The experimental study on dynamic shear modulus ratio and damping ratio of recently deposited soils in the southern area of Jiangsu province in China.

Guoxing, Chen ; Xuezhu, Liu ; Dinghu, Zhu 等

Site soils in the southern area of Jiangsu province in China belong to washland sediment or limnetic facies sediment. The components of sediment are very complex and the process of sedimentation is in disorder. This paper analyzes the test results of free vibration column apparatus on 155 undisturbed samples of recently deposited soils taken from the southern area of Jiangsu province in China, including mucky silty clay, clay, silty clay, silty clay and silty sand interbedded strata, silt, silty sand, fine sand and medium coarse sand, during last four years. The fitting curves of the average relation between dynamic shear modulus ratio G/[G.sub.max] and damping ratio versus shear strain amplitude [gamma] and their parameters in the empirical formula of G/[G.sub.max] ~ [gamma] and D ~[gamma] curves for 8 types of soils mentioned above are presented. The results in this paper will make us understand more about dynamic characteristics of recently deposited soils in this area.

INTRODUCTION

The southern area of Jiangsu province refers to the south flank of the Yangtze Delta and the Taihu Lake region within Jiangsu province, China, mainly including the cities of Zhengjiang, Changzhou, Wuxi and Suzhou. In recent years, large numbers of major projects in this area, such as Jiangyin Bridge, Sutong Bridge and Runyang Bridge on the Yangtze River, one after another is being constructed or has been completed. In the Tenth Five-Year Plan period (2001-2005), many important projects including lifeline engineering projects are being at the stage of programming or construction in the southern area of Jiangsu province, such as freeways, intercity passenger trains, harbors, docks, electric power facilities, water supply works, and so on. The sites of these important engineering projects are located on washland, delta or limnetic facies recently deposited soils in the southern area of Jiangsu province.

The recently deposited soils were formed in the medium or later term of the Holocene Epoch. Generally speaking, the recently deposited soils belong to underconsolidated soils and its strength is quite lower. With different depositional environments, the strength of recently deposited soils differs greatly. As a special type of soils, the engineering geologic property of recently deposited soils is worse with its special engineering characteristic. The concept and classification method of recently deposited soils are often adopted in the realm of Geotechnical engineering and have already been brought into the China Code for the Investigation of Geotechnical Engineering GB50021-2001.

The southern area of Jiangsu province belongs to the delta plain of the Yangtze River or low mountain mound terrain. Soft soils distribute widely in this area. Especially, the component of sediments is very complicated and asymmetrical greatly, and the underground water level is close to ground surface in this area. Based on the typical geologic section on recently deposited soils of the Yangtze delta plain in the southern area of Jiangsu province, from the bottom up, sediment strata are divided into riverbed facies, washland-firth facies and deltaic facies sediment, including medium sand, fine sand and sandy clay with horizontal bedding and cross bedding. Especially, clayey fine sand widely distributes in the south flank of the Yangtze Delta.

The dynamic characteristic of soils is a major factor affecting ground motion characteristic. Dynamic shear modulus and damping ratio of soils, i.e., [G.sub.max], G/[G.sub.max] ~[gamma] and D ~ [gamma], as two parameters of the soil dynamic characteristic, are utilized in the site seismic response analysis when the equivalent linearization model of soils is adopted. Similarly, dynamic shear modulus and damping of soils are indispensable for the seismic safety evaluation of engineering sites. Whether a selection of soil dynamic characteristic parameters matches a practical site condition, there will be a great influence on the reliability of numerical analysis results (Chen guoxing et al.1995). So, whether these parameters are rational or not, they will directly effect on safety and economical efficiency of important projects.

