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Falling cone method to measure the strength of marine clays
Ocean Engineering 31 (2004) 1915–1927 www.elsevier.com/locate/oceaneng
Technical note
Falling cone method to measure the strength of marine clays G. Rajasekaran a,,1, S. Narasimha Rao b a
b
Instrumentation Division, BPL Ltd, Singapore Ocean Engineering Centre and Department of Civil Engineering, Indian Institute of Technology, Madras, India Received 22 August 2000; accepted 20 December 2000
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. # 2004 Published by Elsevier Ltd. Keywords: Marine clays; Shear strength; Fall-cone method; Vane shear test
1. 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
Corresponding author. E-mail address: [email protected] (G. Rajasekaran). 1 Formerly Professional Officer, Dept. of Civil Engineering, National University of Singapore, Singapore 119260. 0029-8018/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.oceaneng.2000.12.001
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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 ¼ K ðQ=h2 Þ
ð1Þ
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.
2. Experimental work 2.1. 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 study. The optimum lime requirement (OLR) of marine clay was estimated by the procedure given by Eades and Grim (1966). It has been estimated that lime column tests require 6% of dry weight of soil, whereas lime injection tech-
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Table 1 Untreated soil properties Physical and chemical properties
Liquid limit (%) Plastic limit (%) Plasticity index pH Organic content (%) Sulphates (%) Chlorides (%) Cation exchange capacity (m.eq/100 g of the soil) Lime content (%) Shear strength of the soil (kN/m2) Compression index (Cc)
Test results Fresh water system
Sea water system
88 33 55 7.06 1.36 0.02 0.05 38 0.19 16 0.87
85 32 53 7.3 1.41 0.20 1.8 42 0.21 18 0.85
nique needs 40% concentration of calcium hydroxide in water (by weight). Initially the lime column work was carried out in a circular tank of diameter 600 mm and height 550 mm (Fig. 1). A homogeneous soil bed was prepared by placing clay lumps under soft consistency in layers of 30 to 50 mm thickness up to a total
Fig. 1. Experimental set up for lime column work.
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height of 500 mm. Each layer was first hand packed and then pressed with a wooden template to remove the entrapped air for ensuring homogeneous packing. In all tests of this series, columns of 50 mm diameter and 500 mm height were installed at the center of the soil bed. To investigate the effectiveness of various chemicals, different combinations of column materials such as calcium sulphate, calcium chloride, sand, sodium sulphate with quicklime were used in the ratio 1:1. Additives such as calcium sulphate and calcium chloride have been used to accelerate the efficiency of lime stabilization, whereas sodium sulphate was used to examine the influence of sodium on the soil strength. Also, the chemicals listed above are common in the sea water as well as in marine clays (Rajasekaran, 1993). Further, hydrated lime column was used as column filler material to bring out the hydration effect of soil strength. The details of various tests and their corresponding column filling materials are given in Table 2. To bring out the lime induced changes in marine clays, seawater (SW) was used in most of the tests for mixing and curing of soil bed. However in one case, fresh water (FW) was used for comparison purposes with land based clay. In the second case, lime injection method was extended in a tank of size, 1000 1000 750 mm filled with marine clay (Fig. 2), and a homogeneous soil bed was prepared as per the procedure explained for lime column series tests. Two tests were carried out to evaluate the effective radial penetration of lime slurry into the soil systems. The soil bed was prepared up to a thickness of 700 mm, and lime slurry (LS) of 40% concentration (dry weight of soil) was carried out in three stages. First, a steel injection pipe of inner diameter 18 mm and length 1250 mm was slowly pushed into the soil strata until it reached the bottom of the soil Table 2 Details of experimental setups used Test programme
Water used for mixing Schedule of and curing of the soil sampling (days)
Lime column work (Set up 1)
Lime injection work (Set up 2)
Column filling material used
Injection material used
Quicklime Quicklime Hydrated lime Quicklime-sand Quicklime-calcium sulphate Quicklime-calcium chloride Quicklime-sodium sulphate –
– – – – –
Fresh water Sea water Sea water Sea water Sea water
2, 7, 15, 2, 7, 15, 2, 7, 15, 2, 7, 15, 2, 7, 15,
–
Sea water
2, 7, 15, 30 and 45
–
Sea water
2, 7, 15, 30 and 45
Hydrated lime slurry
Sea water
2, 7, 15, 30 and 45
Set up 1: Circular tank of diameter, 600 mm and height, 550 mm. Set up 2: Rectangular tank of size, 1000 1000 750 mm.
