Study of Load-Settlement and Consolidation Behavior of Pond Ash and Stone Dust Column Installed In Soft Clayey-Silt Soil

Abstract: An attempt is made to use pond ash and stone dust (stone passing through 600 micron) to form column that will be installed into soft soil. The column made of pond ash and stone dust is stiffer and stronger than the original soil. The properties of soil, pond ash and stone dust are determined in the laboratory. Load test on column installed in clayey-silt soil is conducted by varying the loading plate size through which load is applied on soil column system. The diameter of the column is 20 mm and the length is taken as 200 mm. A tank of size 320 mm × 320 mm × 400 mm is used of as model testing tank. Test is conducted at liquid limit (= 45%) and water content maintained throughout the tests in the range of 45 ̶ 46%. Bulging depth of the pond ash column and stone dust column installed in soft soil is also determined after each loading test. For understanding of consolidation behavior of the soil column system consolidation tests are conducted in laboratory. The effects of pond ash and stone dust used as column on load-settlement behavior and consolidation characteristics are also premeditated. Test results show that column made of stone dust is better than pond ash for soft clayey-silt soil.

indicated that stone columns transfer the load to the surrounding clay through bulging action which helps in mobilizing the passive resistance of the surrounding clay. Till date stone column have been studied by many researchers as well as scientists. It is a pre-requisite to study the properties stone column, sand column which are in use today for developing a column with new material other than stone. In the present study special attention is given to the stone column technique to improve soft soils. An attempt has been made to use pond ash and stone dust (passing through 600 micron) to form columns which installed into soft clayey-silt soil. In this study clayey-silt soils are mixed with water at liquid limit. That is why this clayey-silt soil is considered as soft soil. These types of small column provide reinforcement and drainage facility to the soft clayey-silt soil and improve the strength and deformation properties of this soil. The existence of the column generates a composite material that is stiffer and stronger than the original surrounding soil.

II. DESCRIPTION OF LABORATORY MODEL TESTS A. Materials and Experimental Programme
Three types of material are used in this study i.e., pond ash, stone dust (stone passing through 600 micron) and clayey-silt soil. The physical and engineering properties of these materials are evaluated in laboratory by conducting various laboratory tests. Table 1 shows the properties of pond ash, stone dust and clayey-silt soil. In the present study a unit cell model is used. Generally, in the field there are numbers of columns installed in the soil. In that case centrally placed column is generally considered as unit cell and the same concept implemented when test conducted in the laboratory using single column installed in soil within the testing tank. This is called as unit cell concept. References [1, 2, 5, 7 and 13] have used the unit cell tank in their laboratory model tests. To make the simple design of apparatus and assess the behavior of column made of pond ash and stone dust a rectangular tank is used. To avoid the effect of boundaries of the testing tank on loading a size of 320 mm × 320 mm × 400 mm (height) testing tank is used to represent the unit cell area. The diameter of the columns used for model test is 20 mm and the length of each column is 200 mm. The column is installed at centre of the tank installed in soft clayey-silt soil as shown in Fig. 1. Load is applied on single column in three ways: 1. Through loading plate of diameter equal to column diameter; 2. Through loading plate of diameter equal to twice the column diameter; and 3. Entire area loading (i.e., size of square loading plate is 300 mm × 300 mm).

B. Preparation of Clayey-Silt Soil Bed and Installation of Column
The soft clayey-silt soil is mixed thoroughly with water at liquid limit of 45 %. A 50 mm bed is prepared by compacted sand. The tank is filled with soil water slurry surrounding the column in five layers up to length equal to the column length. . A uniform compaction is given to each layer of soil water slurry in testing tank to achieve a uniform density of 14.7 kN/m 3 at liquid limit. A column is installed at the center of the tank by displacement method. A casing pipe having an inner diameter equal to diameter of the column and 2 mm thickness is used to form the column. The casing pipe is placed at the centre position on the soil bed. After positioning the casing pipe the premeasured amount of materials used to form column is poured into pipe in 3 layers by continuous lifting of the pipe to form the column. Each layer is compacted with 25 numbers of blows with tamping rod. Subsequently, the tank is filled by pouring the estimated amount of soft clayey-silt soil-water slurry in five layers. The top surface of the column is leveled properly. After that the tank is kept in close room for a period of 1 to 2 days. The moisture content of the testing tank is maintained in the range of 45-46%. In the testing tank drainage facility is provided. For every load test, the clayey-silt soil bed is to be laid down afresh in same manner. The plan area of the tank is preferred in such a way that the effect of tank boundaries on loading system has no effect. Column of pond ash and stone dust (stone passing through 600 micron) used is shown in Fig. 2.

C. Load Test on Column
After finishing the installation programme of the column, the entire tank along with soil-column system is placed on Compression Testing Machine is shown in Fig. 4. The column is subjected to vertical load through loading plate of various sizes as mentioned earlier for load-settlement test. Loading test is shown in Fig. 3. The thickness of all the loading plates is 10 mm. The settlement in the column is measured with the help of a dial gauge of accuracy 0.01 mm at suitable intervals and the load is measured through a proving ring of capacity 5 kN.

III. RESULTS
In laboratory, total 12 numbers tests are carried out for both load test and consolidation on single columns installed in soft clayey-silt soil, in which 9 numbers of tests for load-settlement behavior and 3 numbers of tests for consolidation behavior of soil-column system.

