Effect of installing columns from different solid waste materials on the soft clay bearing capacity

. Soft clay deposits cause many problems due to low bearing capacity and high deformation. This paper presents the results of laboratory model tests for studying the improvement of soft clay-bearing capacity using two kinds of soil columns, the first is slag-cement dust columns and the second is bentonite column. Three columns from SCC (slag-cement dust columns) encased by Woven Geotextile are installed beneath a steel plate representing a footing. Four bentonite Columns (BC) encased with non-woven geotextile are installed around the steel plate at a distance of 1 B, where B is steel plate width. The research aims to assess the increase in the SCC length effect on clay-bearing capacity. A series of 4 experimental tests were performed. The results show that, the clay bearing capacity increased with the increase of column length because slag and bentonite can absorb the clay water content, leading to an increase clay shear strength. Floating columns gives better improvement than the end bearing columns. The slag partially replaced with cement dust is weak enough to transfer the total applied stress to a strong soil layer.


Introduction
A wide range of various improvement techniques is implemented to improve the clay-bearing capacity and reduce excess settlement such as stone columns [1], grouting with lime, lime silica [2], and [3]. Some techniques depend on using lime, fly ash, and cement, as presented by [4], [5], [6], [7]. Further improvement to the soft ground by introducing the soil-cement column method as reported by [8]. Alternative methods to enhance the load-bearing capacity of soft soil by soil reinforcement technique are presented by [9]. The bearing capacity of footings on soft clay can considerably be improved by placing a layer of compacted granular fill of limited thickness without/with geotextile or geogrid reinforcement at the sand-clay interface [10]. Some of these techniques are very expensive because of raw material and installation expenses, columns using waste material are one of the most are one of the most effective and economic techniques. Soil columns are installed in the field with the same methods. Although the columns method is widely used, the interaction between the soft soils and the column is not well -understood, but it proved to give excellent results. There are different solid waste materials produced in Egypt due to the industrial factories, and these materials cause environmental pollution. Slag & cement dust are solid waste materials produced from steel and cement production. In this study, Slagcement dust columns (SCC) are installed in soft Clay beneath a steel plate as isolated footing with dimensions 200 mm * 200 mm * 20 mm. Bentonite columns (BC) are installed around the steel plate. Soft Clay used in this research was brought from Mahmoudia city -Alexandria government. SCC is encased by woven geotextile, while a non-woven geotextile encasement was used for the BC. The proposed research studies the effect of floating columns and end-bearing columns on soft clay-bearing capacity and settlement. The columns length was 60%, 80% & 100% from the clay layer thickness.

Soft Clay
The soft soil used in this research was natural deposit soft clay from Alexandria. The physical and mechanical properties of the clay under study are listed in Table 1

Slag
Slag material is considered as a solid waste material from the manufacture of steel. The slag type used in this research was water-cooled Blast furnace slag because, that type can absorb water. Slag mixed with a small amount of cement dust to form the final mixture material was used to form columns beneath the footing. Table 2 shows the laboratory tests for slag.

Cement dust
Cement dust is a solid waste material from the Tora factory in Egypt. A chemical analysis test was performed to determine the cement dust components, as shown in Table 3.

Encasement material
Woven geotextile and non-woven geotextile were used to wrap columns. Woven geotextile was used to confine the SCCs, providing strength against the lateral pressure due to the stress applied. It can interlock with Clay because the openings' sizes are slightly oversized. Non-Woven geotextile was used to confine the bentonite columns, preventing the mixing between Clay and bentonite and allowing the excess water to enter the column to be absorbed by bentonite. Table 4 shows the Geotextile properties.

Trial mixes from slag & cement dust
Different percentages of cement dust & slag were mixed and submerged in water for seven days to determine the optimum mix for using it to form SCC. After seven days, samples were extracted from water and tested. The optimum percentage is corresponding to the maximum compressive strength after seven days. The results in Fig. 1 showed that the optimum percentage for cement dust and slag was 17 % & 83 % slag, respectively. These percentages will be used to form the SCCs.

