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

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).+e soil behavior type index (Ic) obtained fromCPTUs and the standard penetration test index (N63.5), obtained from SPTs, are analyzed for saturated silty sand and silt. +e analysis result reveals a linear relationship between N63.5 and Ic, given by N63.5 � −18.8Ic + 52.0. +e larger the value of Ic is, the greater the viscosity of soil is, and the smaller the value of N63.5 is. According to themethod, liquefaction assessment of saturated silty sand and silt foundation can be conducted by usingN63.5 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.


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][2][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].erefore, 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.e in situ test method is widely used because it involves a small disturbance and is relatively good at representing the liquefaction potential.e 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.
e evaluation method for liquefaction potential using the CPT is becoming preferable because it provides standardized and reliable data [9].rough several stages of modification by , 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].e CPTU can provide three types of data, namely, the pore pressure (u 2 ), the cone tip resistance (q c ), and the sleeve frictional resistance (f s ), and the CPTU gives near continuous parameters of a continuous geological section.At present, the soil behavior type index (I c ) 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 q c , f s , and u 2 were observed after the dynamic compaction, the soil behavior type determined with these CPTU parameters largely remained unchanged.Others have done similar researches about I c [20][21][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.us, some researchers tried to evaluate the soil liquefaction by combining I c with other parameters.However, liquefaction evaluation using only I c based on CPTU has rarely been reported, and none of the existing methods can directly calculate the standard penetration value (N 63.5 ) by using I c .
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 between N 63.5 and I c 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.

Site Description.
e Su-xin highway was constructed in eastern China, and its site location is shown in Figure 1.
e intensity of earthquake is 8 degrees in the area, and the design value of seismic acceleration is 0.20 g. e landforms of test site are mainly flood plain and undulating plain on the Yellow River, and denudation monadnock is often scattered in the site.
e test site lies in the quaternary coastal plain of the abandoned Yellow River.e 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.

Improving the Foundation Using the Resonance Method.
e 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. e resonance device and the field of foundation improvement are shown in Figure 3.

CPTU Design.
e size of test region is 100 m × 40 m, as shown in Figure 4.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#).e depth of each hole is 20 m. e test design is shown in Table 1.
CPTUs were conducted in winter (dry season) after ground improvement using the vibrocompaction method.A typical result of the CPTU is shown in Figure 5, 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 (q t ), the side friction (f s ), and the standard penetration test value (N 60 ).All the maximum values of q t , f s , and N 60 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.

Liquefaction Assessment Using the Standard
Penetration Test Value Based on the CPTU Soil Behavior Type Index  Advances in Civil Engineering degree of saturated sand or silt needs further liquidation evaluation.So the maximum depth of calculation in this paper is set to 20 m. e critical value of the SPT (N cr ) for liquidation evaluation can be calculated using the following equation: where N 0 is the reference value of the SPT for liquidation evaluation ( where Q t is the normalized cone tip resistance; F r is the normalized sleeve frictional resistance in percentage; q t is the total cone tip resistance after correction; σ v0 and σ v0 ′ are the total and effective overburden pressure, respectively; and f s is the measured side friction.e classification of the soil behavior type index after correction is shown in Table 3. Robertson determined the relationship between the ratio of the normalized cone tip resistance (q c /p a ) and standard penetration test value N 60 ((q c /p a )/N 60 ) and the average particle size D 50 (0.001∼1 mm), where N 60 is the standard penetration test value with 60% of the energy (actual hammering energy/total hammering energy).Subsequently, Robertson calculated the ((q c /p a )/N 60 ) value of various types of soils and determined the relationship between I c and (q c /p a )/N 60 as follows: 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   Advances in Civil Engineering SPT is 63.5 kg, the standard penetration test value obtained using the SPT is remembered as N 63.5 .us, the mathematical relation between N 63.5 and N 60 can be obtained according CPTU data and equation ( 5). e mathematical expression is as follows: erefore,  2)-( 4); the relationship between I c and N 60 is shown in Figure 6.
After fitting, a negative linear relationship with a higher goodness of fit can be found between I c and N 60 .Based on comprehensive analysis of the four groups of data (shown in Figure 7), the following mathematical relation between I c and N 60 can be obtained: Equation ( 8) is suitable for saturated silty sand and silt for which the range of I c is 1.5 < I c < 2.5.It can be concluded from equation ( 8) that I c and N 60 are inversely related; that is, the larger the I c value, the greater the viscosity of soil, and the smaller the N 60 value.e result is in accordance with actual engineering properties of saturated silty sand and silt.

3.3.
Correlation between N 63.5 and I c .By simultaneously solving equations ( 7) and ( 8), the following mathematical relation between I c and N 63.5 can be obtained: In this paper, the standard penetration test value is replaced by N 63.5 , and the liquefaction estimation is conducted according to the Code for Seismic Design of Buildings (GB 50011-2010).e following are the concrete steps used in this study.First, calculate the value of N 63.5 using equation ( 9), and then compare N 63.5 with the critical value of the SPT (N cr ) calculated using equation (1).If the calculated value of N 63.5 is greater than N cr , then no liquefaction occurs.Otherwise, the soil is considered to be liquefied.

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.e concrete steps are as follows.
(1) With the data of soil depth, cone tip resistance, and side friction, the I c value can be calculated using equations ( 2)-(4) (2) N 60 is calculated using equation ( 8) and compared with its original value (as shown in Figure 8(a)) (3) N 63.5 is calculated using equation ( 9) and compared with its measured value obtained using the SPT (as shown in Figure 8(b)) (4) N cr is calculated using equation ( 1) and compared with the measured value of N 63.5 obtained using the SPT (as shown in Figure 8(c)) (5) e calculated value of N 63.5 is compared with N cr (as shown in Figure 8(d)) Figure 8 shows that the calculated value of N 60 is basically in line with its original value.e calculated value of N 63.5 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.

Practical Significance of the Method.
Compared with the original methods, this new method has the following advantages.(1) is 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.

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.Advances in Civil Engineering e main conclusions regarding the efficacy of this method are as follows: (1) e 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.3) e method for liquefaction potential in this paper is estimated through the N 63.5 value calculated from I c ; this method has sufficient reliability and practicability and involves a simple calculation.is method is suitable for the saturated silty sand and silt, for which the range of I c is 1.5 < I c < 2.5. is method must be checked further in engineering practice.

Figure 7 :
Figure 7: e fitting curve of I c and N 60 .

Figure 8 :
Figure 8: Verifying the results: (a) calculated value and original value of N 60 ; (b) calculated value and measured value of N 63.5 ; (c) liquefaction estimation by the measured value (m v ) of N 63.5 (N 63.5 (m v ) vs. N cr ); (d) liquefaction estimation by the calculated value (c v ) of N 63.5 (N 63.5 (c v ) vs. N cr ).

Table 2 )
, d s is the penetration depth (m), d w is the depth of the groundwater level (m), ρ c is the percentage of clay (%), and β is the regulation factor.If the measured value of the standard penetration test (N) is greater than N cr , then no liquefaction occurs; liquidation occurs if N is less than N cr .esoil behavior type index (I c ) used in this work follows the generalized definition given by Robertson 2009; this index has been widely applied in the geotechnical literature.e calculation formula of I c is as follows:

Table 3 :
e classification of I c after correction.

Table 2 :
Reference value of the SPT for liquidation evaluation (N 0 ).