Characterization on the correlation between shear wave velocity and piezocone tip resistance of Jiangsu clays

Engineering Geology 171 (2014) 96–103

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Characterization on the correlation between shear wave velocity and piezocone tip resistance of Jiangsu clays Guojun Cai a, Anand J. Puppala b, Songyu Liu a a b

Institute of Geotechnical Engineering, Southeast University, Nanjing, Jiangsu 210096, China Box 19308, Department of Civil Engineering, The University of Texas at Arlington, Arlington, TX 76019, United States

a r t i c l e

i n f o

Article history: Received 5 November 2012 Received in revised form 26 December 2013 Accepted 29 December 2013 Available online 9 January 2014 Keywords: Soft clays In situ testing Shear wave velocity Cone tip resistance

a b s t r a c t The small strain shear modulus of a soil is a fundamental parameter related to the mechanical behavior used in evaluation of dynamic behavior and seismic design of geotechnical structures. The Jiangsu soft clay is a lightly overconsolidated and sensitive clay of high plasticity in nature. A research database of piezocone penetration test (CPTU) and shear wave velocity, Vs, information for Jiangsu soft clays has been collected to study the small strain shear modulus relationships for these soils and to examine the potential use of CPTU and Vs data in combination for the purposes of characterizing these soils. Test data for sites are based on the laboratory testing performed on thin-wall tube samples and high-quality block samples. Improvements have been suggested to existing correlations between the small strain shear modulus, Go, or Vs and index properties for these soils for better prediction of these soil properties. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The piezocone penetration test (CPTU) is a standard in situ test method utilized in the construction projects for geotechnical site characterization works. In recent years, the CPTU has been fitted with sensors to provide measurements of shear wave velocity, Vs and this new device is termed as seismic CPTU (SCPTU) (Campanella et al., 1986; Lunne et al., 1997; Cai et al., 2010, 2011a). The SCPTU has become the most used in situ method for the geotechnical investigations for determining soil properties at small to large strain levels (Mayne, 2007). Shear wave velocity is typically measured in situ by a variety of geophysical test methods, such as crosshole testing (CHT), downhole testing (DHT), suspension logger probing (SLP), surface refraction survey (SFRS), surface reflection survey (SFLS), and spectral analysis of surface waves (SASW) (Robertson et al., 1986; Campanella and Stewart, 1992; Juang et al., 2008; Long, 2008; Long and Donohue, 2010; Ku et al., 2013). The SCPTU is a variant of the DHT method, and the measured shear wave velocity seems to be relatively independent of the technique used and of the operator. Powell and Lunne (2005a), Boylan et al. (2008), Long (2008) and Tiggelman and Beukema (2008) have noted that for CPTU soundings in soft clays, if the pore pressure measurement system is sufficiently well saturated, then the measured pore pressure (u2) parameter is not affected by the equipment variability. Long and Donohue (2010) demonstrated that the corrected piezocone resistance, qt, values show minor variation from one type of equipment to another as compared with u2 in Norwegian marine clays. They noted that the measured E-mail addresses: [email protected] (G. Cai), [email protected] (A.J. Puppala), [email protected] (S. Liu). 0013-7952/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enggeo.2013.12.012

sleeve friction, fs, shows the most variation from one type of equipment to another and these values should be treated with caution. Since CPTU's u2 (and possibly qt) and the shear wave velocity, Vs are two of the reliable parameters that can be obtained from in situ testing, it seems logical then to attempt to use them in combination for the purposes of characterizing and classifying soft clays (Robertson, 2009; Ku et al., 2010; Cai et al., 2011b; Sun et al., 2013). Analysis of the diversity and complexity of soils makes for a difficult task because of their many geologic origins, ages, constituents, grain sizes, mineralogy, fabrics, and histories. Therefore, parameters interpreted from CPT, CPTU or shear wave velocity tests need to be correlated to each other. The objective of the state of knowledge presented herein is not to cover every study performed on CPT, CPTU, SCPTU or Vs measurement tests. Only the elements relevant to the goals of this study, i.e., a comparison between the results generated in clay deposits previously listed, are mentioned herein. In this paper, data from seven soft to firm clay sites are collected and analyzed to investigate the potential use of CPTU and Vs data in combination for the purposes of characterizing these soils. For all these sites, high-quality CPTU and SCPTU data were available (Liu et al., 2008, 2011). In addition, results of laboratory tests on thin-wall tube samples and block samples were available for each site. Analyses of these results are attempted to validate existing relationships between Vs and qt as well as develop new correlations between the same parameters. This paper provides a comprehensive summary of these results. 2. Site descriptions In this study, the seven sites were chosen which contain sensitive clay deposits. All these sites are located in Jiangsu Province of eastern