Test preparation
The following procedures were performed before conducting the experimental tests: 1-The base filter consists of gravel wrapped by non-woven geotextile. The purpose of the installation is to decrease the test consolidation time. The filter is wrapped with geotextile to prevent mixing gravel and Clay. 2-Soft Clay was placed in layers and mixed with water until the clay cohesion reached 12 kPa to reach the initial cohesion. The clay bed thickness is 55 cm. 3-SCC & BC were installed by the displacement method using a 5.0 cm diameter PVC pipe. The geotextile is wrapped around the outer pipe face and installed in Clay. Then the mix was placed inside the pipe using a funnel and compacted until the column material reached the maximum density. During the compaction, the pipe was extracted, and the geotextile is remained in Clay to form SCC and BC columns. after compaction, the density of Slag-cement dust material was density 1.7 t/m3 (Fig. 4). 4-A sand layer with a thickness of 5.0 cm was placed above the SCC to work as a sand filter to decrease the consolidation time during the test. (Fig. 5) 5-After columns installation, a steel plate with dimension 200 mm * 200 mm * 20 mm was used as a footing above SCC. The measuring system is as follow: -a load cell was used for measuring the applied loads on footing.
-LVDT was used for recording the footing settlement during a test. -A strain gauge was used for observing the geotextile hoop strain during the test.
Finally, the test configuration was ready to perform the proposed tests.

3.3
Loading procedure After the test configuration was ready, different loads were applied to the clay bed for each test. An I-beam is responsible for transferring and magnifying the loads on Clay. The beam transfers the load to the load applier rod and magnifies it by a factor of 4.

Experimental program
The main testing program consisted of 4 tests. Table 5 shows the experimental test configuration.

Clay failure shape
After gradual applying of the load., the clay yield failure shape as shown in Fig. 6. During and after the tests, some cracks were created in the clay surface due to the drying process as the installed columns absorbed the clay water content from the surrounding area. These cracks proved that SCCs and BC did their function to absorb water from the surrounding soil. Fig. 7& Fig. 8 show the bentonite and slag-cement dust soil after the test.  The soil columns were observed to be in saturated case. The slag-cement dust seems to be as gravelly soil, because the cement dust material cemented the slag particles with each other, due to the hydration process in the presence of water. Samples from bentonite and slagcement dust mixture were taken before and after the test. Fig. 9 & Fig. 10 &. For slag-cement dust soil, the water content before the test was almost 0%, but the water content varied from 15 to 20 % after the test. The soil's ability to absorption depends on the surrounding water content as the clay water content varies vertically and horizontally. The moisture content of SCCs after the test was nearly around 35% compared to a value of about 13 % before the test.

Effect of column length on bearing capacity
From the test results it was found that, the installation of SCCs and BC in soft clay increase the clay bearing capacity because the installed columns accelerate the exit of water from clay so, the consolidation rate and clay shear strength increased. In case of increasing the columns length, the surface exposed to the columns became greater thus, the treated area become larger. In addition, the friction surface between the columns and the clay increased so the clay bearing capacity increased. In case of end bearing columns (columns length equal 100 % from the clay bed thickness), the clay treated with end bearing columns was enhanced more than clay treated with floating columns but with small difference as the columns soil cannot transfer the total stress to the deeper soil as the column soil is not very hard to transfer loads [13]. Fig. 11 shows the comparison between all tests.

Variation in geotextile hoop strain
As a result of loading soft clay treated with SCCs and BCs, the cross-section of the column was changed because of bulging. The bentonite column volume changed due to swelling. At the end of the test, the SCC's & BC's cross-sections were measured using strain gauges along their lengths, to deduce the difference in cross-section. Fig. 12 shows the horizontal deformation in the SCCs cross-section. The results show that the maximum bulging nearly occurred at the column mid-height. Fig. 13 shows the difference in BC's cross-section. The BCs cross-sections increased because the clay water absorption offset the volume change for BC whole volume.

Conclusions
Depending on the Experimental & Laboratory tests performed, it can be concluded that: -SCCS & BCs enhanced the soft clay ultimate bearing capacity by 241 % because the installed columns absorbed the water content from the surrounding area so, the consolidation rate and clay shear strength increased, and also, the SCCs act as a friction pile due to the interaction between the columns soil and Clay in the presence of woven geotextile. - The increase in column length led to an increase in clay ultimate bearing capacity as the surrounding soil between columns and Clay increased, so the friction surface also increased. -Bentonite and slag have an excellent effect for absorbing clay water content, leading to an increase in hear strength, and consequently the composite ground bearing capacity. - The encasement material provides the SCCs with strength towards lateral loads.