Soil in Liwa, Lampung, Sumatra Island, Indonesia

In general, the southern region of Liwa (West Lampung, Sumatra Island) is covered by the residual soil. Soil residues in this region formed by weathering processes are in-situ in the parent rock without experiencing erosion or transportation. Condition tropical regions resulted in the formation of residual soil in Liwa area controlled by the degree of chemical weathering. Climatic and topographic factors also indirectly affect the soil formation process in Liwa area because of these factors helped determine the level of weathering and the thickness of the residual soil. Soil residues from the Liwa area derived from volcanic rock deposition filled up most of volcano hills and valleys. Residual soil weathering is the result of volcanic material such as tuff which has the highest plasticity and high compressibility. It also has a characteristic level of intensive consolidation (Wesley, 1988).

The residual soil in this area can be divided into two types based on their physical appearance. Brown residue on the soil top layer of 0-3 m, whereas below this layer is a red residual soils with a depth> 3 m. They are on the horizon E and B according to the classification of soil profiles according to Soil Survey Staff. Brown residue in the soil on eluviated horizon (E horizon) is characterized by light brown color, many lost their silicate minerals, clay, iron elements, or aluminum due to the washing process and leaving sand or silt particles of minerals resistant. Ground red residues are on illuviated horizon (B horizon) is characterized by red, mineral concentration washing process results in the form of clay minerals, carbon, sesquioxides of iron and aluminum elements.

DETAILS OF GEOTECHNICAL CHARACTERISTIC IN THIS AREA:

·                     Consolidation and compressibility tests showed a tendency to brown residual soil settlement properties (decrease in building construction) are high enough compared with the residual red soil. In accordance with the thickness, consolidation brown residue on the soil is estimated to occur up to 3 m.

·                     Based on the compaction test, natural water content of soil residues (brown and red) are high and are slightly above the optimum water content. As piling material, it is necessary to obtain some degree of drying for the maximum resistance power.

·                     The flow of water occurs vertically from top to bottom through the soil horizons. This process is triggered so that the leaching process of accumulation of minerals often found in the layer B (red residual soil) as enrichment. Lateral water flow at the surface and the erosion is minimal, so that the process of formation of sedimentary soil less than the residual soil widespread in this region.

·                     Soil residues in this region (brown and red) are loaded with halloysite clay minerals, high plasticity value, so that is sensitive to the effects of vibration and changes in pore water pressure. In saturated conditions can cause instability and prone to landslides, especially on steep slopes.