Utilization of Murrum for Embankment Construction

Appropriate Parameters for Prediction of Swelling Pressure of Expansive Clays

Appropriate Parameters for Prediction of
Swelling Pressure of Expansive Clays

SudhaRani

Associate Professor, Dept. of Civil Engineering, S.V. University, Tirupati–517502, Andhra Pradesh, India.

E-mail:

K.Mallikarjuna Rao

Professor, Dept. of Civil Engineering, S.V. University, Tirupati–517502, Andhra Pradesh, India.
E-mail:

ABSTRACT: Predetermination of swelling characteristics (Swelling Pressure, Swell Potential and Swell Index) is essential for safe and cost-effective design of structures resting on expansive soils. In this paper an attempt has been made to develop a correlation for Swelling Pressure accounting both soil state and soil type representative parameters. The soil state is reflected by environmental factors namely Moisture Content, Dry Density and Surcharge Pressure whereas the soil type is reflected by the compositional parameters namely Liquid Limit and Plasticity Index. Four series of free Swell Odeometer tests were conducted on four soil samples under different soil state conditions (varying initial Moisture Content, initial Surcharge Pressures and initial Dry Density). A regression model was developed relating logarithm of Swelling Pressure with three of the soil state parameters and two of the soil type parameters. Relative influence of each of the five parameters on Swelling Pressure is estimated statistically by partial correlation coefficients and its performance is verified with seventy soils data from literature in the form of graphs. The study revealed that all the five parameters have significant influence on Swelling Pressure.

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Appropriate Parameters for Prediction of Swelling Pressure of Expansive Clays

1.INTRODUCTION

Substantial literature has unequivocally documented and spelled out the severity and extent of damages inflicted by soil deposits of swelling nature in many parts throughout the world (Jones Jones 1987, Chen 1975, Dhowian et al. 1990, Abdul Jawad et al. 1992). Although it has been many years since the swelling phenomenon has been fully recognized, still there is no established method of measuring swelling characteristics of clays. Attempts have been made by several investigators to correlate Swelling Pressure with various index properties (Komarnik David 1969, Vijayvergiya Ghazzaly 1973, Nayak Christensen 1974, Chen 1975, Brackley 1975, Weston 1980, Mowafy etal. 1985, Mallikarjuna Rao 1988 Yusuf Erzin Orhan Erol 2004). Careful study of the proposed equations reveal that the investigators used either Liquid Limit, Clay Fraction, Activity or one of the placement conditions namely natural Moisture Content, initial Void Ratio and initial Dry Density for development of correlations.

The factors considered for development of correlations may be grouped into two broad categories viz., compositional and environmental factors. Liquid Limit, Plasticity Index falls under compositional factors which are known to reflect compositional factors like type and amount of clay minerals, activity and pore water chemistry. whereas initial Void Ratio, initial Moisture Content, initial Dry Density and initial Surcharge Pressure falls under environmental factors. In this investigation an attempt has been made to develop correlations for Swelling Pressure accounting for all the above-identified factors by conducting aseries of laboratory tests on remoulded soil samples.

2. METHODS AND MATERIALS

Expansive clayey soils were collected from four different places namely Panidham, Kurnool, Guntur and Madanapalli of Andhra

Pradesh and are designated as SS1, SS2, SS3 and SS4 for furtherreference. The required amount of soil is collected from the trial pits at a depth of 1.5m below the ground level since the topsoil is likely to contain organic matter and other foreign materials.The soils are collected carefully so that the soil samples are fairly homogeneous.The soil is air dried after transporting the same to the laboratory and is pulverized with a wooden mallet.The soil so prepared is sieved through a
4.75mm sieve and stored in storage bins in the laboratory for further testing.The index properties of these soils are presented in Table 1.

All the soils do contain fine fraction of about 95% and are clay of intermediate to high plasticity.

3. TESTS AND RESULTS

Four series of Free Swell Odeometer tests were conducted on all the four soils samples SS1, SS2, SS3 and SS4 at different placement conditions accounting a total of 46 test samples in order to study the effect of placement conditions on the Swelling Pressure. The typical test results for soil SS1at different placement conditions are given in Table 2.

