# Evaluation Method for the Liquefaction Potential Using the Standard Penetration Test Value Based on the CPTU Soil Behavior Type Index

Authors: Guangyin Du; Changhui Gao; Songyu Liu; Qian Guo; Tao Luo
Journal: Advances in Civil Engineering
 (2019)

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## Abstract

Taking the project of the Su-xin highway treated by using the resonant compaction method as the reference, a new method for the evaluation of liquefaction potential is proposed based on the piezocone penetration test (CPTU) and the standard penetration test (SPT). The soil behavior type index (Ic) obtained from CPTUs and the standard penetration test index (N63.5), obtained from SPTs, are analyzed for saturated silty sand and silt. The analysis result reveals a linear relationship between N63.5 and Ic, given by N63.5=−18.8Ic+52.0. The larger the value of Ic is, the greater the viscosity of soil is, and the smaller the value of N63.5 is. According to the method, liquefaction assessment of saturated silty sand and silt foundation can be conducted by using N63.5 based on the Code of Seismic Design of Building. N63.5 is expressed by a single Ic, which is calculated from the CPTU data. Compared with existing evaluation methods, this method can provide continuous standard penetration test values, moreover, this method involves a simple calculation, and the results obtained using the method are reliable.


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## Body

## 1. Introduction

Liquefaction-induced failure of earth structures, such as road embankments and earth dams, is identified as one of the most dramatic threats of earthquakes [1–3]. Liquefaction evaluation is essential in potentially liquefiable sites [4, 5]. However, liquefaction evaluation is a complex geotechnical engineering task because liquefaction occurrence depends on a large number of factors, such as the mechanical characteristics of the soil layers in the site and the depth of the water table [6]. Therefore, the evaluation of liquefaction potential has attracted widespread attention of geotechnical researchers in the past four decades.At present, there are two major methods for the evaluation of liquefaction potential, namely, laboratory test and in situ test. The in situ test method is widely used because it involves a small disturbance and is relatively good at representing the liquefaction potential. The typical methods of liquefaction evaluation are based on in situ tests, such as the cone penetration test (CPT) and the standard penetration test (SPT), which are preferred by geotechnical engineers to assess liquefaction potential of soils [7, 8]. However, the method for liquefaction evaluation based on the SPT cannot provide continuous parameters of the blow count of the SPT because the measurement is performed every 1.5 m. The evaluation method for liquefaction potential using the CPT is becoming preferable because it provides standardized and reliable data [9]. Through several stages of modification by Seed et al. and others [10–13], more mature and complete implementation of the CPT has been achieved. In this method, liquefaction “loading” is expressed by the CSR (cyclic stress ratio, which represents the cyclic loading on the soil), and the liquefaction “resistance” is expressed by the CRR (cyclic resistance ratio, which is the capacity of a soil layer to resist liquefaction) [14, 15]. In this theory, liquefaction is predicted to occur if CSR ≥ CRR, and no liquefaction is predicted if CRR > CSR. However, the calculation of CSR and CRR is complex and inconvenient to apply in the field.Piezocone penetration test (CPTU) technology, based on the traditional static CPT technology, is a standard and advanced in situ test method utilized in the geotechnical project site and has widely been used in geotechnical engineering because of its high accuracy, good repeatability, and low cost [16, 17]. The CPTU can provide three types of data, namely, the pore pressure (u2), the cone tip resistance (qc), and the sleeve frictional resistance (fs), and the CPTU gives near continuous parameters of a continuous geological section. At present, the soil behavior type index (Ic) calculated by using CPTU data is mainly used to classify soil. Liu et al. [18] established the Chinese soil classification method based on CPTU soil behavior type index by analyzing the CPTU test data of several sample sites in Jiangsu province. Ku and Juang [19] found that, while significant changes in qc, fs, and u2 were observed after the dynamic compaction, the soil behavior type determined with these CPTU parameters largely remained unchanged. Others have done similar researches about Ic [20–22]. In addition, the abovementioned “CRR,” based on the modified cone tip resistance of CPTU test and the soil characteristic of the site, is complex for calculation. And there is no method for liquefaction evaluation directly using CPTU parameters in the Chinese National Standard. Thus, some researchers tried to evaluate the soil liquefaction by combining Ic with other parameters. However, liquefaction evaluation using only Ic based on CPTU has rarely been reported, and none of the existing methods can directly calculate the standard penetration value (N63.5) by using Ic.In view of the shortcomings existing in the two aforementioned methods for liquefaction evaluation and the advantage of the CPTU, this paper gives the relationship betweenN63.5 and Ic and then presents a new and more convenient method to evaluate the liquefaction of the saturated silty sand and silt in the Su-xin highway by combining with the SPT-based method.


