CHAPTER 4: RESULTS AND DISCUSSION
4.15. Comparison of New Relation vs Previous Relations
From the past relationships given by various authors, it is observed that most of the cases qu varies from 10 to 20 times of SPT N value. In this research qu is equal to 16.5 times of SPT N value (Fig.-4.7). The correlation varies for different types of sample for different locations. The correlations that were developed in different times and in this research are linear. The relationship varies with liquid limit, Plasticity Index, compression index, dry density of soil etc. No normalized relation was found (Fig. 4.7).
McEarthy (1977) established the following correlations for silty clay and clay soil.
qu=N/2.5 ksf = 0.4N ksf for silty clay qu=N/2 ksf = 0.5N ksf for clay.
Similar correlation between N and qu was investigated by Murthy (1993) for the preconsolidated silty clay encountered at Farakka in West Bengal, India. The moisture content of the soil was close to the Plastic limit, which for the soil varied from 30 to 40 percent, and the liquid limit from 50 to 100 percent with the preconsolidation ratio in the order of 5. It has been mentioned that there was a considerable scatter of test results and the relation between qu and N shows qu = 10 to 20 N (kPa).
Bowels (1988) suggested a correlation qu = N/4 ksf (12N kPa)
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Categorizing by liquid limit, two types of correlation are found in this study which is shown below:
For lean clay qu = 17.84N Kpa=0.37N ksf; (fig.-4.8) when LL < 50 % For fat clay qu = 9.97N Kpa=0.21N ksf; (Fig.-4.9) when LL ≥ 50 %
In this study, the plastic Limit varies from10 to 23 percent and Liquid limit varies from 17 to 57 percent. It has been found that the test result is scattered and the average value of qu=16.5 N kPa in Bhulta, Narayanganj area. But in Khulna region the soil is too soft and the data which was found from the test result is very scattered.
So in combination of soft and stiff soil, low or high plasticity the value of unconfined compressive strength is
qu=16.5 N kPa = 0.34N ksf (Fig. 4.9)
Comparing the new relationship between qu and N-value with previous correlations done by other authors, it is observed that correlation is not unique. It varies for different type of soils for different locations. It is more or less similar to the other authors.
Azzouz et al. (1976) introduced several correlations between Compression Index and Initial Void Ratio for different types of soil.
Cc = 1.21+1.055(e0-1.87); for Motley clays from Sao Paulo city Cc = 0.208e0+0.0083; for Chicago clays
But the correlations which are found in this study are given below:
Cc = 0.295e0 - 0.028 for stiff clay at Bhulta, Narayanganj (Fig.-4.38) Cc = 0.651e0- 0.310 for soft clay at AMC, Khulna (Fig.-4.39) Cc = 0.309e0 + 0.024 for soft clay at SANH, Khulna (Fig.-4.40) Cc = 0.358e0 - 0.049 for combined correlation (Fig.-4.41)
So, different type of soil shows different correlation. No generalized correlation is found.
In this study, the empirical relationship is found between Compression Index and initial void ratio which is shown in Figure 4.46 including previous study. It is found that all empirical relations including present study are more or less followed the same trend.
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Therefore relation from this study can be used for soils of Bangladesh. So this relation could be used for all type of clay soil.
Azzouz et al. (1976) gave several correlations for the compression index and natural moisture content:
Cc = 0.01wn (for Chicago clays)
Cc = 0.0115 wn (for organic soil, peats etc.)
The correlations of Cc and natural moisture content wn for both type of stiff and soft soil at Narayanganj and Khulna, which are found, are:
Cc = 0.006wn - 0.016 Bhulta, Narayanganj: (Fig-4.42) Cc= 0.021wn- 0.527 AMC, Khulna :( Fig-4.43) Cc = 0.009wn- 0.042 SANH, Khulna: (Fig-4.44)
The combined result of compression index Cc and natural moisture content wn is plotted in Figure 4.45. It was observed that there exists a straight relationship between Cc and wn. Regression analysis of the data yielded the following equation with a correlation coefficient (R2) of 0.902.
Cc = 0.0092wn - 0.068
Similar correlations were introduced by some other authors including the present study in Figure4.47. The previous relationships which were developed are similar in nature of Bhulta and Khulna. Each type of soil shows the linear relationship. So the correlation which was developed for compression index and natural moisture content can be used for all category of soil.
Some correlations were given by Terzaghi and Peak (1967) for the compression index and liquid limit
Cc = 0.009 (wL-10) for the soil where the sensitivity ratio is less than 4
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Cc = 0.007(wL-10) for remolded soil
Serajuddin et al (1967) correlated Cc with wL of a large number of undisturbed plastic silt and clay soil samples of different areas of Bangladesh and obtained the following empirical relationships:
Cc = 0.0078(wL– 14)
In this study, the correlation with Cc & wL which are found:
Cc = 0.002wL + 0.003 Bhulta, Narayanganj; (Fig-4.30) Cc = 0.013wL - 0.3691 AMC, Khulna ;( Fig-4.31) Cc = 0.0196wL - 1.134 SANH, Khulna; (Fig-4.32)
Cc = 0.006(wL- 21.34) Combination of both type of soil ;( Fig-4.33) Similar correlations were found for compression index and liquid limit that were established by some other authors including the present study in Figure 4.48. The linear correlations were developed for different types of soils that vary for stiff or soft soil. But no generalized relation is observed. This correlation is different for different types of soil.
Nacci et al. (1975) tested some natural deep ocean soil samples and correlate with compression index and plasticity index.
Cc = 0.02+0.014 IP
Similarly, Nakase et al. (1998) correlated with compression index. But there is a limitation of this relation, that this correlation is applicable when Plasticity index is IP< 50% and showed a relation:
Cc = 0.046 + 0.0104 IP
By testing a large number of undisturbed soils collected from Narayanganj and Khulna, several correlations are found that are as follows:
Cc = 0.004Ip + 0.021 Bhulta, Narayanganj: (Fig. 4.34) Cc= 0.018Ip - 0.169 AMC, Khulna; (Fig. 4.35)
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Cc = 0.016Ip - 0.446 SANH, Khulna; (Fig. 4.36)
Cc = 0.007Ip- 0.015 Combination of Bhulta & Khulna; (Fig. 4.37) The relationship between compression index and plastic limit that was established by some authors including the present study is shown in Figure 4.49. The relationships that are developed for soft and stiff soil is followed the similar trend. Comparing to the other authors’ correlations (Fig. 4.49), similar type of linear relations are found.