Soil mechanics and investigation

GEOSCIENCE

By MICHAEL JOHN UGLO
WELCOME all to our tenth lecture in the Sciences of the Earth series.
Soil as we know has multiple uses and in geoscience we will explore its engineering properties. Soil principally is defined to its finite constituents to derive its usefulness.
Geotech is the study focused on the mechanics of the soil as well as the mechanics of the rocks. The two fields of geotech and engineering geology overlap in their applications. It is geotech engineering that takes precedence in the particular uses of soil for applications in works such as dam constructions, building embankments, retaining structures for landfills, building foundations for constructions as roads and earthworks.
The mechanics of the soils and rocks are paramount. Apart from engineering geology, in geotechnical engineering soil properties are given foremost consideration. The engineering properties of the soil include the characteristics of soil in its permeability, porosity, the shear strength, the voids, compressibility, the Atterberg limit and the specific weight or the specific unit weight. The particular considerations of the above properties come under the specific presence of the proportion of the predominant mineral, the type of that mineral, its grain size distribution and the proportion of the availability of the mineral, water and air.
Prior to any soil being considered for use in any civil engineering work, it is foremost considered that soil is composed of a three component parts of a particular mineral, water and air.
Shear strength of a particular soil is when a force is applied to the soil that is able to withstand that amount of force without causing any shear failure. The Atterberg limit is used to show the liquid limit, the shrinkage limit as well as the plasticity limit can be a good measure. These are effective stresses leading to the shearing that causes the soil and its mechanics in those certain circumstances to withstand or to fail when going beyond its limit.

Dilation occurs to a mass of consolidated soil particles that are densely packed while in contraction with granular loose materials whether it be rock or soil.
The specific weight or the specific unit for any particular soil is the mass of that soil and the water content within the voids. Normally the air is disregarded because its mass is negligible. The porosity is the measure of the spaces available within the mineral particles. It is related to the measure of voids. The voids and porosity are related to each other in that porosity includes the mineral particles as well as the air and water while voids are the availability only of the spaces and the mineral particles.
Atterberg limit is the shrinkage limit of a particular soil as well as its plasticity limit and its liquid limit. When a particular mineral’s limit in any of these three Atterberg limits are exceeded, it will result in shrinkage, fracture and shear failures. The foundations that are laid in any civil engineering work such as retaining walls for sanitary landfills, building of dams or any foundation structures for any constructions takes into consideration the Atterberg limits to cater for the effective stress. The earth works are very vital considering settlement of soil to the bed rock or to create the soil stability with settlement that can withstand the weight of the structure to be built.
The compressibility is the engineering property of the soil that looks at the water content of the soil contained in its voids or more simply its porosity. When the water is squeezed out of the soil, it is basically called the process of consolidation. Voids and porosity are both related by the formula n = e / 1 + e where n is for the porosity while e represents the voids.
The permeability is the amount of water that flows in a particular soil at a particular rate. That includes the timing and volume of water flow in a particular soil and the rate is measured in Darcies. Different minerals of soil allow different rates of water flow because they have different porosities and voids.
Prior to any civil engineering works, both surface and subsurface considerations are given attention. For surface surveys, a geotech engineer could basically attempt to see the features such as the geomorphology and looking at land features as well as surface rocks and rock structures. An advice from a geologist is much needed when doing a surface survey: https://petgeo.weebly.com/uploads/9/9/1/0/99107128/gp_orig.jpg
Electromagnetic surveys can be done including the resistivity of underlying ground water potentials, the use of magnetometer, as well as the use of ground penetrating radar in subsurface investigation prior to a civil engineering works.
Sub-surface investigation demands digging into the subsurface layer to obtain soil samples to be tested in situ or brought to laboratories to determine their engineering properties.
The soil and rock mechanics and their engineering properties apart from civil engineering and geotechnical engineering can be used in petroleum engineering, mining, coastal engineering, the military as well as offshore constructions.

My Prayer for PNG today is: “Love is His name, love is His law. Hear His command, all who are His: “Love one another, I have loved you” Love only love is His law. “

Next week: Structures in infrastructures and technology

Michael John Uglo is a science textbook author and a lecturer in avionics, auto-piloting and aircraft engineering. Please send comments to: [email protected]