FOUNDATION OF THE STUDY

According to the analysis of the test results of free-vibration columniation apparatus for soil samples of six sorts including clay, silty clay, silty clay and fine sand interbedded strata of recently deposited soils in the Nanjing and its neighboring cites, the dynamic shear modulus and damping ratio were investigated primarily (Chen guoxing and Liu xuezhu 2004). Based on the research results of Chen guoxing and Liu xuezhu (2004), through the test study and theoretical analysis for seven sorts of recently deposited soils from 15 cities in the lower reaches of the Yangtze River including some cities in the southern area of Jiangsu province, the elementary regularity of dynamic shear modulus ratio G/[G.sub.max] and damping ratio D versus shear strain amplitude [gamma] was presented (Chen guoxing et al. 2005). Similarly, the recommend values of the parameters were also presented and they may be used in more areas. Furthermore, for attempting to provide nationwide curves of dynamic shear modulus ratio G/[G.sub.max] and damping ratio D versus shear strain amplitude [gamma] for various soils, only using some limited test data, its representation and creditability ought to be deliberate (Chen guoxing et al. 2005). As a whole, regional mean curves of G/[G.sub.max] ~ [gamma] and D ~ [gamma] various soils through large numbers of experimental investigation should be more credible and referenced. Consequently, with further consideration of the regional character of soils and similarity of depositional environment of soils, this paper investigated detailedly the dynamic characteristic of recently deposited soils in the southern area of Jiangsu province, based on the research results of Chen guoxing and Liu xuezhu (2004) and Chen guoxing et al. (2005). Those results have been applied in the site seismic safety evaluation for some important projects in the southern area of Jiangsu province, such as across Yangtze River bridges, orbit transportation and electric establishments etc.

THE EMPIRICAL FORMULA OF DYNAMIC SHEAR MODULUS RATIO AND DAMPING RATIO ON RECENTLY DEPOSITED SOILS IN THE SOUTHERN AREA OF JIANGSU PROVINCE

The relationship of dynamic shear modulus ratio versus the amplitude of shear strain is expressed as (Martin P.P. and Seed H.B 1982):

G/[G.sub.max] = 1 - H([gamma]) (1)

H([gamma)] = [{([gamma]/[[gamma]).sub.0].sup.2B]/1 + ([gamma]/[([gamma].sub.0]).sup.2B]}.sup.A] (2)

where A, B and [[gamma].sub.0] are fitting parameters related with soil behavior.

The damping ratio of soils D varies with shear strain amplitude. The empirical equation of damping ratio is recommended in this paper as follows:

D = [D.sub.min] + [D.sub.0] + [D.sub.0] [(1 - G/[G.sub.max]).sup.n] (3)

Where n and [D.sub.0] are fitting parameters related with soil behavior. [D.sub.min] is the minimum damping ratio relative to the initial dynamic shear modulus [G.sub.max]. Limited to existing test technique, instrument accuracy and experiment condition, it is difficult to measure accurately minimum damping ratio relative to the initial dynamic shear modulus. So, [D.sub.min] can be treated as fitting parameters related with soil behavior.

These tests were operated with self-developed free vibration column test machine (Chen guoxing et al. 2003). The undisturbed samples of recently deposited soils came from the southern area of Jiangsu province, such as Zhenjiang, Changzhou, Wuxi and Suzhou including mucky silty clay, clay, silty clay, silty clay and silt interbedded strata, silt, silty sand, fine sand and medium coarse sand, and the total number of soil samples is equal to 155. The amount and distribution of soil samples are listed in Table 1.

The test results of G/[G.sub.max] ~ [gamma] curves can be fitted well by empirical Eq.(1) and Eq.(2) (Chen guoxing et al. 2005). In this paper, the test results on G/[G.sub.max] ~ [gamma] of recently deposited soils are also fitted by empirical Eq.(1) and Eq.(2), and the test results on D ~ [gamma] of recently deposited soils are fitted by empirical equation (3). In addition, the fitting parameters on G/[G.sub.max] ~ [gamma] and D ~ [gamma] curves are listed in Table 2. Figure 1 indicates that the fitting results on G/[G.sub.max] ~ [gamma] and D ~ [[gamma] curves are more appropriate under small strain range.

[FIGURE 1 OMITTED]

CONCLUSIONS

Based on the free vibration column test results of 155 undisturbed soil samples of recently deposited soils taken from the southern area of Jiangsu province, the fitting parameters of the mean relationship curves between dynamic shear modulus ratio G/[G.sub.max] and damping ratio versus shear strain amplitude [gamma] are suggested by means of Eq.(1) and Eq.(2) and the recommended empirical equation of damping ratio in this paper. The results will make us understand more about dynamic characters of recently deposited soils in the southern area of Jiangsu province and provide some essential data for seismic safety evaluation of general engineering sites in this area.

