LABORATORY STUDY OF FINE GRAINED SOIL IMPROVEMENT USING LIME MORTAR STONE COLUMNS

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1 International Symposium on Geotechnical Engineering, Ground Improvement and Geosynthetics for Human Security and Environmental Preservation, Bangkok, Thailand LABORATORY STUDY OF FINE GRAINED SOIL IMPROVEMENT USING LIME MORTAR STONE COLUMNS M. H. Bagheripour and M. R. Malek Poor Associate Prof., Department of Civil Engineering, University of Kerman, Kerman, Iran A. R. Ahangar and M. M. Toufigh Graduate Student, Department of Civil Engineering, University of Kerman, Kerman, Iran ABSTRACT An experimental study was conducted to evaluate the performance of mortar stone columns in improving the load bearing capacity of soft fine grained soils. In this study, more than tests were carried out on composite stone columns samples in the laboratory, of surrounding soil and - well graded soil mixtures with various mixing content of and clay and different curing times. To verify the effect of the moisture content on these columns, specimens were tested in three different states of dry, soaked, and unsoaked. Test results were used to train the artificial neural network. Using the neural network it would be possible to predict the behavior of these columns and their resistance to changes in clay and content and curing times Tests results show that the mortar columns, which contain % and 22% clay, increase the soil strength in soft fine grained soils to a noticeable amount. INTRODUCTION Stone column are extensively used to improve bearing capacity of poor ground and reduce settlements of structures built on them. Stone columns increase stiffness of the ground, reduce settlement, facilitate consolidation of soft ground, and minimize the likelihood of liquefaction due to earthquake. Stone columns are cylindrical elements which find their strength stiffness in associate with the surrounding soil. In other words, applied loads cause lateral strains in the soils and stone columns, and the shear strength of the soil which is improved using stone columns gets obtained as a result of the interaction of the soil and the stone column, which is because of the radial strains in the column and the surrounding soil and the passive strength that is developed in the column. Stone column is one of the popular types of ground improvement all over the world. It has been studied by many researchers and been understood to some extent. According to model tests, Hughes and Withers (1974) found that the settlement and failure of stone columns mainly results from bugling of upper part of the columns. Based on model tests on composite specimens with cemented stone columns in a modified triaxial apparatus, Juran and Riccobono (1991) studied the cementation effect on the performance of the granular columns and the group effect on the performance of the granular columns and the group effect on the settlement response of the reinforced soil. Hughes and Withers (197) presented a settlement calculation method in which the volume of stone column comes from its distension. Yan et al. (199) presented that the silty mat between the loading plate and the pile top has important 367

2 effect on the settlement and bearing behavior of the composite with compressible pile or rigid pile. The interaction between stone columns and the surrounding soil in composite ground is an important factor that influenced the bearing capacity and settlement behavior of the composite ground. Gung- Xinli et al. () investigated systematically interaction in soils which are improved using compressible granular piles and also the interaction in soils that are reinforced using gravel columns. The results showed that interaction behaviors of the composite ground with granular columns and that with compressible piles are quite different: In the former case, the interaction is mainly laterally, while in the later case, the interaction is mainly vertically. As part of a research program to study the settlement characteristics and behavior of granular columns, McKelvey (2) found that it was necessary to design and construct equipment to overcome the experimental difficulties. McKelvey et al. (4) proposed a technology to examine the settlement performance of footing supported on soft clay reinforced with granular columns. Black et al. (6) described an innovative design of a newly developed large test setup for testing the performance of footing supported on soft clay reinforced with granular columns. This advanced testing method is used to examine the settlement of footing supported on granular columns. Where stone is not available, may be used for the stabilization of soft soils. The use of columns for soft clay stabilization was studied by Broms and Boman (1979), Paul and Rao (1997), Rajasekaran and Rao (1998), Bell (1988), and Zhou et al. (1). This paper presents the main results of an experimental study focused on the using of mortar stone columns to improvement of soft clayey soils. Some of laboratory tests were conducted under a water soaking condition to investigate the soaking influence. LABORATORY PROGRAMMIG AND PROPERTIES OF THE USED MATERIALS Properties of Clay Samples To compare the strength of the improved soil which is reinforced with mortar stone columns with the strength of the soft soil, samples with dimensions of cm in diameter and 8 cm in height were made using the clay with the field density. The clay used in tests is from a construction site in Kerman-Iran. The Properties of this soil is mentioned in Table (1). Samples are made and tested with three moisture conditions, natural or in situ water content, saturated conditions or test conditions (samples were put in the water for 96 hours before test), and dry condition. test values for saturated soil specimens, specimens with natural moisture content, and dry condition are 2,, and 33 respectively. Table 1 Major physical and mechanical properties of in situ clay soil Depth(m) G s LL PL W (%) γ d ( 3 kg/m ) Average

3 Properties of Lime Mortar Samples After testing clay samples, samples from materials that were used in mortar stone columns are made. These samples were made of the well graded soil contain clay with ratios of 11 and 22% and with different ratios of,, and 2%. The well graded soil which was used in this test is a mixture of gravel, sand, and clay from a region in Kerman, Iran that the grain size distribution for this soil is shown in Figure 1. Special gravity (G s), Coefficient of Uniformity (C u ) and Coefficient of Gradation (C c ) for well graded soil were 2.67,.86 and 1.69 respectively Percent finer Grain Diameter(mm) -3 Fig. 1 Grain size distribution of well graded soil Lime was used in mortar stone columns, was normal hydrated which its chemical properties is mentioned in Table 2. Samples are tested in curing times of 14, 28, and 6 days. To investigate the effect of the moisture content on mortar samples, some of them are tested in soaked conditions (after being 96 hours into the water). Table 2 Chemical grade hydrated Component CaO MgO SiO 2 Fe 2 O 3 Al 2 O 3 S Mn NaCl Loss on Oxides Ignition Composition Composite Samples After making and testing of the clay and mortar samples, and comparing the results, to investigate the simultaneous performance of the clay and mortar stone column and laboratory modeling of the improved site using these columns, composite samples were made which were from surrounding soil which is clay and mortar column. In these samples the 369

4 clay with the field density and water content is compacted in a container with dimensions of cm in diameter and 8 cm in height, and then a hole is made in the central part of the sample using a sampling pipe with the diameter of 3 cm and 7 cm in height, and the soil is replaced with mortar in that hole. In order to make mortar columns, first and water are mixed, and then the well graded soil is added to the mixture using a mechanical mixer. The mixture is stirred up for minutes to make a smooth and homogenous mixture. The mixing was thoroughly with a certain amount of water (the ratio of water over the dry weight of well graded soil and together is.3). The previously used mortar samples had well graded soil with 11% and 22% clay and were used to investigate the appropriate clay content and to find the optimum amount of that for using with. So these samples had, 2, and % clay in addition to the content that they had previously. After mixing the mortar completely, it is placed in the hole in the central part of the clay. As mentioned before, mortar column sample had a diameter of 3 cm and a height of 7 cm (1 cm less than the whole sample). Low A r (the area ratio) was chosen because of the attempt to remove the effect of the walls of the test container on the test results in composite masses, so there is 1 cm clay between the bottom of the container and the sample. The first series of the specimens after curing times of 14, 28, and 6 days are tested in dry condition. The test is done on the second series of the specimens which were kept into plastic covers to prevent them from losing their natural moisture content. Finally, the third series of the specimens after curing times of 14, 28 and 6 days are kept into water for 96 hours and are tested in wet condition. LABORATORY TEST RESULTS Test Resultrs of the Lime Mortar Specimens values of the mortar specimens versus the percentage of the content for dry and soaked conditions are shown in figures 2 to. Lime mortar specimens shown in figures 2 and 3 have 11 weight-percent clay and specimens shown in figures 4 and contain 22 weightpercent clay. 8 Clay =11%, Dry Samples 9 Clay = 22% - Dry Samples Day 28- Day 14- Day Day 28- Day 14- Day Fig. 2 values versus percentage of added for mortar samples Fig. 4 values versus percentage of added for mortar samples 37

5 2 Clay = 11%, Soaked Samples Clay= 22% - Soaked Samples 2 6- Day 28- Day 14- Day 2 Pecentage of added Fig. 3 values versus percentage of added for mortar samples 6- Day 28- Day 14- Day 2 Fig. values versus percentage of added for mortar samples Test Results of the Composite Specimens Test results shown in figures 6 to 8 represents the relationship between the value and the percentage of the added in tested composite specimens for curing times of 14, 28, and 6 days respectively. The clay content in mortar columns in these specimens varies between 11 to percent. Figure 9 is plotted using the data from Figure 7 and clearly depicts the effect of the variations of the clay amount on the strength of the specimens for the curing time of 28 days. 1 Dry Samples - 14 Days DrySamples-6Days Clay=22% Clay=2% Clay=% Clay=% Clay=11% 2 Fig. 6 The relationship between 14 - day value and percentage of added Clay=22% Clay=2% 4 Clay=% Clay=% Clay=11% 2 Fig. 8 The relationship between 6- day value and percentage of added 371

6 Dry Samples - 28 Days 18 Dry Samples - 28 Days 1 Clay=22% Clay=2% Clay=% Clay=% Clay=% 2 Fig. 7 The relationship between 28- day value and percentage of added 9 6 Lime= 2% Lime= % Lime= % Lime= % Without 2 3 Percentage of added clay Fig. 9 The relationship between 28- day value and percentage of added clay Some tests are done to investigate the real behavior of the ground when the surrounding soil around the stone column contains the natural moisture content. After preparing specimens used in these tests, they are kept in plastic covers and are tested after curing times of 14, 28 and 6 days. Test results of these specimens are shown in Figures to 13. values done on specimens composed of the mortar column and the surrounding soil in soaked condition (kept in water for 96 hours) are shown in Figure 14. In these specimens, using the previous test results, the optimum amount for clay content (22%) is chosen. Figure represents the variation of the value versus the percentage of the added for clay content of 22% and for different moisture conditions. Unsoaked Samples - 14 days Unsoaked Samples - 6 days 2 Clay=22% Clay=2% Clay=% Clay=% Clay=11% 2 Fig. The relationship between 14- day value and percentage of added Clay=22% Clay=2% Clay=% Clay=% Clay=11% 2 Fig. 11 The relationship between 6 - day value and percentage of added 372

7 14 Soaked Samples - Clay=22% Unsoaked Samples - 28 days days 28 days 14 days 2 Fig. 12 The relationship between a 28 day value and percentage of added Clay=22% Clay=2% Clay=% Clay=% Clay=11% 2 Fig. 14 The relationship between 28- day value and percentage of added Unsoaked Samples - Clay = 22% Clay = 22% - 6 Days Days 28 Days 14 Days 2 Fig. 13 The relationship between 28- day value and percentage of added 8 4 Dry samples Unsoaked samples Soaked samples 2 Fig. The relationship between 6 day value and percentage of added ANALYSIS OF THE TEST RESULTS As shown in previous results, mortar type of stone columns can be used to improve soft fine grained soils. Using these columns in soft soils increases the load bearing capacity of the ground and consequently causes the settlements of the structures to decrease under applied loads. Using the above mentioned test results, it can be found out that by increasing the content percentage, the strength of the specimens and the relevant value also increase, but this increment in strength for contents of more than percent is not noticeable and is not economically feasible. It must be mentioned that the content used in mortar columns also depends on the amount of clay content of the well-graded fine grained soil used in columns. Any increases in the amount of the clay content of fine grained soil used in mixture increases the amount of the needed for improving the strength. 373

8 Another factor that increases the strength of the specimens is the increase in the amount of clay content used in the well-graded soil used in mortar columns. Increasing of the clay content to 22% increases the value of the specimens but using more clay decreases the strength of the specimens. Increase in the strength of the specimens by increasing the clay content can be relevant to the chemical reaction between the and the clay and the variations in the structure of clay soils and their aptitude to pair with each other to make Cementation Materials. On the other hand by increasing the clay content, finer clay particles fill the spaces between bigger grains and make a solid piece. Adding clay to more than 22% decreases the amount of the granular soil in the mortar column and decreases the strength of the specimens. Curing time also affects the strength of the specimens and the specimens gain a large amount of their strength after 28 days, in a way that the difference between value for 14 and 28 days is more than the difference of that for curing times of 28 and 6 days. For curing time of 14 days specimens have little strength and by increasing the amounts of the and clay increasing of the specimens strength happens slowly, but variations in strength by increasing the amounts of the and clay for curing times of 28 and 6 days are noticeable. The other factor that was considered in the above mentioned tests was the moisture content of the specimens during tests. When the specimens were tested in dry condition, because the surrounding clay was dry and strong, they showed a lot of strength (even more than the equivalent mortar specimens). Putting the specimens into water for 96 hours before testing noticeably decreases their strength. As mentioned before, this big difference between the strength of the composite specimens in dry and soaked conditions shows that clays have very little strength when they are saturated. Lime columns need warm and wet weather to be able to be cured and become strong enough, but in wet places and where there is water they never get enough strength and decay change into a plastic material. ARTIFICIAL NEURAL NETWORK In this section using the specimens laboratory test results, the neural networks are trained in a way to make it is possible to obtain the value for specimens with various variables using these trained networks. Specimens which are investigated using the neural network are divided into two categories; dry and unsoaked specimens. As the MATLAB package is able to design and train neural networks, so in this research MATLAB is used to design all required neural networks. To train the neural networks, the Radial Basis Function Neural Network (RBFNN) and Generalized Regression Neural Network (GRNN) are used. In MATLAB programming environment it would be possible to design a radial basis function and a generalized regression function using the newrbe and newgrnn commands respectively. The applied structure of the above mentioned commands are shown in Equations 1 and 2. net = newrbe(p,t,spread) (1) net = newgrnn(p,t,spread) (2) In the above equations P is the input vectors matrix which contains the percentage of the and clay content and different curing times, and T the objective vectors ( values) matrix, and also SPREAD is the development factor of the network which plays a very 374

