Soil Exploration: Methods, Types & Objectives

What Is Soil Exploration?

Soil exploration is the process of carrying out fieldwork and laboratory tests to get the information about soils needed for the appropriate design and construction of any project. It allows engineers to create soil profiles that show the order of soil layers as well as the related soil properties.

The various engineering properties of the underlying soil have a significant role in the choice of the foundation as well as the depth, bearing capacity, and settlement study. We can collect and evaluate soil information after laboratory testing with the use of various soil investigation techniques.

The primary goals of soil exploration are always to identify the nature of the soil deposits, the depth, and thickness of the various soil layer, and the position of the groundwater table. To obtain soil and rock specimens for testing, and to determine the engineering properties of the rocks and soil layers by laboratory or field assessments which influence the performance of the structural system situated on it.

Stages Of Soil Exploration

The soil exploration is carried out in stages as shown below.

  • Site reconnaissance
  • Preliminary site exploration
  • Detailed exploration
  • Preparation of soil investigation report

Site Reconnaissance

The initial phase of a site assessment is mostly reconnaissance. The primary goal of this exploration stage is to get a general understanding of the local soil type.

The reconnaissance phase also known as the initial stage involves gathering the basic information about the site is collected and it is the most important activity for carrying out a site investigation.

It involves going to the location and looking over the maps and some other essential documents.

In order to decide on future exploration programs, this research aims to obtain a basic soil profile as well as representative samples of the main soil layers and groundwater conditions.

This research must be completed at the lowest possible expense, and no large-scale exploration work is commonly performed at this phase.

Preliminary Site Exploration

The preliminary site research is conducted for smaller constructions, light structures, roads, and air bases.

The primary goal of preliminary exploration is to provide a rough image of subsoil conditions at a minimal expenditure. The subsoil information obtained should be adequate to design and construct light structures.

The basic data obtained during preliminary site exploration is listed below.

  1. Estimated values for the compressive strength of the soil.
  2. Location of the groundwater level.
  3. The depth and extent of the soil layers.
  4. Composition of the soil.
  5. Depth from the ground to the hard stratum.
  6. soil engineering properties

Detailed Exploration

For a detailed analysis of the soil, digging, sampling, and testing are carried out to determine the engineering properties of the soil.

A detailed study is required for complicated projects, major engineering tasks, and heavy constructions including dams, high-rise structures, and bridges. 

The detailed site investigation would cost a considerable amount of money, so it is not recommended for small engineering projects with limited budgets.

For small engineering projects, the information gathered by initial site investigation is sufficient. The various site investigation methods that are utilized for both preliminary and detailed site investigation are described in detail below.

Method Of Soil Exploration

The methods that can be used for soil investigation are

Direct Methods

Test or trial pits

Trial Pit
Test or trial pits

The test pits or trial pits are simple and open systems for exploring the soil. Trial pits are mostly appropriate for shallow-depth investigations. Every type of soil can be tested in a trial pits method.

Test or trial pits are holes that have been dug out to help the geotechnical engineer to check the ground’s suitability for the foundation of the planned structure. Before beginning any work, geotechnical engineers can investigate the composition of the soil by using test pits with an approximate depth that ranges between 3 to 15 feet deep depending on the soil stability.

It is simple to collect soil samples from the subsurface, which can then be utilized for appropriate laboratory testing to determine the strength and some other engineering properties.

Semi-Direct Methods

Boring is the method of creating or drilling holes into the ground to collect soil or rock samples at certain depths. The various types of borings include:

Auger Boring

Auger Boring
Auger Boring

The instrument called a “soil Auger” is useful for drilling a borehole deep into the earth.

The usage of hand-operated augers is restricted to shallow depths only around 3 to 5 meters, whereas the use of power-driven augers is more common at deeper depths.

The soil auger is pushed downward by turning it and simultaneously pushing it into the soil. The auger is taken away when it is fully loaded with soil, and then a specimen of the soil is taken. There are two types of augers: post-hole augers and helical augers.

Auger And Shell Boring

The soil is protected from sliding by using a pipe described as a “Shell” or “Casing” when the hole’s sidewalls cannot be left unsupported. Casings are suitable for both sand and hard clays.

The casing must be driven first, then the auger. The auger must be removed every time the casing needs to be extended, which slows down the work’s progress.

The Power-driven augers are utilized as an instrument named a “Boring Rig”, which can go as deep as 50 meters whereas the hand rig can be suitable for borings up to 25 meters deep.

Wash Boring

Wash Boring
Wash Boring

It is a simple and quick method for drilling holes in the ground. In wash boring, the hole is augured to a shallow depth, and afterward, the casing pipe is pushed down into the soil to keep the bore hole’s walls from collapsing.

The chopping bit connected to the base of a hollow drill rod is used to continue the boring process. Water is pushed with pressure by the drill rod, which is simultaneously elevated, dropped as well as turned.

The jetting and cutting movement loosens the soil. The annular area between both the drill rod as well as the casing is used to push the loosened soil up to the ground level in the condition of a soil water slurry.