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(Fear and Robertson, 1995). However, for a given measurement type, the function that expresses the effective stress effect is relatively constant from one study to another and the qc, qt or Vs values used in the different empirical relationships are first normalized for the same effective stress (100 kPa) to eliminate the effect of this variable (and therefore of the depth). The normalized test values then essentially reflect the void ratio or soil density. The expressions currently used to standardize or correct test results for the effective stress due to overburden pressure are (Robertson and Wride, 1998; Wride et al., 2000): qc1 ¼ qc C Q

Fig. 1. Location map for Jiangsu clay CPTU sites.

China (see Figure 1). These clay deposits are located at Nanjing, Lianyungang, Changzhou, Yancheng, Suzhou, Taizhou and Yangzhou, respectively. A summary of the seven sites is also given in Table 1. In each of the investigated sites, the high quality samples were taken at different depths that corresponded to the depths where shear velocity measurement was made. Tube sample of 76 mm diameter was collected from boreholes, using stainless steel fixed-piston tube samplers below ground level. Once the fixed-piston sampler was withdrawn from the borehole, the soil sample at the end of the tube was excavated for waxing sealing at both ends. The area ratio of the tube samples used in this paper is 11%. The laboratory testing program included basic soil characterization tests such as water content, unit weight, Atterberg limits, grain size distribution, and specific gravity. Soil parameters for the seven study sites, over the depth range for which shear wave velocity and high-quality sample data are available, are summarized in Table 2. 3. Test results and analysis

ð1Þ

where qc1 (MPa) is the cone tip resistance normalized for the overburden stress (100 kPa), qc (MPa) is the cone tip resistance measured in the CPT test, CQ = (Pa/σv0′)n is a correction for overburden stress with a maximum value of 2, and n is the stress exponent that varies with the soil type and ranges from 0.5 in sandy soils to 1.0 in clay soils (Robertson, 2012). It should be noted that Pa is an atmospheric pressure (100 kPa) and σv0′ is the vertical effective stress (kPa). Although cone penetration resistance is often corrected for overburden stress (resulting in the term qc1), the truly normalized cone penetration resistance normalized for overburden stress (qc1N, dimensionless) is given by (Robertson and Wride, 1998)
n ′ qc1N ¼ ðqc =P a2 Þ  P a =σ v0 ¼ qc1 =P a2

ð2Þ

where Pa is a reference pressure in the same units as qc (i.e., equal to 0.1 MPa for qc or qc1 in MPa).

F¼

fs  100% qc −σ v0

ð3Þ

where F is the standardized friction ratio, fs is the sleeve friction measured during penetration in the CPT, and σv0 is the total vertical stress (Robertson, 1990);
0:25 ′ V s1 ¼ V s  P a =σ v0

3.1. Effect of initial effective stress Table 3 presents a few correlations between Vs and qc and between Vs and qt. Several important empirical relationships have been developed over the years to determine the in situ properties of cohesionless soil deposits including the void ratio, relative density, grain shape, and age factor from qc–CPT (Schmertmann, 1976; Jamiolkowski et al., 1985, 2001; Baldi et al., 1986; Kulhawy and Mayne, 1990; Tanizawa et al., 1990), qt–CPTU (Lunne et al., 1997; Mayne, 2007; Robertson, 2009; Schnaid, 2009), and Vs (Hardin and Richard, 1963; Robertson et al., 1995). The in situ measurements can be influenced by a number of factors that are probably interrelated, such as compressibility, particle distribution, mineralogy, and grain shape. Silts are really very fine sands and hence some of the general sand correlations might have application to silts and clayey silts. For a given uncemented soil of Holocene age (b10,000 years), these relationships show that the qc, qt or Vs values depend mostly on the void ratio (or density index) and state of effective stress and compressibility