The test results obtained are analyzed to study the influence of each of the environmental factors and compositional factors on Swelling Pressure. The variation of Swelling Pressure with each of the placement factors while keeping other two placement factors constant among the three (wL, IP,γd), and the variation of PSwith each of the compositional factors (wL,IP) at different placement conditions was examined graphically. Typical plots showing the variation of PS with one of the placement factors (γd) and one of the compositional factors (wL) at different placement conditions are given in Figures 1 & 2.

From the graphical plots showing variation ofPS versus placement factors and PSversus compositional factors. it can be concluded that the Swelling Pressure is dependent on all placement conditions namely initial Moisture Content, initial Dry Density and initial Surcharge Pressure and also the Liquid Limit and Plasticity Index of the soil. Placement conditions reflect environmental factors whereas Liquid Limit and Plasticity Index reflect compositional factors. Hence Swelling Pressure can be said to be dependent on both environmental factors and compositional factors and may be expressed as given below.

PS= f ((wL, IP, γd, mc, Si) (1)

The Swelling Pressure is observed to bear a nonlinear relationship with all the influencing factors (γd, mcand Si). The Swelling Pressure is observed to increase more sharply with Dry Density than with mcand Si. Hence no linear relationship between the independent and dependent variable is possible. In order to make the relationship linear, logarithm of Swelling Pressure (Log PS) and logarithm of initial Dry Density (Log γd) are considered in the development of relationships. Hencerelationship between Swelling Pressure and initial Moisture Content, initial dry density, initial surcharge, Liquid Limit and Plasticity Index can be expressed as given below.

Table 1: Properties of Tested Soils

*VeryHigh (VH), High(H), Medium(M), Low(L)

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Appropriate Parameters for Prediction of Swelling Pressure of Expansive Clays

Table 2: Typical Test Results for Swelling Pressure for Soil SS1

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Appropriate Parameters for Prediction of Swelling Pressure of Expansive Clays

Fig. 1: Swelling Pressure Variation withInitial Dry Density

Fig. 2: Swelling Pressure vs Liquid Limit atVarying Moisture Contents (d= 16 kN/m3,Si= 5 kPa)

Log(PS)=(2)

The values of a0, a1, a2, a3, a4 and a5 can be obtained from multiple regression analysis. Microsoft-Excel software provides a subroutine for multiple regression analysis and the same is used here to obtain the regression coefficients a0, a1, a2, a3, a4 and a5 as well as the regression model and correlation coefficient, R2.The regression model so obtained is given below along with R2 value.

Log (PS) = ((–4.3341) + (0.0071WL) + (0.0006IP)
+(5l.2802Log (γd))– (1.7900mc) –0.0037Si))(3)

Correlation coefficientR2=0.979

Swelling Pressure can be predicted using above equation knowing the placement conditions, Liquid Limit and Plasticity Index. The regression analysis yielded a correlation coefficient of0.979 indicating good correlation between the variables and the Swelling Pressure. Any attempt to correlate Swelling Pressure with either compositional factors alone or environ-
mental factors alone or any other combination other than the one presented in equation 1 did not yield any fruitful regression models. Hence the same were not presented here.

In order to assess the influence of each of the compositional and environmental factors on prediction of Swelling Pressurepartial correlation coefficients are determined. The influence of any given parameter is found out by keeping aside that particular parameter and finding the multiple correlation coefficient thereby the partial correlation coefficient. The partial correlation coefficient is obtained using the following equation

= (1 –((1 –)/(1 –) )) (4)

Where = partial correlation coefficient

=Multiple correlation coefficient between x1 (i.e. PS)and all the independent variables (wL, mc, Log γd ,IPandSi).

= Multiple correlation coefficient between x1 (i.e.PS)and all the independent variables except the chosen xi(choosing one amongwL, mc, Log γd,IPandSias influencing parameter and the remaining four variablesas independent variables).