Percussion Drilling

Percussion Drilling Method
Percussion Drilling

Percussion drilling involves the heavy drill bit hung on a drill rod or cable that is pushed through a series of strokes.

Water is used to ease the cracking of hard rock or soil. At regular intervals, the slurry of the crushed material is drained off. This technique is slow in plastic clay as well as it cannot be utilized in loose sand.

Rotary Drilling

Rotary Drilling
Rotary Drilling

Rotary drilling is mainly adopted for soil exploration tasks when the requirement for deeper boreholes in hard with boulders, broken rock, or water-logged sand occurs.

In this process, a power rig rotates the cutter bit or the core barrel with a coring bit connected to the bottom side of the drill rods.

The bit breaks the material into smaller parts by chipping, cutting, and grinding. The material is consequently removed by pumping water or drilling slurry by using the hollow drilling rod. There is no need for a casing for the hole when drilling mud is utilized.

Read More: What Is Soil Stabilization | Soil Stabilization Methods | Types of Soil Stabilization

In Direct Methods

Sounding Or Penetration Tests

The change in sample or cone penetration resistance is used in sounding methods to determine a few of the physical properties of layers.

Geophysical Methods

Geophysical Method
Geophysical Methods

General limits of drastically different strata are identified by geophysical methods.

The most popular geophysical methods are:

Seismic Or Refraction Method

Seismic Refraction Method for Soil Exploration
Seismic Or Refraction Method

The seismic sound waves that occur from an impact or shock at a spot on or within the ground move through the surrounding soil at speeds according to its elastic properties. The magnitude of the velocity is measured as well as used to describe the material.

The shock can be produced by hitting a strike plate on the ground using a sledgehammer or by detonating a smaller explosive charge at or beneath the ground level.

Electrical Resistivity Method

Electrical Resistivity Method
Electrical Resistivity Method

Resistivity is a phenomenon that every material contains. The electrical resistivity method is focused on the concept that resistivity values in rock and soil layers vary adequately to allow that property to be utilized for recognition.

Electrodes are used to generate electrical currents into the ground surface in order to measure the resistivity at a place.

The resistance among opposing sides of a material’s unit cube is commonly used to describe resistivity. So, every soil contains its own resistivity which depends on the amount of water present, how compacted it is, as well as the composition of the soil.

Types Of Soil Sampling

The quality of soil samples has a significant impact on laboratory test outcomes. From trial pits or boreholes, two major types of soil samples can be collected.

  • Disturbed samples
  • Undisturbed samples

1. Disturbed Samples

The phrase “Disturbed soil sample” describes a soil specimen in which the natural composition of the soil has been disturbed during the sampling procedure.

It is essential to determine the specific gravity, plasticity properties, as well as grain size of the soil using a disturbed soil sample.

2. Undisturbed Samples

In the case that the soil’s natural form and water content are not disturbed throughout the sampling procedure, it means the soil preserved its original structure and moisture content, and these sorts of soil samples are considered undisturbed soil samples.

However, we must keep in mind that it is difficult to obtain a totally undisturbed soil sample. During soil sample collection, the top and bottom of the sample were disrupted.

The engineers avoid the disturbing component of the specimens while the tests are being conducted.

The engineering properties of soils including shear strength, permeability, as well as compressibility are evaluated using undisturbed soil samples.

Read More: Water Content of Soil | Moisture Content of Soil | Oven Drying Method

Objectives Of Soil Exploration

The following list includes the objectives of soil exploration.

  • To determine the overall suitability of the place.
  • The type of soil deposits is determined.
  • To produce a foundation design that is both cost-effective and safe.
  • The measurement of the depth, thickness, as well as a horizontal extension of different soil layers.
  • To understand the nature of each layer as well as the engineering features of the rock and soil that can influence the design and technique of construction of the planned building and foundation.
  • The origin of groundwater and changes in its level.
  • To predict and prepare for any construction-related challenges caused by climatic changes and other factors.
  • Taking samples of rock and soil from different layers.
  • To determine the sources of building materials and the best locations for dumping excess water and materials.
  • Determining in-situ properties through the application of field testing.
  • To assure the safety of the nearby old buildings.
  • To identify issues that could arise with foundations and how to fix them.
  • To choose appropriate soil-improving methods.
  • To determine the soil bearing capacity.
  • To choose the appropriate construction equipment.

Purpose Of Soil Exploration

The purposes of the soil exploration are as follows

  • To Identify the essential properties of the soil, such as compressibility, strength, and hydrological condition, that have an impact on the design as well as the safety of construction.
  • Selecting the appropriate foundation type and depth for a particular construction.
  • To identify the quantity and qualities of the material that will be utilized in the building.
  • The measurement of the foundation’s load-bearing capabilities.
  • To determine the depth of the groundwater as well as any potential corrosive impact of water and soil on the material of the foundation.
  • Estimating how much a building will probably settle.
  • To examine the reasons why existing works have failed.
  • Development of construction techniques for fluctuating subsurface conditions.
  • To evaluate the ground pressure acting laterally against retaining structures as well as abutments.

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