ð4Þ

where Vs is the shear wave velocity measured and Vs1 is the shear wave velocity normalized for the vertical effective stress. 3.2. Correlations between Go and e or w Long and Donohue (2007) attempted to relate Go to natural water content, w, or in situ void ratio, e0, for Norwegian clay sites. Note that Go is directly related to Vs by 2

Go ¼ ρV s

ð5Þ

where ρ is the mass density of the soil. Here, data for seven additional sites is included in an attempt to improve these correlations and investigate which of the index parameters are the most useful. The overall objective of this study is to check that

Table 1 Summary of sites surveyed. Site location

Soil type

Geological origin

Vs measured by

References

Lianyungang Nanjing Yancheng Suzhou Changzhou Taizhou Yangzhou

Soft clay Silty clay Soft clay Soft clay Firm clay Silty clay Soft clay

Marine Backswamp Lagoonal Alluvial and lacustrine Alluvial and lacustrine Alluvial and lacustrine Lagoonal

SCPTU SCPTU, SASW SCPTU, DH SCPTU, CH SCPTU, DH SCPTU, DH SCPTU, SASW

Liu et al. (2008), Liu et al. (2011) Cai et al. (2011a) Cai et al. (2010) Cai et al. (2011b) Cai et al. (2006), Cai et al. (2011b) Cai et al. (2011a) Cai et al. (2010), Cai et al. (2011b)

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G. Cai et al. / Engineering Geology 171 (2014) 96–103

Table 2 Basic information of the investigated sites. Site name

Depth range (m)

γ (kN/m3)

w (%)

wL (%)

Ip

Gs

e

OCR

Ir

Su (kPa)

σv0 (kPa)

σv0′ (kPa)

qt (MPa)

qc1 (MPa)

Ic

LYG 1 LYG 2 LYG 3 LYG 4 NJ 1 NJ 2 NJ 3 NJ 4 NJ 5 NJ 6 YC 1 YC 2 YC 3 YC 4 YC 5 SZ 1 SZ 2 SZ 3 SZ 4 SZ 5 CZ 1 CZ 2 CZ 3 CZ 4 CZ 5 TZ 1 TZ 2 TZ 3 TZ 4 TZ 5 YZ 1 YZ 2 YZ 3 YZ 4 YZ 5

2.0–10.5 3.0–16.8 2.2–14.4 1.4–15.0 2.6–13.4 2.4–23.0 3.0–13.8 5.2–17.6 4.0–15.0 5.0–16.6 1.4–12.6 2.2–13.8 4.4–16.2 1.6–12.0 2.2–12.6 2.0–17.0 2.0–18.3 2.1–13.0 3.5–14.0 1.6–19.4 1.5–18.8 2.0–17.8 1.1–14.8 1.8–16.5 2.1–19.0 1.4–13.5 1.8–19.0 2.4–17.8 2.2–18.9 1.9–16.4 1.5–15.5 1.4–16.2 1.2–18.9 2.3–17.8 1.9–16.4

15.3 16.0 16.8 15.7 17.3 17.6 17.5 17.5 17.4 17.8 16.5 17.8 18.9 18.6 16.9 16.4 16.7 16.5 16.6 16.5 19.4 19.7 19.9 19.5 19.8 17.5 17.6 17.8 17.7 17.6 17.5 18.0 18.2 17.9 18.1

74.2 75.5 76.3 73.4 41.1 42.6 42.2 43.7 44.4 42.6 56.1 57.3 56.3 57.6 58.3 64.7 69.9 65.2 67.4 68.2 23.4 24.3 25.6 23.2 25.1 23.1 25.4 37.8 40.1 26.7 64.1 65.2 73.7 71.1 70.5

