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Soil Engineering

Seismic Refraction Method

It is also called a seismic refraction survey. It is one of the types of geophysical methods used in civil engineering. We know that seismic waves have different velocities in different types of soil or rock. The seismic waves are refracted when they cross the boundary between different types of soils. The seismic refraction method is based on these two facts. This method helps us to determine the types of soil and the approximate depth of boundaries of strata or the bedrock.

The method consists of inducing impact by striking a plate on the soil with a hammer or generating shock by exploding a small charge at or near the ground surface. It radiates the shock waves which are recorded by a device called a geophone. The geophone records the time of travel of the waves. These are installed at suitable known distances on the ground in a line from the source of shock. Some of the waves known as direct or primary waves travel directly from the shock point along the ground surface. These waves are first picked by geophones. The other generated waves travel in a downward direction at various angles to the horizontal. These waves get refracted at the interface of two soil strata if they travel into a stratum at different seismic velocities. These waves are picked up by geophones. As the distance between the source of shock and the geophones increases, the refracted waves reach the geophone earlier than the direct waves. By knowing the time of travel primary and refracted waves at various geophones, the depth of various strata can be evaluated by preparing distance-time graphs and analyzing them.


Electrical Resistivity method for Soil Exploration

It is one of the types of geophysical methods in civil engineering. It is based on the measurement and recording of changes in the mean resistivity or apparent specific resistance of various soil. The resistivity of soils is a complicated function of porosity, permeability, ions present in the pore water, and clay mineralization. The most common electrical methods used in hydrogeological ad environmental investigations are vertical electrical sounding (resistivity sounding) and resistivity profiling. The resistivity ρ is defined as the resistance between the opposite faces of a unit cube of material. The variations in resistivity can be detected significantly between different types of soil strata, above or below the water table, between unfissured rock and soil, between voids and soil or rock.

Zero circle

Keywords: Zero circle, circle of correction, planimeter, area computation using planimeter

We already discussed the instrumental method for area computation i.e., the planimeter method. If you want to freshen your knowledge then read it here.

Zero circle or the circle of correction, is the circle around whose circumference if the tracing point is moved but no rotation of the wheel will be caused.the wheel will slide on the paper without causing any change in the reading.

Measurement of the area using Planimeter

Keywords: Planimeter, measurement of area, zero circle method

Planimeter is an accurate instrumental method used for the computation area. Read more about Planimeter here.

For measuring the area of plots with planimeter, two methods are generally adopted.

Instrumental methods-use of planimeter

Keywords: planimeter, Computation of area, zero circle
Instrumental methods are one of the important technique uses to determine area of a plot. The other methods for area computation are using field notes and from previously plotted plan. We have already discussed them here.
Computation of areas and volumes | Computation of Area from Plan

Significance of Planimeter in Area computation
The planimeter gives more accurate (within one percent) results compared to the other methods except by direct calculation from the field notes. It is used for determining the areas of figures plotted to scale, especially when the boundaries are irregular. Example for such plots are, plot with irregular boundary and plot having a river as boundary on one side.