## 2. In Situ Piezocone Penetration Test

### 2.1. Site Description

The Su-xin highway was constructed in eastern China, and its site location is shown in Figure1. The intensity of earthquake is 8 degrees in the area, and the design value of seismic acceleration is 0.20 g. The landforms of test site are mainly flood plain and undulating plain on the Yellow River, and denudation monadnock is often scattered in the site. The test site lies in the quaternary coastal plain of the abandoned Yellow River. The topsoil of the site is artificial backfill and Qpd, and the lower soils are silt and silty sand (Figure 2). According to Guidelines for the Seismic Design of Highway Bridges (JTG/T B02-01-2008), the problem of liquefaction is widespread in this area.Figure 1
Location of the project.Figure 2
Engineering geological conditions.


### 2.2. Improving the Foundation Using the Resonance Method

The liquefiable foundation was improved using a resonance device, which involves walking machinery, a vibratory hammer, and a cross-shaped vibration wing developed by Institute of Geotechnical Engineering of SEU. The resonance device and the field of foundation improvement are shown in Figure3.Figure 3
The resonance device and the field of foundation improvement.


### 2.3. CPTU Design

The size of test region is 100 m × 40 m, as shown in Figure4. According to the shape of the vibrator, the excitation force, and the spacing of the vibrant points, 5 test regions were divided. With 2 test holes of the CPTU for each region, 10 holes in total were produced (1#∼10#). The depth of each hole is 20 m. The test design is shown in Table 1.Figure 4
The holes position of the CPTU.Table 1
Test design.
Section
Vibrator shape
Vibration hammer
Area (m)2
Spacing (m)
Vibration frequency (Hz)
Vibration time (min)
Depth (m)
Stationing form
A
Cross
DZ60KS
20 × 40
1.5
16.5
20
20
Regular triangle
B-1
Cross
DZ60
15 × 40
1.8
16.5
20
20
Regular triangle
B-2
Cross
DZ60KS
15 × 40
1.8
16.5
20
20
Regular triangle
C
Cross
DZ60KS
30 × 40
2.0
16.5
20
20
Regular triangle
D
Swedish wing
DZ60KS
20 × 40
1.8
16.5
20
20
Regular triangleCPTUs were conducted in winter (dry season) after ground improvement using the vibrocompaction method. A typical result of the CPTU is shown in Figure5, it can be seen that two phases are divided for pore water pressure using 9 m as the dividing line; above is negative pore water pressure, and below is positive pore water pressure. So, 9 m can be regarded as the groundwater table. Parabola relationships are found between the depth and the cone tip resistance (qt), the side friction (fs), and the standard penetration test value (N60). All the maximum values of qt, fs, and N60 are obtained near 9 m. Taking 9 m as the demarcation depth, above this depth is unsaturated soil, and below this depth are saturated silty sand and silt. In this paper, the method for liquefaction evaluation is studied based on the saturated silty sand and silt below 9 m.Figure 5
The typical result of CPTU.


## 2.1. Site Description

The Su-xin highway was constructed in eastern China, and its site location is shown in Figure1. The intensity of earthquake is 8 degrees in the area, and the design value of seismic acceleration is 0.20 g. The landforms of test site are mainly flood plain and undulating plain on the Yellow River, and denudation monadnock is often scattered in the site. The test site lies in the quaternary coastal plain of the abandoned Yellow River. The topsoil of the site is artificial backfill and Qpd, and the lower soils are silt and silty sand (Figure 2). According to Guidelines for the Seismic Design of Highway Bridges (JTG/T B02-01-2008), the problem of liquefaction is widespread in this area.Figure 1
Location of the project.Figure 2
Engineering geological conditions.


## 2.2. Improving the Foundation Using the Resonance Method

The liquefiable foundation was improved using a resonance device, which involves walking machinery, a vibratory hammer, and a cross-shaped vibration wing developed by Institute of Geotechnical Engineering of SEU. The resonance device and the field of foundation improvement are shown in Figure3.Figure 3
The resonance device and the field of foundation improvement.