63.3 64.2 64.3 65.5 37.1 38.5 39.1 35.7 37.8 38.9 23.1 25.3 24.5 27.3 26.4 55.6 56.2 57.8 56.2 55.4 38.4 38.2 39.2 38.4 39.1 27.5 32.7 34.1 29.2 33.1 58.1 59.0 60.2 58.7 60.2

23 24 27 26 16 17 17 15 16 17 24 22 23 21 23 17 20 18 19 18 18 19 18 19 18 8 10 13 11 12 16 17 18 18 16

2.75 2.75 2.74 2.76 2.70 2.69 2.71 2.70 2.69 2.70 2.75 2.75 2.76 2.74 2.73 2.73 2.73 2.72 2.74 2.73 2.73 2.74 2.72 2.73 2.74 2.71 2.70 2.72 2.71 2.72 2.72 2.73 2.71 2.72 2.70

2.1 2.3 2.2 2.4 1.1 1.3 1.2 1.4 1.5 1.6 1.6 2.2 2.0 1.8 1.9 1.5 1.6 1.7 1.6 1.8 0.6 0.7 0.6 0.8 0.7 1.4 1.0 1.3 1.2 1.1 0.9 1.2 1.5 1.3 1.4

0.8 0.9 1.3 1.7 1.0 2.3 3.4 3.3 2.8 1.7 0.9 2.1 1.5 2.4 1.3 1.1 4.6 1.2 2.3 1.9 1.0 3.4 5.8 4.5 2.7 1.0 10.6 7.8 5.4 3.2 1.5 3.6 8.7 5.7 4.8

200 180 220 178 50 45 60 50 58 43 150 148 153 140 160 100 90 120 100 110 50 48 55 45 50 35 36 38 32 35 100 110 98 105 100

16 20 18 25 36 40 43 45 44 37 28 32 30 34 31 32 33 32 34 33 44 48 52 50 46 56 69 73 71 58 38 39 40 38 40

112.5 134.7 121.0 86.5 57.0 49.1 63.2 114.6 81.0 110.2 35.8 55.8 94.3 33.3 43.2 123.5 250.3 240.0 66.0 44.0 58.4 143.5 257.9 96.8 140.2 154.3 234.1 157.8 221.1 289.2 145.2 256.1 178.2 198.2 220.1

87.0 73.2 68.0 50.5 27.0 37.1 59.2 72.6 47.0 66.7 24.3 41.3 51.8 27.8 29.2 93.5 125.3 117.1 63.5 34.0 12.34 78.9 156.2 48.9 78.3 78.2 128.3 78.9 148.1 167.3 77.0 137.2 97.1 102.1 98.2

0.15 0.18 0.2 0.35 1.5 1.5 1.6 1.7 1.4 1.6 0.5 0.4 0.6 0.6 0.3 1.3 1.4 1.8 1.2 1.0 1.8 1.4 1.5 1.6 1.9 1.2 1.3 1.4 1.8 1.4 1.6 1.5 1.4 1.5 1.9

4.1 5.2 3.4 6.5 15 12 13 14 12 11 8 9 7 13 10 10 11 8 9 12 18 19 20 21 18 16 21 25 27 23 14 13 14 16 12

3.7 3.9 3.8 4.1 2.7 2.6 2.8 2.7 2.6 2.7 3.6 3.5 3.7 3.6 3.5 3.8 3.9 3.9 4.0 3.7 2.3 2.2 2.3 2.2 2.1 2.5 2.6 2.4 2.5 2.7 3.7 3.6 3.5 3.7 3.6

γ, unit weight; w, water content; wL, liquid limit; Ip, plasticity index; Gs, specific gravity; e, void ratio; Su, undrained shear strength; OCR, overconsolidation ratio; Ir, rigidity index; σv0, vertical stress; σv0′, vertical effective stress; qt, corrected cone tip resistance;qc1, normalized cone tip resistance; Ic, soil behavior type index.