Table 3 summarizes the regression models developed for logarithm of Swelling Pressure by dropping only one parameter namely Liquid Limit or Initial Moisture Content or Logarithm of Dry Density or Plasticity Indexor Initial Surcharge at a time along with multiple correlation coefficients. The regression models are designated as E1 to E6 in that order. The table also includes regression model presented in equation 3 designated as E1, which considers all of the compositional and environ-
mental factors for the purpose of comparison.The partial correlation coefficients are estimated using the equation 4 choosing wL, IP, Log γd, mcandSias influencing parameters in that order for model . The partial correlation coefficients are 0.4, 0.045, 0.888, 0.943 and 0.870, respectively. Since the partial correlation coefficients are significant for all the three placement factors, it may be concluded environmental factors appear to have a dominating influence on Swelling Pressure than compositional factors. The partial correlation coefficient for Plasticity Index is very low indicating that its influence on Swelling Pressure is not so significant in comparison to other factors. The same is reflected by observing the standard deviation of residuals, where the deviations are more in case of environmental factors when any one of the placement factors is neglected. Hence, it may be concluded that the regression model E1 and E2 in Table 3 can be expected to have
more general applicability. This owes to the fact that
these models include all the influencing parameters namely wL, mc, Logγdand Si as evident from partial correlation coefficients.

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Appropriate Parameters for Prediction of Swelling Pressure of Expansive Clays

Table 3: Regression Models Developed for Prediction ofLogarithm of Swelling Pressure (Log PS)

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Appropriate Parameters for Prediction of Swelling Pressure of Expansive Clays

4.VERIFICATION WITH THE REPORTED DATA

The applicability of the proposed correlation for Swelling Pressure is assessed by comparing the predicted values of Swelling Pressure for the results reported in this investigation as well as using the test results reported for 76 soils from literature by Komarnik David (1969), Ramudu (1994), Mallikarjuna Rao (1988) in the literature. The Swelling Pressure so predicted is plotted against observed Swelling Pressure for the results of this investigation and for the reported data. These plots are shown in Figures 34. The solid line in these plots indicates the line of equality. The points are found to fall close to the line of equality in case of results of this investigation indicating good prediction. This is expected because it is the data used for development of proposed regression model However, for other’s data, though many points are falling close to the line of equality, some of the points are dispersed away from line of equality. In other words, prediction is good for many soils but not for all soils.This may be attributed to the fact that coarse fraction which can influence swelling characteristics has not been accounted in the proposed regression model.

Fig.3: Observed vs Predicted Swelling Pressure
(Results of Present Investigation)

Fig. 4: Observed vs PredictedSwelling Pressure
(Others data)

5.CONCLUSIONS

The factors considered for development of correlations for prediction of Swelling Pressure PS are grouped into two broad categories viz., compositional and environmental factors.
wL, IP falls under compositional factors whereas mc, γd and fallsunder environmental factors. Regression models were developed relating Log(PS)with wL, IP,Log γd, mc andSiseparately as well as in various combinations of these five parameters.wL, Log γd, mc andSiare identified as the parameters having significantinfluence on Log(PS) basedon the partial correlation coefficient. Regression model E1 and E2 are found to have more general applicability as both the models include all the influencing parameters namelywL, Log γd, mcandSias evident from partial correlation coefficients. The Performance ofthe regression modelE1 accounting all the five parameters is found satisfactory when verified by comparing the observed PS and predicted PSvalues for 70 soils test data reported in literature but not used in the development of the models.

REFERENCES

Abdul Jawad, S.N., Hammed, R.A., Al-Sulaimani, G.I, Basunbul, I.A. and Safar, M.M. (1992). “Expansive Soils in Eastern Province of Saudi-Arabia”, In: Proc. 7th Int. Conf. on Expansive Soils, Dallas, USA, Vol. 1, 426–431.

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Ranganatham, B.V. and Satyanarayana, B. (1969). “A Rational Method of Predicting Swelling Potential, for Compacted Expansive Clays”, Proc. 6th Int. Conf. SM and FE, Canada, Vol. 1, pp. 92–96.

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