30 and 45 30 and 45 30 and 45 30 and 45 30 and 45
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Fig. 2. Experimental set up for lime injection work.
bed. The injection pipe had 40 perforations over a length of 300 to 400 mm at the bottom portion. Next, LS was injected through the pipe under a pressure range of 0.2 to 0.3 N/mm2, and the pipe was slowly withdrawn in stages to inject the middle and top layers of 200 mm height. In all test cases, free water of 50 mm was always maintained on the top of the soil bed, and a nominal surcharge pressure of 5 kN/m2 was applied to prevent the free swelling of soil. A number of samples were collected from the middle layer of soil beds (0.2 to 0.6 H; H ¼ thickness of clay layer) at different time intervals of 2, 7, 15, 30 and 45 days. The procedure highlighted by Fohs and Kinter (1972) was used to estimate the lime content of samples collected at a radial distance of 200 mm from the centre of lime column/injected points. Air dried and powdered v samples were used for the XRD analysis with a scanning speed of 3 /min. The presence of clay minerals and formation of various compounds as a result of soillime reactions were identified using ASTM (1991). The changes that occurred in the strength of soil for different lime treated systems were examined by conducting several falling cone tests as per the procedure given by Towner (1973), and Mullins and Fraser (1980). For each reported data, atleast a minimum of three samples were tested and the average of these values have been reported. It has been widely accepted that laboratory vane shear tests can be used to measure the strength of both untreated as well as treated soils. Further, the standard laboratory vane shear
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tests were carried out as per ASTM D 4648-87 (1989) to validate the falling cone test data.
3. Results and discussion 3.1. 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 crowding of excessive calcium ions in the other systems respectively whereas other lime columns as well as injection treated systems show values ranging from 1.3 to 1.5% of lime content. Further, the effective penetration of lime into the soil system up to a radial distance of four times the diameter of column, and 8 times the diameter of lime injection pipe was confirmed. This amount of
Fig. 3. Variation of lime content of different treated soil systems.
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Fig. 4. (a) X-ray diffraction analysis of untreated samples. (Q, quartz; M, montmorillonite; K, kaolinite; C-V-M, chlorite, vermiculite, montmorillonite). (b) X-ray diffraction analysis of quicklime column treated samples in fresh water set up. (CSH, calcium silicate hydrate; CAH, calcium aluminate hydrate; CA/UT, calcium aluminate/unsubstituted tobermorite; CASH, calcium aluminate silicate hydrate; K, kaolinite; M, montmorillonite; Q, quartz). (c) X-ray diffraction analysis of quicklime column treated samples in sea water set up. (Abbreviations as panel (b)).
seeped lime in the soil system is considered adequate to satisfy the full requirements of cation exchange (Rajasekaran, 1993).
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Table 3 Mineralogical composition of untreated soil Mineral present
Composition (%)
Montmorillonite Kaolinite Sepiolite Quartz Chlorite-vermiculite-montmorillonite
15 9 13 36 27
Estimated using the method given by Pierce and Seigel (1969).