9 important role in improving the development capability of the network. The neural network must be trained to be useable. To do this the network outputs for new input vectors, which are not used by the network before, must be simulated which can be done using the command presented as Equation 3. out =sim (net,q) (3) In the above equation q is the input vectors matrix, net is the trained network, and out is the output vectors matrix. In this study the models of the first and second categories are trained using both radial basis function and generalized regression neural networks. The average error amount of the trained network for models of the first category, for radial basis function and generalized regression neural networks, is 4.6% and 4.% respectively, and this figures for the models of the second category are 2.68% and 1.3%, which are acceptable errors in using neural networks. The comparison of the results from laboratory tests and trained neural networks are shown in Figures 16 and 17. Dry Samples Unsoaked Samples Laboratory results RBF Neural Network' s results GR Neural Network' s results 14, 14, 14, 14,2 28, 28, 28, 28,2 6, 6, 6, 6,2 Fig. 16 Comparing laboratory and neural network results for dry specimens Laboratory results RBF Neural Network' s results GR Neural Network' s results 14, 14, 14, 14,2 28, 28, 28, 28,2 6, 6, 6, 6,2 Pecentage of added Fig. 17 Comparing laboratory and neural network results for unsoaked specimens Composite specimens used to train the network contain 22% clay. This specimens are chosen for different content percentages and curing times. In these figures the horizontal axis shows the increment percentage of the content and the numbers which are to the left and on this axis represent different curing times. As it can be seen in the figures, there is a good consistence between the results from laboratory tests and the trained network. CONCLUSIONS Based on the test results and the discussion presented above, the following conclusions may be drawn: 1. Using mortar stone columns increases the bearing capacity and also causes the earth settlements to decrease. 2. Increasing the content in composite specimens of mortar columns increases the strength in specimens. Variations in strength of specimens, first increase by adding the content percentage, but for contents of more than percent strengths 37

10 variations are not noticeable. The content in mortar columns also depends on the amount of the used well-graded clay, and as the amount of clay increases more is needed to obtain better results. 3. Adding clay to the well-graded soil increases the strength of the composite specimens which are treated using mortar stone columns. This increment in strength firstly, is relevant to the chemical reaction of the and the silica content of the clay which increases the strength of the mortar, and the other reason is that with increasing the content, fine clay particles go between coarser grains and makes a strong and solid mortar column. Increasing clay content to more than 22% decreases the strength of the specimens due the decreasing the weight of the existing coarse grains. 4. Water content has a noticeable effect on strength of the specimens which are treated using mortar columns and decreases their strength to a considerable amount. The most important reason of these variations in strength is the great strength variation of clay for different amounts of the water content.. Comparing results for the specimens treated by mortar columns and the untreated ones shows that mortar columns containing % and 22% clay increase the strength of loose clayey soils in dry, unsaturated, and wet conditions, to 6, 6 and times respectively. REFRENCES Balaam, N. P. and Poulos, H. G. (1983). The behavior of foundations supported by clay stabilized by stone columns. Proceedings of the 8 th European Conference on Soil Mechanics and Foundation Engineering, Vol. 1. Black, J., Sivakumar, V. Madhav, M. R. and McCabe, B. (6). An Improved Experimantal Test Set - up to Study the Performance of Granular Columns. Geotechnical Testing Journal, Vol. 29, No. 3, pp Cheng, Z., Jian, H. Y. and Jing, P. M. (2). Bearing capacity and settlement of weak fly ash ground improved using fly-ash or stone columns. Canadian Geotechnical Journal, Vol. 39, pp Gung, X. L. and Wen, F. H. (). Interaction between columns inclusion and surrounding soil in composite ground. Lowland Technology International, Vol. 2, pp Hughes, J. M. O. and Withers, N. J. (1974). Reinforcing soft cohesive soils with stone column. Ground Engineering. Vol. 7, No. 3, pp Hughes, J. M. O., Withers, N. J. and Greenwood, D. A. (197). A field trial of the reinforcing effect of a stone column in soil. Geotechnique. Vol. 2, No. 7. Juran, I. and Riccobono, O. (1991). Reinforced soft soil with artificially cemented compactedsand columns. Journal of Geotechnical Engineering. ASCE. Vol. 117, No. 7. pp

11 Mckelvey, D. (2). Performance of Vibro Stone Column Foundations in Deep Soft Ground. Ph. D. Thesis, Queen s University, Belfast. McKelvey, D., Sivakumar, V., Bell, A. and Graham, J. (4) A Laboratory Model Study of the Performance of Vibro Stone Columns in Soft Clay. Journal of Geotechnical Engineering, Vol. 2, pp Yan, M. L., Wu, C. L. and Yang, J. (199). Design of composite ground of CFG pile. Proceedings of China 4 th Symposium of Ground Treatment, pp

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