Bearing Capacity of very Expansive Soils at Jatinangor Area, West Java, Indonesia

Expansive soil is a kind of soil that has ability to shrinkage and swelling. According to Ronny (2014) Jatinangor area has expansive soil that is so very influential in the planning of infrastructure construction. This research aimed to measure the bearing capacity of the very expansive soils in Jatinangor area and to determine the correlation between activity number of soil and its bearing capacity. The method used is to collect the soil physics and mechanics data. Based on the soil mechanics data, the research location is divided into three zones of allowable bearing capacity, those are zone with allowable bearing capacity < 4 T/m2, zone with allowable bearing capacity 4-7 T/m2, and zone with allowable bearing capacity > 7 T/m2. The correlation between activity number and bearing capacity of soil follows the equation qa = -1.9505(A) + 6.957 with correlation coefficient is -0.7911.


Introduction
Expansive soil is a kind of soil that has ability to shrinkage and swelling. in relation to changes in water content. Soils that exhibit swelling and shrinking behavior contain expansive clay minerals, such as smectite, that absorb water, the more of this clay a soil contains the higher its swell potential and the more water it can absorb (Jones and Jefferson, 2012). The behavior of swelling on the soil can result in the differential settlement of the foundation resulting in damage to the infrastructure.
Jatinangor is an area that has rapidly developed into regional education, industry, government and commerce. It can be seen with a lot of infrastructure construction in this area. Jatinangor region is geologically composed of young volcanic rocks, so the soil produced from weathering is loose. This thing causes landslide around Jatinangor area. In addition, swelling of clay minerals on soil from weathering also influences the frequency of soil movement. This research is located around Padjadjaran University, Jatinangor District, Sumedang, West Java. Ronny (2014) has classified soils at Jatinangor to four classes based on its expansiveness, which are low expansive soil, medium expansive soil, high expansive soil and very high expansive soil. This research aimed to measure the bearing capacity of the very expansive soils in Jatinangor area and to determine the relation between activity number of soil and its bearing capacity.

Regional Geology
Based on the regional geological map of Bandung (Silitonga, 1973), the research location is composed of young volcanic rocks, tuffaceous sand, Lapilli, Lava, and agglomerat originating from Mount Tangkuban Parahu and Mount Tampomas. These rocks belong to the Quarternary volcano rocks.

Methodology
The method used in this paper is to collect the secondary data consists of the soil physics data such as weight of soil content (ᵞ), plasticity index, clay content in percent and soil mechanics data such as cohesion (c) and friction angle (ɸ) of very expansive soils at research location. The expansiveness of the soil can be known through the calculation of activity number (A). The activity number (Skempton, 1958 in Das, 1988) was formulated as a comparison of plasticity index (PI) and clay grain percentage (<0.002 mm) as follows: (1) Unit weight (ᵞ), cohesion (c) and friction angle (ɸ) then used to calculate the allowable bearing capacity of very expansive soils on a continuous foundation with the following equation based on Terzaghi (1943, in Zakaria, 2014): From these data, the zonation map of bearing capacity of very expansive soils is made using Surfer 11 software, where column A is X coordinate, column B is Y coordinate, and column C is the bearing capacity of soils. The graph of the relationship between the activity number and the bearing capacity of soils is made using Microsoft Office Excel 2016 software.

Result and Discussion
The research area is a very expansive soil zone based on an expansive soil zonation map by Ronny (2014). This very expansive soil zone has a plasticity index value ranged from 24.63% to 69.67%, whereas the more plastic a soil sample, the higher the expansive of the soil. Activity number (A) from soil samples ranged from 0.44 to 1.37 where the soil is thought to contain the minerals of illite clay, kaolinitic and montmorillonite.  Zone with allowable bearing capacity > 7 T/m2. This zone is scattered in the southern part of the research area.

Figure 4 Bearing capacity zonation map of very expansive soils
The correlation between activity number and bearing capacity of very expansive soils in research area is shown in graph with equation qa = -1.9505 (A) + 6.957 with R= -0.7911. Based on the value of negative correlation coefficient, it can be interpreted that the bearing capacity of the soil will decrease as the activity rate increases. This is due to the value of the activity number is affected by the value of the plasticity index. The higher the plasticity index, the higher the activity number.

Figure 5
Correlation between activity number and Bearing capacity The correlation between plasticity index and soil bearing capacity at research area shows correlation with equation qa = -0.041 (PI) + 7.162 with R= -0.783. It also can be interpreted that the soil bearing capacity will decrease when the plasticity index increases.

Conclusion
A bearing capacity zonation map of very expansive soils consists of three zones based on the value of allowable bearing capacity, which are zone with allowable bearing capacity < 4 T/m2, Zone with allowable bearing capacity 4-7 T/m2, and zone with allowable bearing capacity > 7 T/m2.
The correlation between activity number and bearing capacity of very expansive in research area is shown in graph with equation qa = -1.9505 (A) + 6.957 with R= -0.7911. Based on the value of negative correlation coefficient, it can be interpreted that the bearing capacity of the soil will decrease as the activity number increases. To build infrastructure on expansive soils, stabilization is required. Stabilization on expansive soils can be divided into 3 types: mechanical stabilization, physical stabilization, and chemical stabilization (Ingles and Metcalf, 1972 All authors want to say thank you to God because of His bless, this paper can be done. Authors also want to thank all people who involved of this paper, Padjadjaran University and also to Bandung Institute of Technology to give us chance to submit this paper.