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Interpretation of Seismic Cone Penetration Testing in Silty Soil

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Interpretation of Seismic Cone Penetration Testing in Silty Soil Interpretation of Seismic Cone Penetration Testing in Silty Soil Rikke Holmsgaard1, Lars Bo Ibsen2, and Benjaminn Nordahl Nielsen3 1PhD. Fellow, Master of Science in Civil Engineering, Aalborg University, Department of Civil Engineering, Sofiendalsvej 11, 9200 Aalborg SV, Denmark, Phone +45 40939994, email: [email protected] 2Professor, Aalborg University, Department of Civil Engineering, Sofiendalsvej 11, 9200 Aalborg SV, Denmark, Phone +45 99408458, email: [email protected] 3Associate Professor, Aalborg University, Department of Civil Engineering, Sofiendalsvej 11, 9200 Aalborg SV, Denmark, Phone +45 99408459, email: [email protected] Corresponding author: Rikke Holmsgaard, email: [email protected] ABSTRACT Five Seismic Cone Penetration Tests (SCPT) were conducted at a test site in northern Denmark where the subsoil consists primarily of sandy silt with clay bands. A portion of the test data were collected every 0.5 m to compare the efficacy of closely-spaced down-hole data collection on the computation of shear wave velocity. A minimum of eight seismic tests were completed at each depth in order to examine the reliability of shear wave velocity data, as well as to assess the impact of the time interval between CPT termination and seismic test initiation on SCPT results. The shear wave velocity was computed using three different methods: cross-over, cross-correlation and cross-correlation “trimmed with window”. In the “trimmed with window” technique the latter part of the signal is clipped off by setting the amplitude to zero. The result showed that more closely-spaced test intervals actually increased the variability of the shear wave velocity and that time interval between seismic tests is insignificant. Correlation between shear wave velocity and cone resistance for silty soils were also determined and assessed relative to other published data on multiple soil types. KEYWORDS: Field Testing, Site Investigations, Strength and Compressibility of Soils, Sampling and Related Field Testing for Soil Evaluations INTRODUCTION In a Seismic Cone Penetration Test (SCPT), a geophone is integrated into the cone, making it possible to determine the small strain shear modulus Gmax (or G0 ) by measuring the shear waves ( S ), and assessing the shear wave velocity. The shear modulus is an important soil parameter that among others is especially useful for wind turbines where the dynamic behavior often drives the design (Campanella et al. 1986). In addition, the shear modulus is also highly applicable for liquefaction - 4759 - Vol. 21 [2016], Bund. 15 4760 analysis and could be used for site classification (Robertson et al. 1995; Lunne et al. 1997). The shear modulus is computed from equation 1: 2 Gmax =Vs ⋅ ρ (1) where ρ is the soil mass density (g g) and Vs is the shear wave velocity. The shear wave velocity is generated at shear strain amplitudes of around10−4% , for which the low strain level dynamics shear modulus, Gmax , is obtained (Campanella et al. 1986; Robertson et al. 1986; Sully and Campanella 1995). As is apparent in equation (1), it is important that shear wave velocity be calculated as accurately as possible since the value is squared to calculate Gmax , and errors would be substantially magnified in the final calculation of the shear modulus. Shear wave velocity is measured by performing a SCPT as either a crosshole tests or a downhole tests. Studies have shown that shear wave velocity results generated by either test are essentially identical (Campanella et al. 1986; Robertson et al. 1986). This paper focuses on the downhole test where the energy source is located at the ground surface and the receiver cone is in the borehole. Normally the downhole SCPT is conducted with seismic tests at every meter in the borehole, which is why Vs (or Gmax ) is a constant at one meter intervals. Execution of an in situ SCPT can be rather time-consuming, and therefore expensive and impractical for low-risk projects. As an alternative, it may be preferable to conduct standard CPTs and apply empirical correlations in order to estimate the dynamic soil parameters. Data gathered by some researchers indicate a direct correlation between the shear wave velocity and the cone resistance. However, since the shear wave velocity is derived from small strain values and the cone resistance is related to peak shear stress strains at failure, questions have been raised as to whether the two parameters can be correlated in any usable manner. Nevertheless, both the shear wave velocity and cone resistance are dependent on, and respond to, many of the same parameters, including confining stress level, K 0 stress state, mineralogy and aging (Mayne and Rix, 1993; Mayne and Rix, 1995; Tonni and