Fig. 4a shows the X-ray diffraction of untreated soil, which indicates the presence of swelling minerals such as montmorillonite and chlorite, and kaolinite and quartz. The mineralogical composition of the untreated soil can be seen in Table 3. The formation of cementation compounds such as calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) can be seen in quicklime column treated soil systems (Fig. 4b,c) cured with both fresh water (FW) and sea water (SW). The presence of compounds of similar nature in both SW and FW lime treated soil systems indicate that the lime induced pozzolanic reactions are the same in both systems. It has been established that the presence of the above compounds in the soil system induces particulate growth and results in aggregation effect of particles, and improves the shear strength of soil (Rajasekaran and Narasimha Rao, 1998). 3.2. Strength behavior of soils The change in strength was evaluated using falling cone method which allows rapid and multiple test measurements on samples. The average of three cone penetrations in each set up was considered to establish the shear strength of the soil system. Fig. 5 shows the variation of strength measurements of different lime treated soil systems for the samples taken at the nearest sampling points. There is not much variation in the strength measurements of both fresh water and sea water mixed quicklime column systems. However, the above results indicate that the presence of sea water does not retard the strength improvement of marine clay with time. This is an encouraging sign to adopt lime treatment technique in marine environment, and the measured values showed a significant strength improvement for up to eight times than that of the untreated soil. The hydrated lime column and lime injection systems showed on the other hand an improvement for up to five times that of untreated soil which is less compared to other treated systems. The addition of calcium chloride and calcium sulphate with quicklime has significantly improved the shear strength of soil when compared to other treated systems. This may be due to the additional supply and higher adsorption of calcium (Ca2+) ions by soil particles as a result of cation exchange phenomena. In all systems, there is not much variation in the improvement after 30 to 45 days of treatment. The lower strength improvement in the hydrated column as well as lime injection systems has
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Fig. 5. Shear strength of lime treated marine clay systems (nearest sampling points).
resulted due to the absence of quicklime hydration effect. It has been reported that the increase in temperature as a result of hydration of quicklime accelerates the formation of cementation compounds which induce several beneficial changes in the soil (Rajasekaran and Narasimha Rao, 2000). Under large depths of water, it is convenient to adopt lime injection technique to stabilize submarine weak soils which can be done from a ship or a suitable vessel. The quicklime-sodium sulphate column treated soil system indicates lower strength improvement that may be due to excessive diffusion and domination of sodium (Na+) ions. With the addition of sodium compound, the crowding of monovalent sodium cations around the particles cannot be avoided. Further, there is a possibility that sodium sulphate reacts with clay minerals and forms ettringite, which weaken the system over a period of time (Mitchell, 1986; Rajasekaran, 1993). The variation of strength for the samples taken from the farthest sampling location is shown in Fig. 6. As expected, the measured values were lower than the samples taken at a radial distance of 80 mm. However, a similar increasing trend of strength improvement among the different systems was observed. The quicklimecalcium chloride system showed a maximum value of 9 times that of untreated soil. This may be due to the diffusion of additional calcium ions in to the soil system, which results in crowding of cations near the soil particles and results in better formation of reaction products as highlighted in the earlier section. The above test results revealed that the strength behavior of marine clay has been improved significantly due to lime treatment. It has been observed that the improvement in strength takes place within the first 15 to 30 days, and later the measured values are stable. The lime induced strength improvement is significant except in quicklime-sodium sulphate system, and improvement can be seen up to a radial distance
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Fig. 6. Shear strength of lime treated marine clay systems (farthest sampling points).
of four times the diameter of lime column, and 10 times the diameter of injection pipe. As explained in the earlier section, formation of cementation compounds such as CSH and CAH in the soil system results in a significant increase in strength with time (Rajasekaran and Narasimha Rao, 1998) Also, it has been established that compounds formed due to soil-lime reactions are quite stable and considered irreversible (Rajasekaran, 1993). Laboratory vane shear tests were carried out to compare the falling cone test results of different lime treated marine clays, and tests were carried out to examine the variation of strength as per the procedure described in ASTM D 1587-83 (1989). The results obtained from the different lime treated systems were compared statistically to determine the relationship between falling cone and laboratory vane shear tests (Figs. 7 and 8). Correlation coefficients and regression line equations were computed to establish a relationship between the above two methods. The test results showed a linear relationship with the falling cone test results, and the variation between the two methods of strength measurement to be insignificant. This aspect encourages the use of falling cone method to measure the strength of lime treated marine clays. The coefficient of correlation values indicate values of 0.94 and 0.86 for the samples taken at nearest and farthest sampling points respectively. Some anomalous results were recorded even though laboratory shear vane penetrated well into the soil, which could be due to the interaction between the shearing soil and base of the vane shear container. In addition, the lime induced strength improvements at the nearest sampling point could be affected considerably than the farthest sampling locations. The above aspect depends on the effective penetration of calcium ions into the soil systems, and physicochemical factors such as cation exchange capacity, pH, pore fluid system chemistry, and soil particles
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Fig. 7. Correlation between falling cone and laboratory shear vane tests (nearest sampling points).
interparticle forces (Sridharan and Prakash, 1998). The use of falling cone is a novel and simple approach to measure the strength of lime treated marine clay. The obtained correlation pertains to the soil used in the present study only. However, extending the use of falling cone method to other types of soils can be determined only after ensuring insignificant deviation of test results through few laboratory vane shear tests.
4. 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 on the type of chemical additives used. Quicklime treated soil systems
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Fig. 8. Correlation between falling cone and laboratory shear vane tests (farthest sampling points).
showed better results of strength increase when compared to hydrated lime treated soil systems, and the presence of sea water had no affect on strength improvements. However, strength improvements are not encouraging in the case of quicklimesodium sulphate systems. The results of this investigation established that both the lime column and lime injection techniques could be conveniently used to improve the behavior of soft marine clay deposits.
References ASTM D 1587-83, 1989. Standard practice for thin-walled tube sampling of soils. Annual Book of ASTM Standards, 04.08, 226–228. ASTM D 4648-87, 1989. Standard test method for laboratory miniature vane shear test for saturated fine-grained clayey soil. Annual Book of ASTM Standards, 04.08, 868–873. ASTM, 1991. Index to the powder diffraction file, Joint Com On Powder Diffraction Standards, Philadelphia, PA. Bjerrum, L., 1973. Geotechnical problems involved in foundation of structures in North Sea. Geotechnique 23, 319–358.
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Campbell, D.J., 1975. Liquid limit determination of arable topsoils using a drop-cone penetrometer. Journal of Soil Science 26, 234–240. Campbell, D.J., 1976. Plastic determination using a drop cone penetrometer. Journal of Soil Science 27, 295–300. Clare, K.E., Cruchley, A.E., 1957. Laboratory experiments in the stabilization of clays with hydrated lime. Geotechnique 7, 97–111. Eades, J.L., Grim, R.E. 1966. A quick test to determine lime requirements for lime stabilization, Highway Res Rec Bull No. 139, Washington DC, 61–72. Fohs, D.G., Kinter, E.B., 1972. Migration of lime in compacted soil. Public Roads 37, 1–8. Hansbo, S. 1957. A new approach to the determination of the shear strength of clay by the fall-cone test. Royal Swedish Geotechnical Institute Proc. No. 14, Stockholm. Mitchell, J.K., 1986. Practical problems from surprising soil behaviour. Journal of Geotechnical Engineering Div ASCE 112, 259–289. Mullins, C.E., Fraser, A., 1980. Use of the drop-cone penetrometer on undisturbed and remoulded soils at the range of soil-water tensions. Journal of Soil Science 31, 25–32. Pierce, J.W., Seigal, F.R., 1969. Quantification in clay minerals studies of sediments and sedimentary rocks. Journal of Sedimentary Petrology 9, 187–193. Rajasekaran, G. 1993. Physico-chemical behaviour of lime treated marine clay, PhD thesis, Ocean Engineering Centre, Indian institute of Technology, Madras, India. Rajasekaran, G., Narasimha Rao, S., 1998. Particle size analysis of lime treated marine clays. International Journal of ASTM, USA 21, 109–119. Rajasekaran, G., Narasimha Rao, S., 2000. Pollutants behaviour and temperature effect on chemical piles treated marine clay. Ocean Engineering 27, 147–166. Sherwood, P.T., Ryley, M.D., 1970. An investigation of a cone-penetrometer method for the determination of the liquid limit. Geotechnique 20, 203–208. Sridharan, A., Prakash, K., 1998. Liquid limits and fall cones: discussion. Can Geotech J 35, 793–798. Towner, G.D., 1973. An examination of the fall-cone method for the determination of some strength properties of remoulded agricultural soils. Journal of Soil Science 24, 470–479.
Technical note
Falling cone method to measure the strength of marine clays G. Rajasekaran a,,1, S. Narasimha Rao b a
b
Instrumentation Division, BPL Ltd, Singapore Ocean Engineering Centre and Department of Civil Engineering, Indian Institute of Technology, Madras, India Received 22 August 2000; accepted 20 December 2000
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. # 2004 Published by Elsevier Ltd. Keywords: Marine clays; Shear strength; Fall-cone method; Vane shear test
1. 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
Corresponding author. E-mail address: [email protected] (G. Rajasekaran). 1 Formerly Professional Officer, Dept. of Civil Engineering, National University of Singapore, Singapore 119260. 0029-8018/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.oceaneng.2000.12.001
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G. Rajasekaran, S. Narasimha Rao / Ocean Engineering 31 (2004) 1915–1927
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 ¼ K ðQ=h2 Þ
ð1Þ
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.
2. Experimental work 2.1. 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 study. The optimum lime requirement (OLR) of marine clay was estimated by the procedure given by Eades and Grim (1966). It has been estimated that lime column tests require 6% of dry weight of soil, whereas lime injection tech-
G. Rajasekaran, S. Narasimha Rao / Ocean Engineering 31 (2004) 1915–1927
1917
Table 1 Untreated soil properties Physical and chemical properties
Liquid limit (%) Plastic limit (%) Plasticity index pH Organic content (%) Sulphates (%) Chlorides (%) Cation exchange capacity (m.eq/100 g of the soil) Lime content (%) Shear strength of the soil (kN/m2) Compression index (Cc)
Test results Fresh water system
Sea water system
88 33 55 7.06 1.36 0.02 0.05 38 0.19 16 0.87
85 32 53 7.3 1.41 0.20 1.8 42 0.21 18 0.85
nique needs 40% concentration of calcium hydroxide in water (by weight). Initially the lime column work was carried out in a circular tank of diameter 600 mm and height 550 mm (Fig. 1). A homogeneous soil bed was prepared by placing clay lumps under soft consistency in layers of 30 to 50 mm thickness up to a total
Fig. 1. Experimental set up for lime column work.
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G. Rajasekaran, S. Narasimha Rao / Ocean Engineering 31 (2004) 1915–1927
height of 500 mm. Each layer was first hand packed and then pressed with a wooden template to remove the entrapped air for ensuring homogeneous packing. In all tests of this series, columns of 50 mm diameter and 500 mm height were installed at the center of the soil bed. To investigate the effectiveness of various chemicals, different combinations of column materials such as calcium sulphate, calcium chloride, sand, sodium sulphate with quicklime were used in the ratio 1:1. Additives such as calcium sulphate and calcium chloride have been used to accelerate the efficiency of lime stabilization, whereas sodium sulphate was used to examine the influence of sodium on the soil strength. Also, the chemicals listed above are common in the sea water as well as in marine clays (Rajasekaran, 1993). Further, hydrated lime column was used as column filler material to bring out the hydration effect of soil strength. The details of various tests and their corresponding column filling materials are given in Table 2. To bring out the lime induced changes in marine clays, seawater (SW) was used in most of the tests for mixing and curing of soil bed. However in one case, fresh water (FW) was used for comparison purposes with land based clay. In the second case, lime injection method was extended in a tank of size, 1000 1000 750 mm filled with marine clay (Fig. 2), and a homogeneous soil bed was prepared as per the procedure explained for lime column series tests. Two tests were carried out to evaluate the effective radial penetration of lime slurry into the soil systems. The soil bed was prepared up to a thickness of 700 mm, and lime slurry (LS) of 40% concentration (dry weight of soil) was carried out in three stages. First, a steel injection pipe of inner diameter 18 mm and length 1250 mm was slowly pushed into the soil strata until it reached the bottom of the soil Table 2 Details of experimental setups used Test programme
Water used for mixing Schedule of and curing of the soil sampling (days)
Lime column work (Set up 1)
Lime injection work (Set up 2)
Column filling material used
Injection material used
Quicklime Quicklime Hydrated lime Quicklime-sand Quicklime-calcium sulphate Quicklime-calcium chloride Quicklime-sodium sulphate –
– – – – –
Fresh water Sea water Sea water Sea water Sea water
2, 7, 15, 2, 7, 15, 2, 7, 15, 2, 7, 15, 2, 7, 15,
–
Sea water
2, 7, 15, 30 and 45
–
Sea water
2, 7, 15, 30 and 45
Hydrated lime slurry
Sea water
2, 7, 15, 30 and 45
Set up 1: Circular tank of diameter, 600 mm and height, 550 mm. Set up 2: Rectangular tank of size, 1000 1000 750 mm.
30 and 45 30 and 45 30 and 45 30 and 45 30 and 45
G. Rajasekaran, S. Narasimha Rao / Ocean Engineering 31 (2004) 1915–1927
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Fig. 2. Experimental set up for lime injection work.
bed. The injection pipe had 40 perforations over a length of 300 to 400 mm at the bottom portion. Next, LS was injected through the pipe under a pressure range of 0.2 to 0.3 N/mm2, and the pipe was slowly withdrawn in stages to inject the middle and top layers of 200 mm height. In all test cases, free water of 50 mm was always maintained on the top of the soil bed, and a nominal surcharge pressure of 5 kN/m2 was applied to prevent the free swelling of soil. A number of samples were collected from the middle layer of soil beds (0.2 to 0.6 H; H ¼ thickness of clay layer) at different time intervals of 2, 7, 15, 30 and 45 days. The procedure highlighted by Fohs and Kinter (1972) was used to estimate the lime content of samples collected at a radial distance of 200 mm from the centre of lime column/injected points. Air dried and powdered v samples were used for the XRD analysis with a scanning speed of 3 /min. The presence of clay minerals and formation of various compounds as a result of soillime reactions were identified using ASTM (1991). The changes that occurred in the strength of soil for different lime treated systems were examined by conducting several falling cone tests as per the procedure given by Towner (1973), and Mullins and Fraser (1980). For each reported data, atleast a minimum of three samples were tested and the average of these values have been reported. It has been widely accepted that laboratory vane shear tests can be used to measure the strength of both untreated as well as treated soils. Further, the standard laboratory vane shear
1920
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tests were carried out as per ASTM D 4648-87 (1989) to validate the falling cone test data.
3. Results and discussion 3.1. 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 crowding of excessive calcium ions in the other systems respectively whereas other lime columns as well as injection treated systems show values ranging from 1.3 to 1.5% of lime content. Further, the effective penetration of lime into the soil system up to a radial distance of four times the diameter of column, and 8 times the diameter of lime injection pipe was confirmed. This amount of
Fig. 3. Variation of lime content of different treated soil systems.
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Fig. 4. (a) X-ray diffraction analysis of untreated samples. (Q, quartz; M, montmorillonite; K, kaolinite; C-V-M, chlorite, vermiculite, montmorillonite). (b) X-ray diffraction analysis of quicklime column treated samples in fresh water set up. (CSH, calcium silicate hydrate; CAH, calcium aluminate hydrate; CA/UT, calcium aluminate/unsubstituted tobermorite; CASH, calcium aluminate silicate hydrate; K, kaolinite; M, montmorillonite; Q, quartz). (c) X-ray diffraction analysis of quicklime column treated samples in sea water set up. (Abbreviations as panel (b)).
seeped lime in the soil system is considered adequate to satisfy the full requirements of cation exchange (Rajasekaran, 1993).
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Table 3 Mineralogical composition of untreated soil Mineral present
Composition (%)
Montmorillonite Kaolinite Sepiolite Quartz Chlorite-vermiculite-montmorillonite
15 9 13 36 27
Estimated using the method given by Pierce and Seigel (1969).
Fig. 4a shows the X-ray diffraction of untreated soil, which indicates the presence of swelling minerals such as montmorillonite and chlorite, and kaolinite and quartz. The mineralogical composition of the untreated soil can be seen in Table 3. The formation of cementation compounds such as calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) can be seen in quicklime column treated soil systems (Fig. 4b,c) cured with both fresh water (FW) and sea water (SW). The presence of compounds of similar nature in both SW and FW lime treated soil systems indicate that the lime induced pozzolanic reactions are the same in both systems. It has been established that the presence of the above compounds in the soil system induces particulate growth and results in aggregation effect of particles, and improves the shear strength of soil (Rajasekaran and Narasimha Rao, 1998). 3.2. Strength behavior of soils The change in strength was evaluated using falling cone method which allows rapid and multiple test measurements on samples. The average of three cone penetrations in each set up was considered to establish the shear strength of the soil system. Fig. 5 shows the variation of strength measurements of different lime treated soil systems for the samples taken at the nearest sampling points. There is not much variation in the strength measurements of both fresh water and sea water mixed quicklime column systems. However, the above results indicate that the presence of sea water does not retard the strength improvement of marine clay with time. This is an encouraging sign to adopt lime treatment technique in marine environment, and the measured values showed a significant strength improvement for up to eight times than that of the untreated soil. The hydrated lime column and lime injection systems showed on the other hand an improvement for up to five times that of untreated soil which is less compared to other treated systems. The addition of calcium chloride and calcium sulphate with quicklime has significantly improved the shear strength of soil when compared to other treated systems. This may be due to the additional supply and higher adsorption of calcium (Ca2+) ions by soil particles as a result of cation exchange phenomena. In all systems, there is not much variation in the improvement after 30 to 45 days of treatment. The lower strength improvement in the hydrated column as well as lime injection systems has
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Fig. 5. Shear strength of lime treated marine clay systems (nearest sampling points).
resulted due to the absence of quicklime hydration effect. It has been reported that the increase in temperature as a result of hydration of quicklime accelerates the formation of cementation compounds which induce several beneficial changes in the soil (Rajasekaran and Narasimha Rao, 2000). Under large depths of water, it is convenient to adopt lime injection technique to stabilize submarine weak soils which can be done from a ship or a suitable vessel. The quicklime-sodium sulphate column treated soil system indicates lower strength improvement that may be due to excessive diffusion and domination of sodium (Na+) ions. With the addition of sodium compound, the crowding of monovalent sodium cations around the particles cannot be avoided. Further, there is a possibility that sodium sulphate reacts with clay minerals and forms ettringite, which weaken the system over a period of time (Mitchell, 1986; Rajasekaran, 1993). The variation of strength for the samples taken from the farthest sampling location is shown in Fig. 6. As expected, the measured values were lower than the samples taken at a radial distance of 80 mm. However, a similar increasing trend of strength improvement among the different systems was observed. The quicklimecalcium chloride system showed a maximum value of 9 times that of untreated soil. This may be due to the diffusion of additional calcium ions in to the soil system, which results in crowding of cations near the soil particles and results in better formation of reaction products as highlighted in the earlier section. The above test results revealed that the strength behavior of marine clay has been improved significantly due to lime treatment. It has been observed that the improvement in strength takes place within the first 15 to 30 days, and later the measured values are stable. The lime induced strength improvement is significant except in quicklime-sodium sulphate system, and improvement can be seen up to a radial distance
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Fig. 6. Shear strength of lime treated marine clay systems (farthest sampling points).
of four times the diameter of lime column, and 10 times the diameter of injection pipe. As explained in the earlier section, formation of cementation compounds such as CSH and CAH in the soil system results in a significant increase in strength with time (Rajasekaran and Narasimha Rao, 1998) Also, it has been established that compounds formed due to soil-lime reactions are quite stable and considered irreversible (Rajasekaran, 1993). Laboratory vane shear tests were carried out to compare the falling cone test results of different lime treated marine clays, and tests were carried out to examine the variation of strength as per the procedure described in ASTM D 1587-83 (1989). The results obtained from the different lime treated systems were compared statistically to determine the relationship between falling cone and laboratory vane shear tests (Figs. 7 and 8). Correlation coefficients and regression line equations were computed to establish a relationship between the above two methods. The test results showed a linear relationship with the falling cone test results, and the variation between the two methods of strength measurement to be insignificant. This aspect encourages the use of falling cone method to measure the strength of lime treated marine clays. The coefficient of correlation values indicate values of 0.94 and 0.86 for the samples taken at nearest and farthest sampling points respectively. Some anomalous results were recorded even though laboratory shear vane penetrated well into the soil, which could be due to the interaction between the shearing soil and base of the vane shear container. In addition, the lime induced strength improvements at the nearest sampling point could be affected considerably than the farthest sampling locations. The above aspect depends on the effective penetration of calcium ions into the soil systems, and physicochemical factors such as cation exchange capacity, pH, pore fluid system chemistry, and soil particles
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Fig. 7. Correlation between falling cone and laboratory shear vane tests (nearest sampling points).