A. Load -settlement test:
The load-settlement tests for single column are carried out on column up to a maximum settlement of 40 mm. The values of load carried by column using pond ash and stone dust (stone passing through 600 micron) corresponding to a settlement of 40 mm are in the range of 0.374 to 1.017 kN and 0.473 to 1.547 kN respectively for all the three sizes of loading plate of this study. Loads carried by clayey-silt soil alone without any column is in the range of 0.309 to 0.950 kN for all the three sizes of loading plate. Details are shown in Table 2. B. Consolidation test To understand the consolidation behavior of soil-column system consolidation tests are carried out in laboratory. These tests are carried out at liquid limit of 45 %. Table 3 shows the consolidation properties of soft clayey-silt soil and column of different materials installed in soft clayey-silt soil. DISCUSSIONS Load tests and consolidation tests are carried out on columns made of pond ash and stone dust installed in soft clayey-silt soil in testing tank. Effects of materials, size of the loading plates, failure surface and consoilidation characteristics (i.e., C c , and c v ) are discussed herein.

A. Effect of materials of columns on clayey-silt soil
Different materials are used to form column i.e., pond ash and stone dust (stone passing through 600 micron). Figure 5 shows the effect of material on load-settlement behavior of soil-column system using single column in testing tank. In the laboratory test result on single column it is seen that stone dust (passing through 600 micron) column improves the bearing capacity of soft clayey-silt soil more than that of column made of pond ash in clayey-silt soil. This may be due to stiffness of the materials used in this study. The stiffness ratio of the stone dust and pond ash are 0.0164 kN/mm and 0.0110 kN/mm respectively. The material having higher stiffness ratio is stiffer materials than others. Stiff material can take more loads at a particular settlement installed in soft clayey-silt soil. That is why stone dust column carries more loads than pond ash column. The improvement of soft clayey-silt soil using column made of pond ash is less because of low density of pond ash and also as the weight of individual particles in pond ash less. Ultimately it is observed that in case of clayey-silt soil with either column made of pond ash or stone dust, load carrying capacity of soil increases, irrespective of size of loading plates.

B. Effect of size of loading plate on columns
The diameter of the loading plate affects the load-settlement behavior of the clayey-silt soil. Load is applied through the plate having the diameter of column, twice the diameter of column and over whole area of the testing tank on pond ash and stone dust column installed in soft clayey-silt soil. By varying the plate size through which load is applied on soil-column system, loading condition of the model test has changed. At different loading condition bulging effect of column, load carried by column at different settlement, effect of diameter and materials of column on load-settlement behavior of soil-column system are studied. Figure 6 shows the effect of loading plate on load-settlement behavior of soil-column system. It is noticed that the larger loading plate offers more load at any settlement than the other plate. Load over entire area for column of pond ash and stone dust shows maximum load carrying capacity. It is due to contact area between loading plate, columns and surrounding soils. Large size plate contacts more surface area of soil-column system in testing tank than the plate equal to column diameter which is in smaller size. In case of plate of diameter d, load is applied on column only where as for plate of diameter 2d, not only the column but the surrounding soil also takes the load. That is why load obtained in case of plate diameter 2d is higher than the d. For same reason, maximum load for same materials at same settlement is obtained using the plate size of 300 mm×300 mm.

E. Effect of column on settlement of clayey-silt soil
It is observed that pond ash column installed in soft clayey-silt settles more as compare to stone dust column. Stone dust is poured into soft clayey soil in dry condition to form the column of 20 mm diameter where as the water content of the surrounding soil is 45% (i.e., liquid limit). stone dust helps to expelled out the water from voids of the surrounding soft clayey-silt soils quickly than pond ash as void ratio of stone dust is more than that of pond ash and clayey-silt soil (table 1). Figure 10 shows the e-log p curve for pond ash and stone dust column installed in soft clayey-silt soil.

F. Effects compression index and coefficient of consolidation on column installed in soft clayey-silt soil
C c value of clayey-silt soil is higher than that of pond ash and stone dust column as shown in Fig. 11(a). This is because of inserting the stiff material into the soil compare to the surrounding soft clayey-silt soil. Pond ash is stiffer than soft clayey-silt soil and stone dust is stiffer than pond ash. Due to C c, values, settlement of the stone dust column installed in soft clayey-silt soil is smaller than the pond ash column and than that of soft clayey-silt soil. Coefficient of consolidation (c v ) also effect the settlement of soft clayey-silt soil. Figure 11(b) shows the effect of c v on soft clayey-silt soil, pond ash and stone dust column installed in soft clayey-silt soil. As c v increases time required for settlement decreases. Because of stiffer material and more voids in particles, stone dust settles within a short time than pond ash and soft clayey-silt soil. On the other hand the individual particles of pond ash is thin and light in weight and cannot expelled out the water from surrounding soft clayey silt soil easily than that of stone dust; take more time to complete the consolidation process compare to stone dust column. soft clayey-silt soil take maximum time to consolidate as the particles are fully saturated and there is no additional material in to it to help in flow of water. c v value is higher for clayey-silt soil with stone dust column than that of pond ash column and soft clayey-silt soil without column.