## 2.3. CPTU Design

The size of test region is 100 m × 40 m, as shown in Figure4. According to the shape of the vibrator, the excitation force, and the spacing of the vibrant points, 5 test regions were divided. With 2 test holes of the CPTU for each region, 10 holes in total were produced (1#∼10#). The depth of each hole is 20 m. The test design is shown in Table 1.Figure 4
The holes position of the CPTU.Table 1
Test design.
Section
Vibrator shape
Vibration hammer
Area (m)2
Spacing (m)
Vibration frequency (Hz)
Vibration time (min)
Depth (m)
Stationing form
A
Cross
DZ60KS
20 × 40
1.5
16.5
20
20
Regular triangle
B-1
Cross
DZ60
15 × 40
1.8
16.5
20
20
Regular triangle
B-2
Cross
DZ60KS
15 × 40
1.8
16.5
20
20
Regular triangle
C
Cross
DZ60KS
30 × 40
2.0
16.5
20
20
Regular triangle
D
Swedish wing
DZ60KS
20 × 40
1.8
16.5
20
20
Regular triangleCPTUs were conducted in winter (dry season) after ground improvement using the vibrocompaction method. A typical result of the CPTU is shown in Figure5, it can be seen that two phases are divided for pore water pressure using 9 m as the dividing line; above is negative pore water pressure, and below is positive pore water pressure. So, 9 m can be regarded as the groundwater table. Parabola relationships are found between the depth and the cone tip resistance (qt), the side friction (fs), and the standard penetration test value (N60). All the maximum values of qt, fs, and N60 are obtained near 9 m. Taking 9 m as the demarcation depth, above this depth is unsaturated soil, and below this depth are saturated silty sand and silt. In this paper, the method for liquefaction evaluation is studied based on the saturated silty sand and silt below 9 m.Figure 5
The typical result of CPTU.


## 3. Liquefaction Assessment Using the Standard Penetration Test Value Based on the CPTU Soil Behavior Type Index

### 3.1. Correlation between the Standard Penetration Test Value and the Soil Behavior Type Index

According to the survey results of the code compilation group in China, the liquefaction phenomenon does not exist when the soil depth is more than 20 m. And according to the Code for Seismic Design of Buildings (GB 50011-2010 (Standardization Administration of China)), the SPT-based method should be adopted for the soil within 20 m from ground when the degree of saturated sand or silt needs further liquidation evaluation. So the maximum depth of calculation in this paper is set to 20 m. The critical value of the SPT (Ncr) for liquidation evaluation can be calculated using the following equation:(1)Ncr=N0βln0.6ds+1.5−0.1dw3ρc,ds<20,where N0 is the reference value of the SPT for liquidation evaluation (Table 2), ds is the penetration depth (m), dw is the depth of the groundwater level (m), ρc is the percentage of clay (%), and β is the regulation factor. If the measured value of the standard penetration test (N) is greater than Ncr, then no liquefaction occurs; liquidation occurs if N is less than Ncr.Table 2
Reference value of the SPT for liquidation evaluationN0.
Design value of seismic acceleration
0.10 g
0.15 g
0.20 g
0.30 g
0.40 g
N
0
7
10
12
16
19The soil behavior type indexIc used in this work follows the generalized definition given by Robertson 2009; this index has been widely applied in the geotechnical literature. The calculation formula of Ic is as follows:(2)Ic=3.47−lgQt2+1.22+lgFr2,(3)Qt=qt−σv0σv0′,(4)Fr=fsqt−σv0×100%,where Qt is the normalized cone tip resistance; Fr is the normalized sleeve frictional resistance in percentage; qt is the total cone tip resistance after correction; σv0 and σv0′ are the total and effective overburden pressure, respectively; and fs is the measured side friction.The classification of the soil behavior type index after correction is shown in Table3.Table 3
The classification ofIc after correction.
Range ofIc
Classification of the soil in China
I
c ＞ 3.45
Muck and mucky soil
3.00 <Ic < 3.45
Clay
2.80 <Ic < 3.00
Silty clay, clay
2.60 <Ic < 2.80
Silt, silty clay
2.40 <Ic < 2.60
Silt
2.10 <Ic < 2.40
Silty sand, silt
1.87 <Ic < 2.10
Fine sand
I
c < 1.87
Medium sandRobertson determined the relationship between the ratio of the normalized cone tip resistanceqc/pa and standard penetration test value N60qc/pa/N60 and the average particle size D50 (0.001∼1 mm), where N60 is the standard penetration test value with 60% of the energy (actual hammering energy/total hammering energy). Subsequently, Robertson calculated the qc/pa/N60 value of various types of soils and determined the relationship between Ic and qc/pa/N60 as follows:(5)qc/paN60=8.51−Ic4.6.After 62 SPTs were performed using a drill pipe of 42 mm in diameter by Liao et al. [23], the result showed the energy ratio of the SPT is approximately 85%, with a lower variation coefficient of 0.03. Because the weight of the hammer in the SPT is 63.5 kg, the standard penetration test value obtained using the SPT is remembered as N63.5. Thus, the mathematical relation between N63.5 and N60 can be obtained according CPTU data and equation (5). The mathematical expression is as follows:(6)N63.5×0.85=N60×0.60.Therefore,(7)N63.5=0.71N60.


