Elsevier

Ocean Engineering

Volume 31, Issues 14–15, October 2004, Pages 1915-1927
Ocean Engineering

Technical note
Falling cone method to measure the strength of marine clays

https://doi.org/10.1016/j.oceaneng.2000.12.001Get rights and content

Abstract

An attempt has been made to use falling cone technique for measuring the shear strength of lime treated marine clays. The amount of lime seeped into different lime treated marine clays with duration was estimated, and laboratory vane shear tests were carried out to compare with falling cone strength data. X-ray diffraction (XRD) technique was used to examine the nature of compounds formed in different lime treated soil systems. Test results revealed that strength of different lime treated systems increased by 8 to 10 times of untreated soil. Further the obtained results indicated a linear relationship between the falling cone and laboratory vane shear tests.

Introduction

Construction of various structures on marine clays greatly increased the need for tapping natural marine resources from the seabed. It has been reported that the construction of coastal and offshore structures in marine deposits are confronted with many geotechnical problems (Bjerrum, 1973). There is a need to develop a fast and reliable laboratory measurement of shear strength since in-situ measurements at offshore are time-consuming and expensive. In view of the above, the use of a simple laboratory testing technique for evaluating the engineering behavior of marine clay has been encouraged. Further, the use of conventional soil stabilization techniques for marine clays are not effective due to hostile wave conditions and large depths of water in the ocean environment. In such cases, the engineering behavior of marine clays can be improved using lime column and lime injection techniques (Rajasekaran, 1993). It is well established that the use of lime in fine grained soils makes the system less sensitive to changes in stress and other environmental factors (Clare and Cruchley, 1957).

The fall-cone method was first developed to measure the liquid as well as plastic limits of cohesive soils (Sherwood and Ryley, 1970). Several investigators reported that measuring the Atterberg limits of soils using falling cone technique was simple and reliable (Campbell, 1975, Campbell, 1976). The presence of dominant clay mineral and its amount in the soil system has considerable influence on the falling cone results (Sridharan and Prakash, 1998). Few investigators extended the use of falling cone method to measure the strength of land based soils (Towner, 1973, Mullins and Fraser, 1980). They reported that the strength of soils depends on a factor of proportionality (K), which can be influenced by the cone apex angle and degree of remolding soil (i.e. moisture content and soil texture). Hansbo (1957) developed a semi-empirical analysis in relating the depth of cone penetration (h in mm) and undrained shear strength (Cu in N/mm2) of soil as follows. Cu=(Q/h2)where K is the factor of proportionality (7×103 for clay), and Q is the mass of the cone (grams). From the literature, it has been noted that there is not much information available on the study of lime treated marine clay using falling cone method. Hence, an attempt has been made to examine the above aspect, and subsequently laboratory vane shear tests were carried out to validate the results of the falling cone test. In addition, migration of lime from the centre of columns and injected points into the clay and its associated new compounds formation have been examined.

Section snippets

Soil and chemicals used

Marine clay samples were procured from the east coast of Madras, India using an open trench excavation method during low tide period, and the untreated soil properties can be seen in Table 1. For weak marine clays under considerable depths of water, lime column method is better suited, whereas for clays under significant depth, lime slurry injection technique can be used successfully. In view of the above reasons, both lime column and lime slurry injection techniques were attempted in this

Lime migration studies

Fig. 3 indicates the variation of lime content in the middle layer of treated soil systems. There is an increase in the concentration of diffused lime content of different treated soil systems. Quicklime-sand, quicklime-calcium sulphate and quicklime-calcium chloride columns showed a maximum lime content penetration of 2.2 to 2.3% up to a radial distance of 4 times the diameter of the column. This could be due to the use of good drainage material (sand) in the case of quicklime-sand column, and

Conclusions

The diffusion of lime and its related strength improvements in lime treated marine clay was confirmed. The above test results indicated that falling cone method could be used to evaluate strength improvements in lime treated marine clay. Considering its high performance and simplicity factors, the cone penetrometer proved to be an excellent alternative method to determine the shear strength of soils. Lime treatment showed a significant improvement in the strength of marine clay, which depends

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    Therefore, the skeletal structure can easily collapse as a result of changes in stress caused by landform alteration or construction activity; the quick clay then liquefies, which can cause serious clay slides (Geertsema et al., 2006; L'Heureux et al., 2012). Torrence (1996) defined quick clay as soil exhibiting sensitivity >30 and remolded shear strength of <0.5 kPa based on the falling-cone test (Rajasekaran and Narasimha Rao, 2004). Karlsrud et al. (1984) conducted an undrained triaxial compression test, and described the extremely fragile behavior of quick clay as follows: the peak resistance was reached at an axial strain of only 0.3% and the shear resistance was reduced to 50% of the peak strength with a strain of 3%.

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1

Formerly Professional Officer, Dept. of Civil Engineering, National University of Singapore, Singapore 119260.

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