interparticle forces (Sridharan and Prakash, 1998). The use of falling cone is a novel and simple approach to measure the strength of lime treated marine clay. The obtained correlation pertains to the soil used in the present study only. However, extending the use of falling cone method to other types of soils can be determined only after ensuring insignificant deviation of test results through few laboratory vane shear tests.
4. 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 on the type of chemical additives used. Quicklime treated soil systems
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Fig. 8. Correlation between falling cone and laboratory shear vane tests (farthest sampling points).
showed better results of strength increase when compared to hydrated lime treated soil systems, and the presence of sea water had no affect on strength improvements. However, strength improvements are not encouraging in the case of quicklimesodium sulphate systems. The results of this investigation established that both the lime column and lime injection techniques could be conveniently used to improve the behavior of soft marine clay deposits.
References ASTM D 1587-83, 1989. Standard practice for thin-walled tube sampling of soils. Annual Book of ASTM Standards, 04.08, 226–228. ASTM D 4648-87, 1989. Standard test method for laboratory miniature vane shear test for saturated fine-grained clayey soil. Annual Book of ASTM Standards, 04.08, 868–873. ASTM, 1991. Index to the powder diffraction file, Joint Com On Powder Diffraction Standards, Philadelphia, PA. Bjerrum, L., 1973. Geotechnical problems involved in foundation of structures in North Sea. Geotechnique 23, 319–358.
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Campbell, D.J., 1975. Liquid limit determination of arable topsoils using a drop-cone penetrometer. Journal of Soil Science 26, 234–240. Campbell, D.J., 1976. Plastic determination using a drop cone penetrometer. Journal of Soil Science 27, 295–300. Clare, K.E., Cruchley, A.E., 1957. Laboratory experiments in the stabilization of clays with hydrated lime. Geotechnique 7, 97–111. Eades, J.L., Grim, R.E. 1966. A quick test to determine lime requirements for lime stabilization, Highway Res Rec Bull No. 139, Washington DC, 61–72. Fohs, D.G., Kinter, E.B., 1972. Migration of lime in compacted soil. Public Roads 37, 1–8. Hansbo, S. 1957. A new approach to the determination of the shear strength of clay by the fall-cone test. Royal Swedish Geotechnical Institute Proc. No. 14, Stockholm. Mitchell, J.K., 1986. Practical problems from surprising soil behaviour. Journal of Geotechnical Engineering Div ASCE 112, 259–289. Mullins, C.E., Fraser, A., 1980. Use of the drop-cone penetrometer on undisturbed and remoulded soils at the range of soil-water tensions. Journal of Soil Science 31, 25–32. Pierce, J.W., Seigal, F.R., 1969. Quantification in clay minerals studies of sediments and sedimentary rocks. Journal of Sedimentary Petrology 9, 187–193. Rajasekaran, G. 1993. Physico-chemical behaviour of lime treated marine clay, PhD thesis, Ocean Engineering Centre, Indian institute of Technology, Madras, India. Rajasekaran, G., Narasimha Rao, S., 1998. Particle size analysis of lime treated marine clays. International Journal of ASTM, USA 21, 109–119. Rajasekaran, G., Narasimha Rao, S., 2000. Pollutants behaviour and temperature effect on chemical piles treated marine clay. Ocean Engineering 27, 147–166. Sherwood, P.T., Ryley, M.D., 1970. An investigation of a cone-penetrometer method for the determination of the liquid limit. Geotechnique 20, 203–208. Sridharan, A., Prakash, K., 1998. Liquid limits and fall cones: discussion. Can Geotech J 35, 793–798. Towner, G.D., 1973. An examination of the fall-cone method for the determination of some strength properties of remoulded agricultural soils. Journal of Soil Science 24, 470–479.