Subsurface Exploration | What is subsurface exploration?
Introduction:
Subsurface exploration is the process of identifying the soil deposits that underlie the proposed structure and their physical characteristics
In contrast, geotechnical engineers deal naturally with Materials (different types of rocks and mud) are their engineering The characteristics are diverse and unknown. Therefore, The geotechnical engineer must identify and inspect Objects in each project site (Subsurface Exploration). Before designing the foundations/structures
Purpose of the subsurface exploration:
The purpose of underground exploration is to obtain information It helps the geotechnical engineer:
- Selecting the type and depth of foundation for a given structure
- Evaluation of foundation bearing capacity
- Estimating the potential origin of the structure
- Determine potential foundation issues (for example, elaborate Soils, Collapsible Soils, Sanitary Landslides, etc.)
- Determine the location of the groundwater floor
- Determination of Lateral Earth Pressure to Maintain Determination Walls, Foil-like Structures
- Pile bulkheads and braced cuts Installation of construction methods to change ground conditions
Subsurface exploration program:
Rotary drills are either skid-mounted, trailer mounted, or, in the case of larger types, mounted on trucks (Figure 7.7). They are used to drill through stone, although they can be broken off by taking samples from the soil.
Rotary percussion drills are designed for rapid drilling on the rock (Figure 7.8). The rock is subjected to rapid velocity effects while rotating the bit, which causes compression and shear in the rock. Full-face bits are used to produce an open hole.
These are usually studded, cruciform, or Triton roller bit types (Fig. 7.9). This technique is most effective in fragile materials because it relies on chipping the rock.
The rate at which drilling continues depends on the type of rock, especially its strength, hardness, and fracture index; Type of drill and drill bit; Flushing media and pressures used; Plus the experience of the drilling crew. Compressed air, water, or soil can be used as a flush. If drilling operations are standardized, differences in penetration rate may reflect differences in rock types.
Drill flushings should be modeled at regular intervals, changes in the physical appearance of the flushing, and significant changes in penetration rates. The definition of rotary percussion drill holes should be related to the nearest cord drill hole.
Rotary percussion drilling is sometimes used as a tool to drill a hole at low and high speeds between intervals requiring core drilling. For many engineering purposes, testing requires a solid and continuous rock core as possible.
The core is cut with a bit and housed in a core barrel (Fig. 7.10). The bit is set with diamonds or tungsten carbide inserts. Inset bits, diamonds are set on the face of the matrix.
Rough surface set diamond and tungsten carbide tip bits are used in soft structures. These bits are usually used with air rather than with a flush of water. The inserted bits contain a matrix filled with diamond dust and their grinding action makes them suitable for hard and broken structures.
Most core drilling is performed using diamond bits, the type of bit used is controlled by the type of rock to be drilled. In other words, the harder the rock, the smaller the size, and the higher the quality of the diamonds needed in the bit. Tungsten bits are not suitable for drilling in very hard rocks.
Thick-walled bits are more robust but penetrate more slowly than thin-walled bits. The latter produces a larger core for a given hole size. This is important where several reductions in size have to be made. Core bits vary in size, and accordingly, core sticks range between 17.5 and 165 mm in diameter. Other factors apart, generally the larger the bit, the better is the core recovery.
Various core barrels are available for rock sampling. The simplest kind of core barrel is a single tube, but it is only suitable for hard bulk boulders, which are rarely used.
In the single-tube barrel, the barrel rotates the bit and flushes over the flush core. In double tube barrels, the flush passes between the inner and outer tubes. Double tubes can be of rigid or swivel type. The disadvantage of a rigid barrel is that the inner and outer tubes rotate together and in the soft rock it can break the core as it enters the inner tube.
Therefore, it is only suitable for hard rock structures. In the double-tube swivel-core barrel, the outer tube rotates but the inner tube remains stable (Figure 7.11). It is ideal for use in medium and hard rocks and offers improved core recovery in soft friable rocks.
The face-ejection barrel is a variety of double-tube swivel-type in which the flushing fluid does not affect the end of the core. This type of barrel is a minimum requirement for badly shattered, weathering, and soft rock formations.
Triple-tube barrels are used to obtain cores from the softest rocks and the most joint and slit rocks. This type of core barrel has an inner triple tube that is split into two halves. Therefore, when the core is withdrawn from the barrel’s casing, the core can be observed and explained without the risk of disturbance.
Both the bit and the core barrel are attached to the drill by rods, through which they are rotated. Water or air is used as a flush. It is pumped through the drill rods and ejected into the bit. The flushing agent acts to cool the bit and remove the cut side from the drill hole.
Bentonite is sometimes added to a flush of water. This facilitates lubrication of the drive and pulls off the casing, which can hold chippings in the suspension and promote drill hole stability by increasing flush returns through the formation of filter skin on the pore walls. When the core is removed from the barrel the core may become obstructed.
Most rock cores should be removed by hydraulic extruders while the tube is held horizontally. To reduce disturbance during extrusion the inner tube of double-core barrels can be lined with a plastic sleeve before drilling commences. On completion of the core run, the plastic sleeve containing the core is withdrawn from the barrel.
If the casing is used for drilling operations, then it is drilled into the ground using a tungsten carbide- or diamond-tipped casing shoe with air, water, or mud flush. The casing may be inserted down a hole drilled to a larger diameter to act as conductor casing when reducing and drilling ahead in a smaller diameter, or it may be drilled or reamed in a larger diameter than the initial hole to allow continued drilling in the same diameter.
Many machines will drill the core at any angle between vertical and horizontal. Unfortunately, The sloping drill holes go offline, the problem is enhanced in more joint structures. In deep drilling, the sag of the rods causes the hole to deflect. Drillhole deflection can be measured by the inclinometer.
Weak seams are usually of great interest, but these are the most difficult to obtain and worsen after extraction. Shales and mudstones are particularly prone to deterioration and can break down completely if some are allowed to dry. Wrapping the core material with aluminum foil or plastic sheeting can reduce suspected rock degradation.
The core material may be photographed before it is removed from the site. Zones of core loss or no recovery must be recorded as these could represent problem zones. Hawkins (1986) introduced the concept of lithology quality designation, LQD, which he defined as the percentage of solid core present greater than 100 mm in length within any lithological unit.
He also recommended that the total core recovery, TCR, and the maximum intact core length, MICL, should be recorded. A simple but nonetheless important factor is the labeling of the core material. This must record the site, the drill hole number, and the position in the drill hole from which material was obtained.
The labels themselves must be durable and properly secured. When rock samples are stored in a core box, the depth of the top and bottom of the core contained and of the separate core runs should be noted both outside and inside the box. Zones of core loss should be identified.
Also Read: