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 (-) γ ( 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). The zones refer to: 1-sensitive fine grained, 2-organic material, 3-clay, 4-silty clay to clay, 5-clayey silt to silty clay, 6-sandy silt to clayey silt, 7-silty sand to sandy silt, 8-sand to silty sand, 9-sand, 10-gravelly sand to sand, 11-very stiff fine grained, 12-sand to clayey sand

EXPERIMENTAL PROGRAM The SCPT equipment (Geotech AB, Sweden) included a cable system with a10cm 2 probe with a 60° tip angle. The S-waves were triggered by a shock between a sledgehammer and a steel plate at the surface. A triggering cable was connected to the sledgehammer and the steel plate (with crocodile clamps) and a SCPT signal conditioning unit. Two steel plates were positioned and aligned on each site of the sounding hole in order to generate a “right” and “left” polarized shear wave (Figure 4). The “L” shaped plates were equipped with transvers “teeth” in order to ensure good ground contact. In the sounding hole a SCPT adapter (accelerometer) was connected to the cone. When a polarized shear wave was triggered by the hammer striking the steel plate, the time required for the shear wave to travel a known distance to the sounding hole was measured. The distance between the sounding hole and where the shear waves were generated (where the hammer hits the steel plate) was 1.4 meters (Figure 4). A special hammer arrangement was constructed and added to the test apparatus in order to generate rapid, repeatable blows to the steel plate. The hammer arrangement was constructed with a fixed point to ensure identical hammer swing. The device also prevented the steel plate from shifting Vol. 21 [2016], Bund. 15 4764 away from its original location following the strike. Securing the plate is critical since allowing the plate to move would dissipate some of the energy from the blow, preventing the full force from traveling into the ground, and making the production of repeatable seismic shear waves problematic (Butcher et al. 2005). To ensure that the seismic measurements were not affected by any background noise, the CPT rig was stopped during the execution of the tests. Because the CPT rig was turned off, a separate generator was used as a power source. This generator was placed approximately 50 m from the sounding hole in order to reduce as much noise as possible. To acquire data on the inhomogeneous silty soils and improve shear wave computation, for two of the five CPTs the probe was stopped every 0.5 meter to carry out seismic measurements.

Figure 4: Schematic design of the Seismic Cone Penetration Tests.

In order to compute the shear wave velocity, collecting one left polarized and one right polarized measurement is generally held to be sufficient. However, according to Liao and Mayne (2006), many commercial firms conduct two strikes on each side in order to confirm the repeatability of the wave source and signal. In order to examine the reliability of the shear wave velocity a minimum of eight left strikes and eight right strikes were conducted every 0.5 m. In addition, by performing a minimum of eight strikes on each side, it is possible to determine if there is a difference in shear wave velocity if the seismic tests are conducted immediately after the CPT is halted or about 5-10 min after stopping the CPT. There is approximately 1 min between each set of strikes, i.e. from the first left stroke to the eighth left stroke, it takes about 8 min. When the CPT is haltered a dissipation process starts, and since the dissipation process in the silty soil typically takes about 1-5 min (Figure 5) it’s possible to determine if the seismic tests are affected by the dissipation process. Vol. 21 [2016], Bund. 15 4765

Figure 5: Typically dissipation curves for the silt.

Even though the test setup was exactly the same among repeated hammer strikes, the signals triggered by the shock were found to be of very different quality (Figure 6). Of the examples shown in Figure 6, three were of good quality (those in bold) and three were of poor quality (not bold). Signals of poor quality were excluded from the analysis. The SCPTs were conducted at a field close to an urban environment from where a relatively large road runs (about 100 m from the testing area). This may explain the signals of poor quality. The signals of good quality, however, also look a bit noisy. Vol. 21 [2016], Bund. 15 4766

Figure 6: Example of three signals of good quality and three signals of poor quality.

Computation of shear wave velocity

Shear wave velocity, Vs , is calculated using equation 2 (Campanella and Steward 1992; Sully and Campanella 1995; Howie and Amini 2005):

L2 − L1 ∆L Vs = = (2) t2 − t1 ∆t

where L2 and L1 are the slant distance between the source beam and the cone sensor for the first and second depth, respectively (e.g. depth interval of 1 m), and t2 and t1 are the shear wave arrival time for the first and second depth, respectively. The time interval ( ∆t = t2 − t1 ) can be computed in different ways. In this paper three widely known methods have been applied to compute the time interval and hence the shear wave velocity. The methods are cross-over (or reverse polarity), cross- correlation, and cross-correlation “trimmed with window”. By using both a left and right steel plate, the shocks from the hammer blows on the plates generate reversed shear wave signals, in which the amplitude of the measured signals is reversed. This signal pattern makes it relatively easy to identify the first cross-over point as the point where the Vol. 21 [2016], Bund. 15 4767 main shear waves arrive and changes signs. The first clear cross-over point of the two shear waves is identical to the arrival time of the shear wave. The time interval is determined by subtracting the arrival time for the first depth from that of the second depth. (Robertson et al. 1986; Campanella et al. 1989; Campanella and Steward 1992; Areias and Impe 2004; Liao and Mayne 2006) The use of two shear wave measurements to determine the cross-over shear wave velocity should increase the reliability of the value, although the utilization of only a single point from the shear wave signals to compute the time intervals does lessen its reliability. In addition, the method is dependent on considerable personal judgment and it is time consuming to manually identify the cross-over point (Campanella and Steward 1992; Sully and Campanella 1995; Areias and Impe 2004; Liao and Mayne 2006). The cross-correlation method uses the entire shear wave signal to compute the time interval and therefore the shear wave velocity. The time interval is determined by shifting the lower signal relative to the upper signal in steps equal to the time interval between the digitized points of the signals, which is 0.2 ms for the current research. For every time shift the coefficient of determination, R 2 , is calculated, and all are then plotted against the time shift. The shift yielding the highest R 2 corresponds to the best fit for the time interval for the shear waves. This time interval is then used to determine the shear wave velocity (Campanella and Steward 1992; Sully and Campanella 1995; Liao and Mayne 2006). The cross-correlation method can be automated, substantially reducing the analysis time relative to the cross-over method. Because it uses the entire shear wave signal it should be more reliable than the cross-over method (Campanella and Steward 1992; Areias and Impe 2004; Liao and Mayne 2006). However, if the shear wave signal is very scattered or noisy after the arrival of the main shear wave (e.g. Figure 6) R 2 could be too low, thus diminishing the utility of the relationship. For this reason, Campanella and Steward (1992) and Liao and Mayne (2006) proposed selecting only the main part of the shear wave for use in the cross-correlation method. This is called the cross-correlation “trimmed with window” technique whereby the latter part of the signal is clipped off by setting the amplitude to zero, and cross-correlation analysis includes only the main part of the shear wave signals, so the signal is clipped off after the arrival of the main part of the shear wave. The resulting estimate of shear wave velocity is more accurate, with a potentially higher R 2 .

Measured shear wave velocity Computed shear wave velocities for one of the SCPTs, with measurements every 0.5 m, were calculated using cross-correlation (Figure 7a and 7b), cross-correlation “trimmed with window” (Figure 7c and 7d) and cross-over (Figure7). Vol. 21 [2016], Bund. 15 4768

Figure 7: Shear wave velocity results for one of the Seismic Cone Penetration Tests with seismic measurements approximately every 0.5 m. CC: Cross-correlation; CC w window: Cross-correlation “trimmed with window”; CO: Cross-over. “Left” and “Right” refer to left signals and right signals, respectively.

These data illustrate the wide variability in shear wave velocity with depth, even within the calculation method. Plotting the R 2 determined for cross-correlation (Y-axis) against the R 2 determined for cross-correlation “trimmed with window” (X-axis) (Figure 7f) shows that in most situations the latter values were higher (most markers are below the diagonal line), suggesting that cross-correlation “trimmed with window” is the preferred estimation method. It is important to note that at any given depth nothing changed during testing. Soil conditions, equipment and test setup were the same. The figure only contains results from set of data where the coefficient of determination, Vol. 21 [2016], Bund. 15 4769

R 2 , is higher than 0.5 computed on the basis of the Cross-correlation “trimmed with window”. In some of the depths, there are no results because the measurements either are of poor quality or the coefficient of determination, R 2 , are too low. The shear wave velocity for the same SCPT (as illustrated in Figure 7) using data from 1.0 m (rather than 0.5 m) and computed with that same three methods is shown in Figure 8. For these calculations, every second dataset is skipped and is therefore representative of more typical (data collected every 1.0 m) test sets. As in Figure 7, Figure 8 presents cross-correlation (Figures 8a and 8b) cross-correlation “trimmed with window” (Figures 8c and 8d) and cross-over (Figure 8e). The variability of the shear wave velocity measurements improves (decreases) with the 1.0 m measurements, as compared to the 0.5 data (compared to Figure 7). This is somewhat counter-intuitive as it was expected that 0.5 m measurements would be preferable due to the inhomogeneous silty soil, and therefore closely-spaced measurements would yield less variable data. That does not, however, appear to the case in this study, although. It should be noted that 1.0 m measurements are still a gross simplification of the shear wave velocity in these silty soils. A comparison of the R 2 values for cross-correlation (Y-axis) and cross-correlation “trimmed with window” (Figure 8f) shows that, as in Figure 7f, the latter method increases the R 2 and most data points fall below the line. Regardless of the value of R 2 , utilizing the “trimmed with window” method appears to have no significant impact on the computed shear wave velocity. The shear wave velocity calculated using the cross-over method (Figure 8e) gives the most reliable and consistent results. This may be due to the fact that the signals are noisy and the cross-over method only uses the point where the main shear wave arrives and not the subsequent signal which may be very different for each test. However, this method is operator-dependent and therefore can vary with individual and level of experience. Vol. 21 [2016], Bund. 15 4770

Figure 8: Shear wave velocity results for the Seismic Cone Penetration Tests with seismic measurements approximately every 1.0 m. CC: Cross-correlation; CC w window: Cross- correlation “trimmed with window”, CO: Cross-over. “Left” and “Right” refer to left signals and right signals, respectively.

Both left and right (relative to the hole position) shear wave signals were obtained. While the left and right signals should produce consistent results, that does not appear to be the case for the data collected in this study (Figure 9). Neither the left nor the right signals produce consistently higher values than the other. In addition, the left and right signals do not generate more consistent results if the cross-correlation “trimmed with window” method is applied. Vol. 21 [2016], Bund. 15 4771

Figure 9: Comparison of the shear wave velocity for the left and right shear wave signals. The figure only contains results measured approximately every 1.0 m.

At each depth where the CPT was stopped, a minimum of eight seismic tests were conducted with approximately 8 min between the first and eight left strikes. The dissipation process that begins as soon as the CPT is stopped could potentially impact the subsequent seismic measurement if it is taken too soon (e.g., immediately after termination of the CPT versus 5 min later). Figure 10 shows the computed shear wave velocity as calculated by the cross-correlation “trimmed with window” method. The sequence number is written above each seismic test. The computed shear wave velocity did not increase or decrease according to the order in which the seismic tests were conducted. These data indicate the shear wave velocity is independent of time, and hence the dissipation process. The measured shear wave velocities have a large variation which is unexpected since the geophone is at the exact same location in the ground and the measured velocities are not related to dissipation (according to Figure 10). The variation could be the result of the urban environment (e.g. the road near the test area) since only time changes in Figure 10. Vol. 21 [2016], Bund. 15 4772

Figure 10: Calculated shear wave velocity illustrating the measurement sequence. There is approximately one minute between each measurement (i.e. approximately 8 minutes between measurements 1 and 8). “Left” (top graph) and “Right” (bottom graph) refer to left signals and right signals, respectively.

Uncertainty related to the time interval Collecting seismic measurements every 0.5 m rather than every 1.0 m did not improve the reliability of the collected data, as demonstrated by comparing Figures 7 and 8. The lower reliability of the 0.5 m measurements is related to the uncertainty in determining the time interval, ∆t , (equation 2), which is constant and independent of the distance between the measurements. At each depth there were eight left strike seismic measurements and eight right strike seismic measurements. By comparing, for example, the first left strike with the fifth left strike, or the third right strike with the eight right strike, the time interval, ∆t , between the signals should be exactly zero since the depth and soil condition are the same. This was not always the case. The impact of the time interval at multiple depths using the cross-correlation method is shown in Figure 11. If more than one result yields the same value within the time interval, it is shown in Figure 11 with a larger circle. Vol. 21 [2016], Bund. 15 4773

Figure 11: Uncertainty surrounding the time interval for different measurements. A larger circle indicates that more of the measurements produce the same value at different time intervals. Therefore, most of the signals have a time interval equal to zero. This figure illustrates only random selected depths showing the typical trend (4, 4.5, 6 and 7 m depth).

While most of the signals had a time interval equal to zero, there were some that produced time interval values of approximately 0.2 ms, 0.4 ms and as high as 1-2 ms. For this reason, it was concluded that the shear wave signals have an uncertainty of about 0.2-0.6 ms. Figure 12 illustrates the time interval uncertainty at different depths as the difference between the calculated mean time interval and the calculated time interval for randomly selected seismic measurements conducted every 0.5 or 1.0 meter. Vol. 21 [2016], Bund. 15 4774

Figure 12: Uncertainty surrounding the time interval for the seismic measurements conducted every 0.5 and 1.0 m. The figure illustrates only random selected depths showing the typical trend.

The uncertainty for the time interval, ∆t , is approximately the same regardless of whether the seismic measurements were collected at the same depth or from the separate 0.5 or 1.0 m measurements points. Because there appears to be no demonstrable spatial impact of collecting at 0.5 or 1.0 m the time interval takes on increased importance for calculation of the shear wave velocity since the magnitude of the time interval is larger when the depths between the measurements are increased. Because the silty soil is inhomogeneous, it would be preferable to have seismic measurements with spatial intervals less than approximately 1.0 m. However, because of the uncertainty on the time interval, the reliability on the shear wave velocity is too low if the seismic measurements are conducted every 0.5 m.

Shear modulus for the silty soil

Accurate determination of the shear wave velocity, Vs , is important since the shear wave velocity is squared in order to calculate Gmax (equation 1), and inaccuracies are subsequently magnified. The mean and standard deviation of shear wave velocity and shear modulus for one of the SCPTs with seismic measurements conducted every 0.5 and 1.0 m are shown in Figure 13. These data Vol. 21 [2016], Bund. 15 4775 illustrate that there is greater uncertainty in the shear modulus generated every 0.5 m than the results collected every 1.0 m. The shear wave velocity for the silty soil fall between 150 and 250 m/s, whereas the shear modulus is about 50-100 MPa.

Figure 13: Mean (black lines) and standard deviation (grey lines) of the shear wave velocity and shear modulus for one of the SCPTs with seismic measurements conducted every 0.5 and 1.0 m. CORRELATION BETWEEN SHEAR WAVE VELOCITY AND CONE RESISTANCE There is a correlation between shear wave velocity and cone resistance for the Dronninglund silt, as shown when the mean shear wave velocity at each depth for all five SCPTs (e.g. Figure 1) is plotted against cone resistance (Figure 2b). The best fit for the Dronninglund silt is a power function given in Equation (3).

0.428 Vs = 99.45qt ( qt in MPa) (3)

Vol. 21 [2016], Bund. 15 4776

The cone resistance used in Equation (3) is as previously mentioned from a standard CPT where no seismic tests were conducted. This potential variability of subsoil is assessed negligible since the geological formation is identical for the test area. Several researchers have proposed correlations between cone resistance and shear wave velocity for silts and intermediate soils (Table 2). How well those correlations fit the Dronninglund silt is shown in Figure 14. The expression suggested by Prakoso (2010) is not included in Figure 14 as it makes use of qc instead of qt , however if the difference between qc and qt is assumed to be negligible, (because it is silt) the correlation from Prakoso (2010) is most similar to Equation (3).

Table 2: Correlation between Vs and qt for silty soils, from this and other studies Reference Correlation Silt type 037 Prakoso 2010 Vs =111.21⋅ qc ( qc in MPa) Silt-clay residual soil 0.627 Trevor et al. 2010 Vs =1.75 ⋅ qt ( qt in kPa) Clayed silt soils 0.319 −0.0466 Trevor et al. 2010 Vs =12.02 ⋅ qt ⋅ f s ( qt in kPa) Sandy silt soils Silts – silt mixtures. Transitional Tonni and Simonini (2013) V =104.1⋅ q ( q in MPa) s t t soils 0.428 Equation 3 (current study) Vs = 99.45qt ( qt in MPa) Sandy silt with clay stripes

Figure 14: Correlation between Vs and qt for silty soils. Vol. 21 [2016], Bund. 15 4777

CONCLUSION Five Seismic Cone Penetration Tests (SCPT) were conducted at a test site in Denmark to examine the shear wave velocity in silty soil. At each depth a minimum of eight seismic tests were conducted in order to examine the reliability of individual event data as well as to examine the potential impacts of the time interval between ending the CPT and initiating the seismic measurements. Two of the SCPTs were conducted approximately every 0.5 m down-hole. It was found that, although the silty soil is inhomogeneous and it would presumably provide less variable data to minimize the distance between each measurement, the more closely spaced test intervals actually increased variability and therefore significantly reduced the reliability of the shear wave velocity data. This unexpected result revealed that the uncertainty of the time interval is the same regardless of the measurement spacing was 1.0 or 0.5 m. It was also determined that timing of seismic tests initiation relative to CPT termination is not important (e.g., immediately after CPT or waiting 5 min) since the seismic measurements are unaffected by the dissipation process. Shear wave velocity was computed using the cross-over method, cross-correlation method and the cross-correlation “trimmed with window” method. Even though cross-correlation “trimmed with 2 window” resulted in a higher R it did not increase the reliability of the data. The cross-over method provided the most reliable results because it only uses a single cross-over point, giving an advantage in inhomogeneous media, such as silty soil. A disadvantage of the cross-over method, however, is that it is operator dependent and therefore may vary with the experience of the individual collecting the data. Consequently, it should be used with caution. Finally, a correlation between shear wave velocity and cone resistance was established for the Dronninglund silt, and compared to correlations found by other researchers for silty soil.

ACKNOWLEDGMENTS The project is funded by DONG Energy and associated with the EUDP program “Monopile cost reduction and demonstration by joint applied research” funded by the Danish energy sector. The funding is sincerely acknowledged.

REFERENCES Areias, L., and Impe, W. V. (2004) “Interpretation of SCPT data using cross-over and cross- correlation methods”, Engineering geology for infrastructure planning in Europe: A European perspective. Springer, Berlin, Heidelberg, pp. 110-116. Butcher, A. P., Campanella, R. G., Kaynia, A. M., and Massarsch, K.R. (2005) “Seismic cone downhole procedure to measure shear wave velocity – a guideline”, Report of Technical Committe TC10, Geophysical Testing, International Society of Soil Mechanics & Geotechnical Engineering, ISSMGE 5. Campanella, R.G., Baziw, E.J. and Sully, J.P. (1989) “Interpretation of seismic cone data using digital filtering techniques”, Proceedings of 12th International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro, Brazil, Vol. 1, pp. 195-199. Campanella, R. G., Robertson, P. K., & Gillespie, D. (1986) “A seismic cone penetrometer for offshore applications”, Oceanology. Springer Netherlands, pp. 479-486. Vol. 21 [2016], Bund. 15 4778

Campanella, R.G., and Steward, W. P., 1992, “Seismic cone analysis using digital signal processing for site characterization”, Canadian Geotechnical Journal, Vol. 29(3), pp. 477-486. Gadeikis, S., Dundulis, K., Žaržojus, G., Gadeikytė, S., Urbaitis, D., Gribulis, D., Šliaupa, S., and Gabrielaitis, L. (2013) “Correlation between shear wave velocity and cone resistance of Quaternary glacial clayey soils defined by Seismic Cone Penetration Test (SCPT), Lithuania”, Journal of Vibroengineering, Vol. 15(2), pp. 992-998. Hegazy, Y.A., and Mayne, P.W. (2006) “A global statistical correlation between shear wave velocity and cone penetration data”, Site and geomaterial characterization, ASCE, pp. 243-248. Howie, J. A., and Amini, A., 2005, “Numerical simulation of seismic cone signals”, Canadian Geotechnical Journal, Vol. 42(2), pp. 574-586. Karray, M., Lefebvre, G., Ethier, Y., and Bigras, A. (2011) “Influence of particle size on the correlation between shear wave velocity and cone tip resistance”, Canadian Geotechnical Journal, Vol. 48(4), pp. 599-615. Liao, T., and Mayne, P.W. (2006) “Automated post-processing of shear wave signals”, Proceedings of the 8th U.S. National Conference on Earthquake Engineering, San Francisco, CA, USA, 18-22 April 2006, paper no. 460. Long, M., and Donohue, S. (2010), “Characterization of Norwegian marine clays with combined shear wave velocity and piezocone cone penetration test (CPTU) data”, Canadian Geotechnical Journal, Vol. 47(7), pp. 709-718. Lunne, T., Robertson, P.K., and Powell, J.J.M. (1997) “Cone penetration testing in geotechnical practice”, Spon Press, New York, NY, USA.

Mayne, P.W., and Rix, G.J. (1993) “ Gmax − qc relationship for clays”, Geotechnical Testing Journal, Vol. 16(1), pp. 54-60. Mayne, P.W., and Rix, G.J. (1995) “Correlations between shear wave velocity and cone tip resistance in natural clays. Soils and Foundations”, Japanese Society of Soil Mechanics and Foundation Engineering, Vol. 35(2), pp. 107-110. McNeilan, T.W., and Bugno, W.T. (1985) “Cone penetration test results in offshore California silts”, Strength testing of marine sediments: laboratory and in-situ measurements, ASTM STP 883, Edited by R.C. Chaney and K.R. Demars, ASTM, Philadelphia, Pennsylvania, pp. 55-71. Paoletti, L., Hegazy, Y., Monaco, S., and Piva, R. (2010) “Prediction of shear wave velocity for offshore sands using CPT data – Adriatic sea”, Proceedings of 2nd International Symposium on Cone Penetration Testing (CPT’10), Vol. 2, Huntington Beach, CA, USA, 9-11 May 2010, pp. 361-368. Poulsen, R., Ibsen, L.B, and Nielsen, B.N. (2012a) “Difficulties regarding determination of plasticity index of silty soils by use of casagrande and fall cone methods”, Proceedings of Nordic Geotechnical Meeting, Copenhagen, Denmark, 9-12 May 2012. Prakoso, W.A. (2010) “Correlation of cone resistance and shear wave velocity for residual soils” Proceedings of 2nd International Symposium on Cone Penetration Testing (CPT’10), Vol. 2, Huntington Beach, CA, USA, 9-11 May 2010, pp. 425-430. Vol. 21 [2016], Bund. 15 4779

Robertson, P.K. (2009) “Interpretation of cone penetration tests - a unified approach”, Canadian Geotechnical Journal, Vol. 46(11), pp. 1337–1355. Robertson, P.K., Campanella, R.G., Gillespie, D., Grieg, J. (1986), “Use of piezometer cone data”, Use of In Situ Tests in Geotechnical Engineering, Proceedings of In Situ ’86, ASCE, Blacksburg, VA, USA, pp. 1263-1280. Robertson, P.K., Campanella, R.G., Gillespie, D., Rice, A. (1986), “Seismic CPT to measure in-situ shear wave velocity”, Journal of Geotechnical Engineering, Vol. 112(8), pp. 791- 803 Robertson, P.K., Sasitharan, S., Cunning, J.C., and Segs, D.C. (1995) “Shear wave velocity to evaluate flow liquefaction”, Journal of Geotechnical Engineering, Vol. 121(3), pp. 262- 273. Robertson, P.K., and Wride, C.E. (1998) “Evaluating cyclic liquefaction potential using the cone penetration test”, Canadian Geotechnical Journal, Vol. 35(3), pp. 442-459. Sully, J. P., and Campanella, R. G. (1995) “Evaluation of in situ anisotropy from crosshole and downhole shear wave velocity measurements”, Géotechnique, Vol. 45(2), pp. 267- 282. Tonni, L., and Simonini, P. (2013) “Shear wave velocity as function of cone penetration test measurements in sand and silt mixtures”, Engineering Geology, Vol. 163, pp. 55-67. Trevor, F.A., Paisley, J.M., and Mayne, P.W. (2010) “Cone penetration tests at active earthquake sites”, Proceedings of 2nd International Symposium on Cone Penetration Testing (CPT’10), Vol. 2, Huntington beach, CA, USA, 9-11 May 2010, pp. 519-526.

© 2016 ejge

Editor’s note. This paper may be referred to, in other articles, as: Rikke Holmsgaard, Lars Bo Ibsen, and Benjaminn Nordahl Nielsen: “Interpretation of Seismic Cone Penetration Testing in Silty Soil” Electronic Journal of Geotechnical Engineering, 2016 (21.15), pp 4759-4779. Available at ejge.com.