Methods of Site Exploration
Site exploration can be carried out using several techniques, which are generally classified into the following categories:
- Open Excavations
- Borings
- Subsurface Soundings
- Geophysical Methods
1. Open Excavation (Trial Pits)
Open trial pits are a cost-effective method for exploring shallow soil layers and are suitable for a variety of soil types. This technique involves excavating pits at the site to expose and study the subsoil layers directly. During the process, soil samples are collected from different depths for analysis. The main advantage of this method is the ability to visually inspect soil layers in their natural state and to easily obtain both disturbed and undisturbed soil samples.
A typical trial pit measures about 1.2 m by 1.2 m in width and can reach depths of around 2.4 m. The cross-section of a trial pit often shows distinct soil layers such as silt, fine sand, coarse sand, and gravel. While this method is practical for depths up to approximately 3 m, the cost of excavation increases significantly with deeper pits. Additionally, for excavations that extend below the groundwater table or in permeable soils, precautions such as lateral support and groundwater control measures may be required to ensure stability.
2. Boring Methods
Several methods of boring are commonly employed for subsurface exploration. These methods include:
(i) Auger Boring
(ii) Auger and Shell Boring
(iii) Wash Boring
(iv) Percussion Boring
(v) Rotary Boring
(i) Auger Boring
Auger boring is widely used in cohesive soils and other soft soils above the water table. Augers can be operated either manually or mechanically. Hand-operated augers are effective up to depths of around 6 meters, while mechanically operated augers can reach greater depths and handle more challenging conditions, such as gravelly soils. There are two primary types of augers:
- Spiral Auger
- Post-hole Auger
The soil samples collected using augers are typically highly disturbed, making them useful primarily for soil identification rather than detailed analysis. Auger boring is an effective method for shallow-depth explorations and is commonly used for exploratory borrow pits.
(ii) Auger and Shell Boring
The auger and shell boring method utilizes cylindrical augers or shells equipped with a cutting edge or teeth at the lower end, allowing for deep borings. Hand-operated rigs can be used for depths up to 25 meters, while mechanized rigs can reach depths of up to 50 meters. This method is suitable for a range of soil types: augers work well in soft to stiff clays, shells are effective in very stiff to hard clays, and sand pumps or shells are used in sandy soils
When small boulders, thin soft rock layers, or cemented gravel are encountered, they can be broken up with chisel bits attached to the drill rods. Typically, a casing is required to stabilize the borehole.
(iii) Wash Boring
Wash boring is a quick and straightforward technique for drilling holes in a variety of soil types, though it is not suitable for boulders or rock. This method begins by driving a casing into the ground, through which a hollow drill rod equipped with a sharp chisel or chopping bit is inserted. Water is pumped under pressure through the drill rod, and the rod is raised, dropped, and rotated. This combined action of chopping and jetting breaks up the soil.
The soil cuttings are carried to the surface as a soil-water slurry through the space between the drill rod and the casing. Changes in soil stratification can be roughly identified based on the rate of drilling progress and the color of the wash water. However, the samples retrieved in wash boring are highly disturbed, making them of limited value for accurate geotechnical analysis. A typical setup for wash boring is shown in Figure below.
(iv) Percussion Boring
Percussion boring is a technique where soil and rock are fractured through repeated impacts from a heavy chisel or bit attached to a cable or drill rod. During the process, water is added to the borehole if it is not already present, creating a slurry with the pulverized material, which is periodically removed. This method is versatile and can penetrate various soil types, boulders, and rock. However, the repeated impact tends to disturb the soil and rock formations, limiting the accuracy of the stratigraphy analysis.
(v) Rotary Boring
Rotary boring, or rotary drilling, is an efficient and rapid method for drilling through both soil and rock. In this technique, a drill bit attached to the end of drill rods is rotated by a chuck and maintained in continuous contact with the bottom of the borehole. Drilling mud, typically a water-bentonite solution with or without additives, is pumped down through the hollow drill rods. As the mud flows back up, it carries the soil and rock cuttings to the surface, eliminating the need for casing in most cases.
For sampling rock cores, rotary core barrels equipped with diamond-studded bits or steel bits with shot are used. This variation of the method, known as core boring or core drilling, enables both drilling and core sample collection. Water is circulated down the drill rods during this process to aid in cooling and transporting cuttings.
3. Subsurface Soundings
Subsurface sounding methods involve assessing soil resistance at various depths using a penetrometer under static or dynamic loading. This penetrometer can be equipped with a sampling spoon, a cone, or other tools, depending on the test requirements. The resistance measured during penetration is empirically linked to certain engineering properties of the soil, including density, consistency, and bearing capacity. The accuracy of these tests relies heavily on the extensive experience and data behind them.