these soils fall into the framework well established for other materials and also allow engineers working on future projects to make rapid estimates of Go for preliminary design or for verification of in situ or laboratory measurements. Soil stiffness depends on interactions of state (bonding, fabric, degree of cementation, stress level), strain level (and the corresponding effects of destructuration), stress history and stress path, timedependent effects (aging and creep) and type of loading (monotonic or dynamic) (Hardin, 1978). Despite these complex interactions, the characteristic response of clay with respect to small strain modulus

and stiffness non-linearity can be directly assessed from in situ crosshole and downhole tests (Mayne et al., 2009; Schnaid, 2009; Sun et al., 2013). As for the cone tests, cone tip resistance is insensitive to most of these factors and correlations between cone tip resistance and soil stiffness are unreliable (Schnaid, 2009; Chittoori and Puppala, 2011; Puppala and Chittoori, 2012). Assessment of soil stiffness is not a matter of simply selecting appropriate correlations. In fact, it is a demanding area that requires a thorough knowledge of material behavior. To investigate the effects of void ratio and effective confining stress, a series of torsional vibration tests was conducted by Hardin and Black

Table 3 Correlations between qc (CPT), qt (CPTU) and shear wave velocity, Vs. Authors

Correlation

Soil

Notes

Jamie and Romo (1988) Bouckovalas et al. (1989) Rix and Stokoe (1991)

Vs = 0.1qc Go = 2.8q1.4 c  −0:75
 Go c ¼ 1634 pqffiffiffiffiffiffi ′ q

Mexico City clays Greek clays Mortar sand and sand from Heber road research site in California

Go and qc in kPa. Go and qc in kPa. Go, σv0′ and qc in kPa.

Robertson et al. (1992) Fear and Robertson (1995) Karray et al. (2011) Mayne and Rix (1993) Mayne and Rix (1993)

Vs1 = 102q0.23 c1 Vs1 = 135q0.23 c1 Vs1 = 149q0.205 c1 Go = 2.78q1.335 c

Peribonka data Drammen and Onsøy Strong dependence of Go upon e0

Vs1 in m/s and qc1 in kPa. Vs1 in m/s and qc1 in kPa. Vs1 in m/s and qc1 in kPa. Go and qc in kPa. pa and qc in kPa.

Reduce scatter the correlation should be between qc and Vs

qc in kPa.

Norwegian soft clays

qt in kPa.

Venice lightly over-consolidated mixed deposits

−u0 ¼ qΔu Bq ¼ qu2−σ v0

c

Mayne and Rix (1995) Long and Donohue (2010) Simonini and Cola (2000) Long and Donohue (2010)

average

σ v0

99:5p0:305 q0:695 a c e1:13 0 0.627 Vs = 1.75qc Vs = 9.44q0.435 e−0.532 c 0 Vs = 2.944q0.613 t Vs = 65.00q0.150 e−0.714 t 0 Go = 21.5q0.79 (1 + Bq)4.59 t

Go ¼

(1 + Bq)2.53 Go = 4.39q1.225 t Vs = 1.961q0.579 (1 + Bq)1.202 t

t

Norwegian soft clays

net

qt in kPa. −u0 ¼ qΔu Bq ¼ qu2−σ v0 t

qt in kPa.

net

G. Cai et al. / Engineering Geology 171 (2014) 96–103

(a)

(b)

(c)

(d)

99

Fig. 2. Relationship between (a) Go normalized by σv0′ and void ratio, e, and (b) Go normalized according to Hardin (1978) and Hight and Leroueil (2003) and e.

(1968) on remolded specimens of kaolinite clay with a plasticity index of Ip = 21. The specimens were consolidated normally to specified effective confining stress and then put to a state of resonance in torsional vibration. It can be concluded that, for normally consolidated clays, the small strain shear modulus at a given confining stress tends to decrease with increase in void ratio. Hardin and Black (1968) and Hardin (1978) identified various factors governing the Go value and proposed a general expression:
 ′ Go ¼ f σ v0 ; eo ; OCR; Sr ; C; K; T