Simonini, 2013). It is reasonable, therefore, to observe a usable correlation between shear wave velocity and cone resistance. Site-specific correlations between cone resistance and shear wave velocity have been reported for medium dense sand (Paoletti et al. 2010) and clayey soil (Gadeikis et al. 2013). Mayne and Rix (1995) proposed empirical correlations to estimate the shear wave velocity in clay soils on the basis of cone resistance and the void ratio, e0 . The void ratio, however, requires tests on undisturbed soil and the data are often not available. Karray et al. (2011) examined coarse and fine sands and suggested that shear wave velocity is related to both cone resistance and mean grain size, D50 . Long and Donohue (2010) proposed a correlation for soft clay depending on both cone resistance and pore pressure parameter, Bq . In order to account for all soil types Hegazy and Mayne (2006), Robertson (2009) and Tonni and Simonini (2013) proposed a global correlation that depends on the normalized cone resistance, qc1N or Qnt , stress level, σ 'v0 or σ v0 , and the soil behavior type index, I c (Robertson and Wride 1998). This paper presents the results of several field seismic tests on inhomogeneous sandy silt with clay bands. The tests were conducted on soil from northern Denmark at a site where the subsoil is primarily silt. While downhole seismic tests are normally conducted every 1 m, for some of these tests the distances between successive seismic tests were reduced to 0.5 m in order to assess the impact of the lack of soil homogeneity. Besides reducing the distance between the tests, a minimum Vol. 21 [2016], Bund. 15 4761 of eight tests in each depth were conducted which also allowed for assessment of the reliability of the measured shear wave velocities. Also examined was the degree to which measured shear wave velocity is dependent on the length of time between when the CPT rods are stopped and the actual seismic tests measurements are taken. METHODOLOGY The test site was located near the town Dronninnglund, situated in the northern part of Denmark. The experimental program consisted of five downhole SCPTs with seismic measurements from approximately 4 to 8 m depth, one standard CPT to measure key parameters i.e. cone resistance, sleeve friction and pore pressure and two soil strata boring to identify soil type (Figure 1). Figure 1: Coordinates of the CPT, SCPTs and borings. SITE DESCRIPTION The soil at the test site was identified by the two soil strata borings and several classification tests in the laboratory, e.g. water content, specific gravity and grain size distribution. The soil consists of silty sand from the ground surface to approximately 4.5 m below ground level. From approximately 4.5 to 11.4 m below ground level the soil consists of sandy silt with clay bands; below 11.4 m the soil consists of silty clay with the number of clay bands gradually increasing with depth. In general, the soil is inhomogeneous and consists of multiple bands or pockets of sand, silt and clay (Figure 2a). Groundwater was encountered at approximately 0.2-0.6 m below ground level. The soil data are found in Table 1. Vol. 21 [2016], Bund. 15 4762 Figure 2: Soil profile at the test site (a) and cone resistance, sleeve friction and pore pressure (b). Table 1: Characterization of soil samples from different depths (see Poulsen et al. 2012a). Depth Soil type Water Specific Soil unit Grain size distribution (%) content gravity weight (m) (%) 3 Sand Silt Clay w Gs (-) g ( kN m ) 3.7-4.7 Silty sand 21.1 2.70 - 58 36 6 4.7-5.7 Sandy silt 23.2 2.67 - 43 46 11 5.7-6.7 Sandy silt 22.9 2.69 20.1 45 51 4 6.7-7.7 Silt/sand 20.0 2.67 20.2 51 40 9 7.7-8.7 Sandy silt 24.0 2.68 20.4 41 46 13 The seismic measurements were only conducted from approximately 4 to 8 depth since this is where the silt layer is located. Even though it is possible to measure the standard CPT parameters (cone resistance, sleeve friction and pore pressure) in the same borehole in which the seismic tests are conducted, the measurements are not considered reliable since McNeilan and Bugno (1985) found that whenever a stop occurs, the excess pore water pressure starts to dissipate and the cone resistance increases. As a result cone resistance would be higher and pore pressure would be lower than what would normally be the case for a silty soil after each stop during the SCPT test. Therefore, the Vol. 21 [2016], Bund. 15 4763 standard CPT parameters were determined from a standard CPT conducted at the test site (e.g. Figure 2b). The CPT parameters plotted according to the soil classification charts developed by Robertson et al. (1986) are illustrated in Figure 3. Both Figure 2b and Figure 3 emphasize that the soil is quite inhomogeneous and stratified. (a) (b) Figure 3: Results of the standard CPTs plotted in the qt , Bq (a) and qt , R f (b) classification charts from Robertson et al (1986).
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