Sounding tests are particularly valuable for general exploration of irregular soil layers, determining the depth to bedrock or a stable stratum, and estimating the strength and properties of soils. This is especially useful for cohesionless soils, where undisturbed samples are challenging to collect. The most commonly used subsurface sounding tests are the Standard Penetration Test (SPT) and the Cone Penetration Test (CPT).
4. Geophysical Methods
Geophysical methods are ideal for deep exploration and situations where rapid investigation is crucial. These techniques detect variations in the physical properties of subsurface geological layers and were initially developed for mineral and oil prospecting. In civil engineering, geophysical methods help identify key characteristics of subsurface formations over large areas quickly and efficiently.
The primary geophysical methods include:
- Gravitational Methods
- Magnetic Methods
- Seismic Refraction Method
- Electrical Resistivity Method
Among these, the seismic refraction and electrical resistivity methods are most commonly applied in civil engineering. They offer valuable information for understanding subsurface conditions, aiding in the design and planning of structures where deep foundation support or complex geological conditions are present.
i. Seismic Refraction Method
The seismic refraction method involves generating shock waves at or below the soil surface by either detonating a small charge or striking a plate with a hammer. These shock waves radiate through the soil and are detected by vibration sensors called geophones or seismometers, which record the wave travel time. Multiple geophones are placed along a line extending from the shock point (shown in figure below) to capture waves as they travel through the soil.
Two types of waves are generated: direct (primary) waves that travel along the ground surface directly from the shock point, and refracted waves that pass through the soil layers and bend at the boundary between different soil strata. When the underlying soil layer is denser, refracted waves travel faster than direct waves. As the distance between the shock point and the geophone increases, refracted waves can reach the geophones faster than the direct waves.
By analyzing the travel times of primary and refracted waves at different geophones, the depth and thickness of each soil layer can be estimated using distance-time graphs and analytical calculations. This method is particularly effective and reliable in mapping the profiles of strata when each successive layer is denser and thus has a higher wave velocity.
Seismic refraction is useful for identifying general material types (like gravel, clay, hardpan, or rock) based on characteristic seismic velocities observed in the distance-time graphs. However, it cannot provide specific material identification. Therefore, it is often complemented with boring, soundings, and sample collection for a more detailed geotechnical analysis.
ii. Electrical Resistivity Method
The electrical resistivity method measures and records variations in the mean resistivity of different soil types. Each type of soil exhibits a unique resistivity, influenced by its water content, compaction, and composition. For instance, saturated silt has low resistivity, while dry gravel or solid rock has high resistivity.
In this test, four metal spikes (electrodes) are inserted into the ground along a straight line, evenly spaced. A direct voltage is applied across the outer electrodes, and the potential drop is measured between the inner electrodes. The mean resistivity WW (in ohm-cm) is calculated using the formula:
W=(2πDE)/I
where:
- D = distance between electrodes (cm)
- E = potential drop between inner electrodes (volts)
- I = current flowing between outer electrodes (amperes)
The depth of exploration is roughly proportional to the electrode spacing. To study vertical changes in the subsurface layers, the electrode spacing is gradually increased outward from a central point to a distance approximately equal to the target exploration depth, a process known as resistivity sounding.
To accurately interpret resistivity data and understand the nature and distribution of soil layers, preliminary trial or calibration tests on known soil types are essential. These calibration tests improve the reliability of identifying subsurface formations based on their resistivity values.
Choice of Exploration Method
Selecting the appropriate site exploration method depends on several key factors
1. Nature of Ground:
- In clayey soils, borings are suitable for deep exploration, while trial pits work well for shallow exploration. In sandy soils, borings are effective, though special equipment is needed for representative sampling below the water table. Such samples can also be taken from trial pits, provided groundwater is managed effectively.
- For hard rock formations, borings are more suitable, whereas in soft rocks, trial pits are preferred. Core borings are valuable for rock type identification, although they cannot capture joint or fissure data, which are better examined in pits or large-diameter borings.
- When exploring deep or large construction areas, geophysical methods—especially electrical resistivity—offer efficient coverage. Nonetheless, borings at specific locations should be conducted to calibrate data. In extensive soft soil areas, sounding methods may be used for rapid coverage.
2. Topography:
- In hilly areas, the choice between vertical openings (such as borings and trial pits) and horizontal openings (like headings) depends on the geological structure. Steeply inclined strata are best explored by headings, while horizontal strata are more effectively examined with trial pits or borings. Swamps and water-covered areas are typically explored by boring, sometimes from floating platforms if necessary.
3. Cost:
- For deep exploration, borings are generally preferred, as deep shafts are more expensive. In vast areas, geophysical or sounding methods may supplement borings to minimize costs. For shallow exploration in soil, the choice between pits and borings depends on soil characteristics and the level of information required. For shallow rock exploration, core drills are only cost-effective if multiple holes are needed; otherwise, trial pits provide a more economical option.