ð6Þ

where σv0′ is the vertical effective stress, eo is the initial void ratio, OCR is the overconsolidation ratio, Sr is the degree of saturation, C is the grain characteristics, K is the soil structure, and T is the temperature. Hardin (1978) suggested that, for reconstituted clays, Go depends on the in situ (or applied) stress (σ′), e, and overconsolidation ratio (OCR) by laboratory resonant column tests. It has, however, been shown that the effects of OCR are, to a large extent, taken into account by the effect of e and could be neglected (Leroueil and Hight, 2003). An empirical equation to describe the influence of the controlling factors on Go can be written as follows:
n ′ ′ ð1−2nÞ Go ¼ SF ðeÞ σ v0 σ h0 P a

ð7Þ

where S is a dimensionless “structure” parameter characterizing the considered soil, F(e) is a void ratio function. F(e) can take the form of (1/e), (1 + e), 1/(e − emin) or (2.17 − e)2/(1 + e), σv0′ and σh0′ are

the vertical and horizontal effective stresses, respectively, n is a parameter indicating the influence of stress, and Pa is the atmospheric pressure (100 kPa). Further research shows that F(e) has a very different mathematically function (Pestana and Whittle, 1995; Jefferies and Been, 2000). As can be seen in Fig. 2a, Go/σv0′ typically varies between 200 and 900 and as expected, Go/σv0′ decreases with increasing e in a similar manner to that described by others, e.g., Jamiolkowski et al. (1991), for a variety of soils. In Fig. 2b, the data have been normalized as suggested by Hardin (1978) and Hight and Leroueil (2003) (Eq. (7)). A line has been added corresponding to S = 700, F(e) = 1/e1.3, K0 = 0.6, and n = 0.25. It can be seen that the fit is fairly good, confirming that Go values for Jiangsu clays are consistent with a large volume of other published experimental data. Fig. 2c and d gave the effect of OCR and pore pressure parameter Bq within the reported Go/σv0′ data. 3.3. Correlations between qt and Vs As discussed by Mayne and Rix (1993) and others, Go depends on e0, σv0′, and OCR. As measured cone resistance (qc) also depends on σv0′ and OCR, previous researchers have sought a relationship between Go and qc despite the fact that they are operable at different ends of the strain spectrum. Mayne and Rix (1993) summarized site-specific correlations between Go or Vs and qc. For example, Jaime and Romo (1988) and Bouckovalas et al. (1989) found that, for Mexico City clays and Greek clays, respectively, V s ¼ 0:1qc

ð8Þ

100

G. Cai et al. / Engineering Geology 171 (2014) 96–103 1:4

Go ¼ 2:8qc :

ð9Þ

Mayne and Rix (1993) established a database from 31 different sites in Europe and North America, where CPT and SASW or seismic cone penetration test data were available. All were clay sites with varying OCRs, strengths, and stiffnesses. Two of the sites were the same as used in this study, namely Drammen and Onsøy. The equation of the best-fit regression line from an assumed log–log relationship was found to be 1:335

Go ¼ 2:78qc

ð10Þ

which is very similar to the expression derived by Bouckovalas et al. (1989) (see Eq. (9)). Mayne and Rix (1993) also found that the strong dependence of Go upon e0, however, requires that qc be only successful as a profiler of Go when e0 is included in the correlation, and they derived empirically the formula

4. New correlations for Jiangsu clay database

99:5p0:305 q0:695 a c Go ¼ 1:13 e0

ð11Þ

where pa and qc are in units of kPa. In a later paper, Mayne and Rix (1995) argued that to reduce scatter the correlation should be between qc and Vs, as these are both directly measured parameters. In the earlier study (Mayne and Rix, 1993), Go had to be calculated from Vs using Eq. (5). Mayne and Rix (1995) derived the empirical formulae 0:627

ð12Þ

0:435 −0:532 e0 :

ð13Þ

V s ¼ 1:75qc V s ¼ 9:44qc

As there was only one small change in the resulting correlation coefficient (associated with Eqs. (11) and (13)), Powell and Lunne (2005b) suggested that Eqs. (11) or (13) are only slightly better than the simpler ones based only on qc. Another important issue with both Mayne and Rix (1993, 1995) equations is that they make use of the uncorrected cone resistance, qc, rather than the corrected value, qt. Mayne and Rix (1993, 1995) used a non-piezocone to obtain qc. There is no correction for pressures acting behind the cone tip. As the reconstruction of the in situ void ratio profile can be a difficult task, particularly given the cost of high-quality undisturbed sampling, Simonini and Cola (2000) suggested that the CPTU pore pressure parameter Bq could be used to replace e0 in the correlation. The standard definition of Bq (Lunne et al., 1997) is Bq ¼