### 3.2. Correlation betweenN60 and Ic

CPTU data are collected from 4 test regions, A (1#, 2#), B-1 (3#, 4#), B-2 (5#, 6#), and C (7#, 8#), with each test regions having 50, 51, 51, and 45 sets of data, respectively. Each set of data includes soil depth, cone tip resistance, side friction, and standard penetration test valueN60. Ic can be calculated using equations (2)–(4); the relationship between Ic and N60 is shown in Figure 6.Figure 6
The relationship betweenIc and N60. (a) A region. (b) B-1 region. (c) B-2 region. (d) C region.
(a)
(b)
(c)
(d)After fitting, a negative linear relationship with a higher goodness of fit can be found betweenIc and N60. Based on comprehensive analysis of the four groups of data (shown in Figure 7), the following mathematical relation between Ic and N60 can be obtained:(8)N60=−26.5Ic+73.3,R2=0.92.Figure 7
The fitting curve ofIc and N60.Equation (8) is suitable for saturated silty sand and silt for which the range of Ic is 1.5 < Ic < 2.5. It can be concluded from equation (8) that Ic and N60 are inversely related; that is, the larger the Ic value, the greater the viscosity of soil, and the smaller the N60 value. The result is in accordance with actual engineering properties of saturated silty sand and silt.


### 3.3. Correlation betweenN63.5 and Ic

By simultaneously solving equations (7) and (8), the following mathematical relation between Ic and N63.5 can be obtained:(9)N63.5=−18.8Ic+52.0.In this paper, the standard penetration test value is replaced byN63.5, and the liquefaction estimation is conducted according to the Code for Seismic Design of Buildings (GB 50011-2010). The following are the concrete steps used in this study. First, calculate the value of N63.5 using equation (9), and then compare N63.5 with the critical value of the SPT Ncr calculated using equation (1). If the calculated value of N63.5 is greater than Ncr, then no liquefaction occurs. Otherwise, the soil is considered to be liquefied.


### 3.4. Method Validation

To verify the reliability of the method presented in this paper, 49 sets of data from the D region (9#, 10#) are selected for validation calculation. The concrete steps are as follows.(1)
With the data of soil depth, cone tip resistance, and side friction, theIc value can be calculated using equations (2)–(4)(2)
N60 is calculated using equation (8) and compared with its original value (as shown in Figure 8(a))(3)
N63.5 is calculated using equation (9) and compared with its measured value obtained using the SPT (as shown in Figure 8(b))(4)
Ncr is calculated using equation (1) and compared with the measured value of N63.5 obtained using the SPT (as shown in Figure 8(c))(5)
The calculated value ofN63.5 is compared with Ncr (as shown in Figure 8(d))Figure 8
Verifying the results: (a) calculated value and original value ofN60; (b) calculated value and measured value of N63.5; (c) liquefaction estimation by the measured value (mv) of N63.5 (N63.5(mv) vs. Ncr); (d) liquefaction estimation by the calculated value (cv) of N63.5 (N63.5(cv) vs. Ncr).
(a)
(b)
(c)
(d)Figure8 shows that the calculated value of N60 is basically in line with its original value. The calculated value of N63.5 is also consistent with its measured value. Because the results of liquefaction estimation through this method is the same as those estimated using the SPT, the proposed method is a reliable for performing liquefaction prediction of saturated silty sand and silt.


