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3. Engineer Detachment (Terrain), 579th

Title:	Geology/soils survey of the Mekong Terrace
Authors:	United States. Army. Engineer Detachment (Terrain), 579th
Keywords:	Soils--Vietnam--Mekong Basin
Publisher:	United States. Army. Engineer Detachment (Terrain), 579th United States. Army. Engineer Command, Vietnam Engineer Research and Development Center (U.S.)
Abstract:	Summary: The Mekong Terrace, comprising much of III CTZ, is a level to rolling plain which gradually increases in elevation from the Mekong Delta in the southwest to the Annam Mountains in the northeast. The present surface topography of the Terrace is the result of random erosion of a tilted surface of deposition. In the southwest, the rivers flow in broad, flat alluvial valleys. In the northeast, the streams have been entrenched into the surface, build valley profiles are steep. Vegetation consists of ricelands in the southern portion of the Terrace and along the major river valleys which flow from the north, rubber plantations on the higher ground near the major roads and villages, and dense broadleaf evergreen forests in the northern and eastern portion of the Terrace. There are only a few limited outcrops of hard rock exposed in the Terrace. The very old bedrock surface has been filled in by a few hundred feet of Ancient Alluvial Sediments, Recent Alluvial Sediments, and Basalt Flows. The Ancient Alluvial Sediments are predominantly grey clayey fine sands (SC); they cover approximately 2/3 of the Terrace. These soils are sensitive to pumping 11 when wet but serve as a stable subbase or base course if sealed and qrained. The clayey fine sands must be washed in order to be used as a fine aggregate or a select free-draining material. Recent Alluvial Sediments (plastic clays (CH) and sandy silts (NL-CL)) nave been deposited in the Mekong Delta, and in the major river valleys which drai.11 the Mekong Terrace. These soils are usually soft and wet and require drying in order to be used in a compacted fill. Their plasticity and low strength make them unsuited for use as a base course. Basalt flows are found in the eastern portion of the Terrace, at the edge of the Southern Annam Mountains. Much of the basalt has weathered to a red-brown clayey silt (ML-MH) soil. The soils are soft and slippery when wet but stable in D. compacted fill. Thicknesses of the silt range from one or two feet to a few hundred feet over the basalt. Where exposed, basalt and weathered basalt fragments can be excavated by dozers for use as a gravel fill or a low-grade base course. The weathered basalt fragments tend to disintegrate under traffic. The best source of select materials are the laterites which cover much of the Terrace. Hard massive laterite is found in the Ancient Alluvium; its formation is related to the tilting of the Terrace. Hard pellet laterite is found in the basalt soils and is probably forming today. Both types of laterite are used as a base course and surface course for roads and airfields. When excavated the laterite breaks down into a gravel with some clay (GC). The percentage of fines, plasticity of the fines, and hardness of the gravel particles determine its suitability for construction purposes. Quarry sites are limited to isolated hills of granitic and metamorphic rock. Granite rock is usually deeply-weathered; surf ace boulders are common. Clean sand and gravel sources are extremely limited in the Mekong Terrace. Clean fine to medium sands may be obtained by dredging the major rivers.
Description:	Geology/Soils Survey
Rights:	Approved for public release; distribution is unlimited.
URI:	http://hdl.handle.net/11681/21686
Appears in Collections:	Engineer Detachment (Terrain), 579th