Fig. 3. qt versus Vs for Jiangsu soft clay database.

u2 −u0 Δu ¼ qt −σ v0 qnet

Data for the seven Jiangsu soft clay sites, plotted simply in terms of qt and Vs, are shown in Fig. 3. To permit later normalization or correlation versus index properties, each data point represents a single high-quality sample. The best fit power correlations is shown 0:403

V s ¼ 7:954qt

:

ð17Þ

Regression analysis gives a moderate R2 of 0.631. Those data that show the greatest scatter are from Yangzhou, where OCR values are relatively high, and Taizhou, where sensitivity, St, is high. As illustrated in Table 3, the form of the empirical relationships established between Vs and qt varies somewhat from one study to another. A purely statistical analysis of the Jiangsu soft clay database led to another relationship (Vs1 = 38q0.61 c1 ) with a form that probably differs from those in Table 3 and could be valid for soils similar to those encountered in Jiangsu area (see Figure 4). Robertson, Wride, and their collaborators have presented the results for the studies (Robertson et al., 1992; Fear and Robertson, 1995; Wride et al., 2000) which led to Vs–qc relationships having simple and identical forms, i.e., a

1=a

V s1 ¼ Y ðqc1 Þ or qc1 ¼ ðV s1 =Y Þ

where Y is a constant determined from the experimental data. For the above-mentioned studies (Robertson et al., 1992; Fear and Robertson, 1995; Wride et al., 2000), exponent a ranges from 0.23 to 0.25 and exponent 1/a ranges from 4 to 4.35. In the paper presented

ð14Þ

where u0 is the in situ equilibrium pore pressure and σv0 is the total overburden stress. When considering relatively lightly overconsolidated mixed deposits from Venice, an appropriate correlation between qt and Go was obtained when incorporating Bq as follows: 0:79

Go ¼ 21:5qt


4:59 1 þ Bq :

ð15Þ

Long and Donohue (2010) proposed a better correlation (R2 = 0.799) between qt and Go when incorporating Bq for Norwegian marine clays as follows: 1:225

Go ¼ 4:39qt




1 þ Bq

2:53

:

ð16Þ

ð18Þ

Fig. 4. Vs1 as a function of qc1 for Jiangsu clays.

G. Cai et al. / Engineering Geology 171 (2014) 96–103

for the interpretation of in situ test results from the CANLEX sites, Wride et al. (2000) ended up using 0.25 for the exponent a. Measured Vs values and those predicted by the original Mayne and Rix (1995) expression (Eq. (13)) are shown in Fig. 5a. It can be seen that in general the Mayne and Rix (1995) expression predicts Vs for Jiangsu soft clays by some ±10%. Note that here e0 has been reliably determined from high-quality block samples. The correlation coefficient, R2, is 0.597, which, consistent with the comments made by Powell and Lunne (2005b), is not a significant improvement to that derived from the simple Vs–qt relationship. The data points that show the most scatter are again from the high OCR Yangzhou site and the high sensitivity Taizhou site. The relationship can be improved using multiple regression analysis, as shown in Fig. 5b, which resulted in the following improved formulation: 0:101 −0:663 2 e0 with R

V s ¼ 90qt

¼ 0:794:

101

(a)

ð19Þ

A similar exercise has been carried out using the Simonini and Cola (2000) formula (Eq. (15)) and Long and Donohue (2010) formula (Eq. (16)), shown in Fig. 6a and b. Here, Go has been calculated from the measured Vs value from SCPTU and the density measurements from laboratory tests on samples. It can be seen that a much better correlation with coefficient of determination R2 value of 0.847 (compared

(b)

(a)

(c)

(b)

Fig. 6. Vs measured and predicted from (a) original Simonini and Cola (2000) expression, (b) Long and Donohue (2010) expression and (c) modified expression for Jiangsu soft clays. Fig. 5. Vs measured and predicted from (a) original Mayne and Rix (1995) expression and (b) modified version of this expression.