### 3.5. Practical Significance of the Method

Compared with the original methods, this new method has the following advantages. (1) This method can provide a continuous parameter of the standard penetration test value via CPTU compared with that via SPT method and increases the reliability of liquefaction estimation; (2) this method involves a simpler calculation compared with the corrected seed method; and (3) this method is easy to use and apply in engineering practice.


## 3.1. Correlation between the Standard Penetration Test Value and the Soil Behavior Type Index

According to the survey results of the code compilation group in China, the liquefaction phenomenon does not exist when the soil depth is more than 20 m. And according to the Code for Seismic Design of Buildings (GB 50011-2010 (Standardization Administration of China)), the SPT-based method should be adopted for the soil within 20 m from ground when the degree of saturated sand or silt needs further liquidation evaluation. So the maximum depth of calculation in this paper is set to 20 m. The critical value of the SPT (Ncr) for liquidation evaluation can be calculated using the following equation:(1)Ncr=N0βln0.6ds+1.5−0.1dw3ρc,ds<20,where N0 is the reference value of the SPT for liquidation evaluation (Table 2), ds is the penetration depth (m), dw is the depth of the groundwater level (m), ρc is the percentage of clay (%), and β is the regulation factor. If the measured value of the standard penetration test (N) is greater than Ncr, then no liquefaction occurs; liquidation occurs if N is less than Ncr.Table 2
Reference value of the SPT for liquidation evaluationN0.
Design value of seismic acceleration
0.10 g
0.15 g
0.20 g
0.30 g
0.40 g
N
0
7
10
12
16
19The soil behavior type indexIc used in this work follows the generalized definition given by Robertson 2009; this index has been widely applied in the geotechnical literature. The calculation formula of Ic is as follows:(2)Ic=3.47−lgQt2+1.22+lgFr2,(3)Qt=qt−σv0σv0′,(4)Fr=fsqt−σv0×100%,where Qt is the normalized cone tip resistance; Fr is the normalized sleeve frictional resistance in percentage; qt is the total cone tip resistance after correction; σv0 and σv0′ are the total and effective overburden pressure, respectively; and fs is the measured side friction.The classification of the soil behavior type index after correction is shown in Table3.Table 3
The classification ofIc after correction.
Range ofIc
Classification of the soil in China
I
c ＞ 3.45
Muck and mucky soil
3.00 <Ic < 3.45
Clay
2.80 <Ic < 3.00
Silty clay, clay
2.60 <Ic < 2.80
Silt, silty clay
2.40 <Ic < 2.60
Silt
2.10 <Ic < 2.40
Silty sand, silt
1.87 <Ic < 2.10
Fine sand
I
c < 1.87
Medium sandRobertson determined the relationship between the ratio of the normalized cone tip resistanceqc/pa and standard penetration test value N60qc/pa/N60 and the average particle size D50 (0.001∼1 mm), where N60 is the standard penetration test value with 60% of the energy (actual hammering energy/total hammering energy). Subsequently, Robertson calculated the qc/pa/N60 value of various types of soils and determined the relationship between Ic and qc/pa/N60 as follows:(5)qc/paN60=8.51−Ic4.6.After 62 SPTs were performed using a drill pipe of 42 mm in diameter by Liao et al. [23], the result showed the energy ratio of the SPT is approximately 85%, with a lower variation coefficient of 0.03. Because the weight of the hammer in the SPT is 63.5 kg, the standard penetration test value obtained using the SPT is remembered as N63.5. Thus, the mathematical relation between N63.5 and N60 can be obtained according CPTU data and equation (5). The mathematical expression is as follows:(6)N63.5×0.85=N60×0.60.Therefore,(7)N63.5=0.71N60.


