Whereas, both the above two factors control the reliability of the. CSR Normalized tip resistance CRR Factor of safety 2 4 FS = 1 Depth (m) Figure 4. Comparison of results of the two selected methods after improvement 3.3 Comparison of estimating CRR of silt A comparison was made between the estimated CRR of the silt based on the Robertson (2009) and s, as shown in Figure 5. The data points before ground improvement are scattered around the y = x line, indicating that the differences are relatively slight. After ground improvement, the scatter increases with CRR. Generally, the CRR values per are one to two times larger than those of the. This indicates that the Juang et al. (2008) method seems to be less conservative and more sensitive to the compaction impact than the Robertson (2009) method. The influence of compaction may contribute to the change of two main factors, the normalization of cone tip resistance and the correlation between the soil behavior type indexes and fines content. The mechanism of compaction and its influence on CPTu measurements still need to be further confirmed. 878

7 CRR of silts per the y = 2x y = x Before improvement After improvement CRR of silts per the Figure 5. Comparison of CRR of silt per the two methods 4 CONCLUSIONS This paper presents an experience centered on the use of simplified procedures based on piezocone tests, to evaluate the cyclic liquefaction potential of silt. Vibro-compaction was conducted at the site to improve the liquefaction resistance. Piezocone tests were performed both before and after the improvement to evaluate the efficiency of vibro-compaction. It is shown that the Juang et al. (2008) and the Robertson (2009) methods can give similar results of liquefaction potential of silts before ground improvement. After the vibro-compaction, a large scatter was observed between the CRR of and those of. Generally, the CRR values per are one to two times larger than those of the. This indicates that the Juang et al. (2008) method may be less conservative and more sensitive to the compaction impact than the Robertson (2009) method. The mechanism of vibro-compaction needs to be investigated to better understand its influence on CPTu measurements. The reliability of simplified procedures for ground improvement also needs to be further confirmed. Besides, CPT/CPTu proves to be capable of evaluating the potential for liquefaction because of its repeatability, reliability, continuous data and cost effectiveness. 5 ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China (Grant No ), the "Twelfth Five-Year" National Science and Technology Support Plan (Grant No. 2012BAJ01B02), the Key Project of Natural Science Foundation of Jiangsu Province (Grant No. BK ) and the Foundation for Excellent Young Teachers (Grant No ). These financial supports are gratefully acknowledged. 879

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