102

G. Cai et al. / Engineering Geology 171 (2014) 96–103

with 0.663 and 0.718) can be achieved by modifying the constants in the expression and using Bq in Fig. 6c. The resulting expression is: 0:31

Go ¼ 30:1qt


3:14 1 þ Bq :

ð20Þ

Logically, a new expression can be developed that relates Vs directly with qt and Bq as follows and as shown in Fig. 7. This relationship yields an R2 value of 0.825 for the Jiangsu soft clays. 0:487

V s ¼ 4:541qt


0:337 1 þ Bq

ð21Þ

An issue with the most commonly used correlation by Mayne and Rix (1995) is that it relies on the measured cone resistance (qc) rather than the corrected one (qt). It is well known that in soft clays the correction can be significant, perhaps of the order of 15% as noted by Long and Donohue (2010). Secondly, it also relies on the in situ void ratio (e0) as an input parameter. This parameter can be difficult to estimate as it is highly susceptible to sampling disturbance. Hence, in this paper a database comprising high-quality samples and CPTUs has been assembled to minimize these uncertainties and improve the Mayne and Rix (1995) correlation for use in Jiangsu soft clays. Unfortunately, this new correlation (Eq. (19)) also relies on e0 as input. This parameter is not always readily available, especially at an early stage in the investigation, as sampling and laboratory testing tasks are required to estimate this parameter. Therefore, two additional correlations have been proposed for these materials that do not need laboratory data as input. The first, which involves the pore water pressure parameter (Bq), is a modification of the Simonini and Cola (2000) expression (Eq. (20)) and the second (Eq. (21)) is a new expression that relates qt and Bq directly to Vs rather than to Go. All three formulae have similar correlation coefficients and are considered equally reliable. 5. Conclusions A database of high quality CPTU and shear wave velocity data for Jiangsu soft clays has been assembled to study the small strain modulus relationships for these materials and examine the potential use of CPTU and Vs data in combination for the purposes of characterizing these soils. In general, the small strain modulus behavior of Jiangsu soft clays follows the framework published for other soils. It is possible to get satisfactory estimates of Go using correlations with void ratio (e). It would seem that the influence of overconsolidation ratio on Go is not completely taken into account through normalization by void ratio. A purely statistical analysis of the Jiangsu soft clays database led to another relationship (Vs1 = 38q0.61 c1 ), where Vs1 is in m/s, qc1 in MPa. Reasonable estimates of Vs can be obtained from correlation with CPTU qt using

Fig. 7. Vs measured and predicted from the new expression (Eq. (15)) involving qt and Bq.

modified versions of the Mayne and Rix (1995) or Simonini and Cola (2000) formulae or from a new expression involving qt and Bq. It would seem that use of Bq as a substitute for e0 leads to an improvement in the predictions for Jiangsu soft clays. This relationship was found identical for all practical purposes, whether it is determined only from the Jiangsu Province database of China. Hence, it should be noted that the results of our study are most applicable to the Jiangsu Province of China. However, such an evaluation is difficult and frequently affected by a degree of inaccuracy. Using different investigation techniques is then useful, especially if reliable correlations exist between the results of the different types of investigation. Acknowledgments Majority of the work presented in this paper was funded by the National Natural Science Foundation of China (grant no. 41202203), the “Twelfth Five-Year” National Science and Technology Support Plan (grant no. 2012BAJ01B02), the New Century Excellent Talents of China (NCET-13-0118) and the Foundation for Excellent Young Teachers (grant no. 3221003202). The authors are grateful to Dr. Liyuan Tong and Guangyin Du for assistance with CPTU truck used, and for their discussions on the field data analysis. The first author received financial support from the China Scholarship Council to attend the University of Texas at Arlington as a visiting research scholar under the guidance of Dr. Anand J. Puppala. The comments provided by Dr. Yanjun Du are greatly appreciated. The authors would like to express their appreciations to the editor and anonymous reviewer for their valuable comments and suggestions. 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