## 3.2. Correlation betweenN60 and Ic

CPTU data are collected from 4 test regions, A (1#, 2#), B-1 (3#, 4#), B-2 (5#, 6#), and C (7#, 8#), with each test regions having 50, 51, 51, and 45 sets of data, respectively. Each set of data includes soil depth, cone tip resistance, side friction, and standard penetration test valueN60. Ic can be calculated using equations (2)–(4); the relationship between Ic and N60 is shown in Figure 6.Figure 6
The relationship betweenIc and N60. (a) A region. (b) B-1 region. (c) B-2 region. (d) C region.
(a)
(b)
(c)
(d)After fitting, a negative linear relationship with a higher goodness of fit can be found betweenIc and N60. Based on comprehensive analysis of the four groups of data (shown in Figure 7), the following mathematical relation between Ic and N60 can be obtained:(8)N60=−26.5Ic+73.3,R2=0.92.Figure 7
The fitting curve ofIc and N60.Equation (8) is suitable for saturated silty sand and silt for which the range of Ic is 1.5 < Ic < 2.5. It can be concluded from equation (8) that Ic and N60 are inversely related; that is, the larger the Ic value, the greater the viscosity of soil, and the smaller the N60 value. The result is in accordance with actual engineering properties of saturated silty sand and silt.


## 3.3. Correlation betweenN63.5 and Ic

By simultaneously solving equations (7) and (8), the following mathematical relation between Ic and N63.5 can be obtained:(9)N63.5=−18.8Ic+52.0.In this paper, the standard penetration test value is replaced byN63.5, and the liquefaction estimation is conducted according to the Code for Seismic Design of Buildings (GB 50011-2010). The following are the concrete steps used in this study. First, calculate the value of N63.5 using equation (9), and then compare N63.5 with the critical value of the SPT Ncr calculated using equation (1). If the calculated value of N63.5 is greater than Ncr, then no liquefaction occurs. Otherwise, the soil is considered to be liquefied.


## 3.4. Method Validation

To verify the reliability of the method presented in this paper, 49 sets of data from the D region (9#, 10#) are selected for validation calculation. The concrete steps are as follows.(1)
With the data of soil depth, cone tip resistance, and side friction, theIc value can be calculated using equations (2)–(4)(2)
N60 is calculated using equation (8) and compared with its original value (as shown in Figure 8(a))(3)
N63.5 is calculated using equation (9) and compared with its measured value obtained using the SPT (as shown in Figure 8(b))(4)
Ncr is calculated using equation (1) and compared with the measured value of N63.5 obtained using the SPT (as shown in Figure 8(c))(5)
The calculated value ofN63.5 is compared with Ncr (as shown in Figure 8(d))Figure 8
Verifying the results: (a) calculated value and original value ofN60; (b) calculated value and measured value of N63.5; (c) liquefaction estimation by the measured value (mv) of N63.5 (N63.5(mv) vs. Ncr); (d) liquefaction estimation by the calculated value (cv) of N63.5 (N63.5(cv) vs. Ncr).
(a)
(b)
(c)
(d)Figure8 shows that the calculated value of N60 is basically in line with its original value. The calculated value of N63.5 is also consistent with its measured value. Because the results of liquefaction estimation through this method is the same as those estimated using the SPT, the proposed method is a reliable for performing liquefaction prediction of saturated silty sand and silt.


## 3.5. Practical Significance of the Method

Compared with the original methods, this new method has the following advantages. (1) This method can provide a continuous parameter of the standard penetration test value via CPTU compared with that via SPT method and increases the reliability of liquefaction estimation; (2) this method involves a simpler calculation compared with the corrected seed method; and (3) this method is easy to use and apply in engineering practice.


## 4. Conclusions

Taking the foundation reinforced project using the resonance compaction method in Su-xin highway as the background, a new and convenient and reliable method for liquefaction estimation based on CPTU data was presented. The main conclusions regarding the efficacy of this method are as follows:(1)
The CPTU is an advanced in situ test method that can obtain continuous parameters of a geological section. As a result, more reliable and comprehensive information can be used to identify and evaluate the soil liquefaction.(2)
The mathematical relation between standard penetration test valueN63.5 and soil behavior type index Ic established for the saturated silty sand and silt is a linear function of N63.5=−18.8Ic+52.0. Using this relationship, by calculating Ic through CPTU data, N63.5 is obtained.(3)
The method for liquefaction potential in this paper is estimated through theN63.5 value calculated from Ic; this method has sufficient reliability and practicability and involves a simple calculation. This method is suitable for the saturated silty sand and silt, for which the range of Ic is 1.5 < Ic < 2.5. This method must be checked further in engineering practice.



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*Source: 5612857-2019-03-12.xml*