For seismic response analysis of general engineering sites in the southern area of Jiangsu province, when there is a lack of relationship curves of G/[G.sub.max] [gamma] and D ~ [gamma] test data of soils, the typical values of the mean relationship curves of G/[G.sub.max] ~ [gamma] and D ~ [gamma] recommended in this paper can be used. But, because of comparatively complex depositional environment of soils in the southern area of Jiangsu province, there are some dispersion in the test results of given site soils comparing with the mean relationship curves of G/[G.sub.max] ~ [gamma] and D ~ [gamma] curves. So, quoting the mean relationship curves of G/[G.sub.max] ~ [gamma] and D ~ [gamma] in this paper should be deliberative for important projects in this area. In order to insure the result reliability of site seismic response analysis, model parameters or representative value on the curves of G/[G.sub.max] ~ [gamma] and D ~ [gamma] for engineering site soils should be obtained by experiment with free vibration column apparatus or dynamic triaxial apparatus.

REFERENCES

Chen guoxing, Xie junfei and Zhang kexu (1995). "The empirical of soil modulus and damping ratio for dynamic analysis". Earthquake engineering and engineering vibration, Vol. 15, No. 1, 73-84.

Chen guoxing and Liu xuezhu (2004). "Testing study on ratio of dynamic shear modulus and ratio of damping for recently deposited soils in Nanjing and its neighboring areas". Chinese Journal of Rock Mechanics and Engineering, Vol.23, No.8, 1403-1410.

Chen guoxing, Chen jihua and Liu xuezhu, et al. (2005). "Experimental study on dynamic shear modulus ratio and damping ratio of recently deposited soils in the lower reaches of the Yangtze river". Journal of Disaster Prevention and Mitigation Engineering. Vol. 1, 49-57.

Martin P.P. and Seed H.B (1982). " One dimensional dynamic ground response analysis". Journal of geotechnical engineering. ASCE, Vol.108, No.7, 935-954.

Chen guoxing, Zhu dinghua and He qizhi (2003). "Development and property test of GZZ-1 free vibration column test system". Earthquake engineering and the engineering vibration, Vol. 1, 110-114.

CHEN GUOXING

Institute of Geotechnical Engineering, Nanjing University of Technology, Nanjing, Jiangsu, China

LIU XUEZHU, ZHU DINGHU, HU QINGXING

Institute of Geotechnical Engineering, Nanjing University of Technology, Nanjing, Jiangsu, China

Table 1. Locations of recently deposited soil samples from the
southern area of Jiangsu province

 Quantity of samples in
Soils different areas Amount

 Zhenjiang Changzhou Wuxi Suzhou

Mucky silty clay 4 4 6 5 19
Clay 2 2 3 9 16
Silty clay 16 4 7 26 49
Silty clay and silt 3 -- 4 6 13
 interbedded strata
Silt 3 3 2 11 16
Silty sand 4 6 3 12 21
Fine sand -- 4 3 7 14
Medium coarse sand 1 3 -- 4 8

Table 2. Parameters of eight recently deposited soils in
the southern area of Jiangsu province

Soils A B [[gamma].sub.0]
 (x [10.sup.-14])

Mucky silty clay 1.06 0.47 2.7
Clay 1.10 0.44 3.4
Silty clay 1.09 0.44 3.0
Silty clay and silt 1.06 0.44 2.9
 interbedded strata
Silt 1.03 0.43 3.3
Silty sand 0.93 0.43 5.5
Fine sand 0.99 0.34 6.9
Medium sand 0.90 0.45 5.6

 Parameters

Soils n [D.sub.0] (%)

 Mean Range

Mucky silty clay 1.06 20.9 19.8 ~ 23.6
Clay 0.90 19.1 17.4 ~ 22.1
Silty clay 0.95 18.4 15.6 ~ 23.7
Silty clay and silt 1.09 19.3 18.2 ~ 23.3
 interbedded strata
Silt 1.02 19.1 17.9 ~ 23.7
Silty sand 0.91 19.5 17.9 ~ 23.3
Fine sand 0.98 19.8 18.2 ~ 22.2
Medium sand 0.98 18.4 18.0 ~ 21.0

 Parameters

Soils [D.sub.min] Amount
 (%)

Mucky silty clay 1.06 19
Clay 1.16 16
Silty clay 1.25 47
Silty clay and silt 1.27 12
 interbedded strata
Silt 1.09 14
Silty sand 0.49 19
Fine sand 0.80 14
Medium sand 0.63 8