Drilling
GEOTECHNICAL DRILLING
Geotechnical engineers and drillers play a crucial role in the construction industry, as they are responsible for taking samples from site for laboratory testing and evaluating the suitability of a proposed construction site. The success of any construction project depends heavily on the stability and reliability of the ground it will be built upon, which is why geotechnical engineers are called upon to assess the site before construction begins.
Geotechnical drilling and sampling is an essential tool used by geotechnical engineers in determining the suitability of a site. This process involves the sampling and testing of organic materials found within the site. This allows engineers to identify any foundation soils that may be unsuitable for construction, such as loose coarse-grained soils, peats, or soft fine-grained soils. These findings can be crucial in alerting contractors to any potential problems they may face during the construction process.
In addition to geotechnical drilling, a geological review is conducted alongside the drilling process. This review maps out the rock formations, ground water levels, and different types of soil found throughout the site. The information gathered from the geological review provides further insight into the foundation requirements for the upcoming construction project. This includes the types of tools and manpower that may be needed to ensure the stability and reliability of the foundation.
Types of Geotechnical Sampling Equipment
There are several types of drilling rigs that can be used to obtain soil samples, each with its advantages and limitations. Here we will review the various types of drilling rigs based on cost and their capabilities.
Spade
The spade is the simplest and most cost-effective way of obtaining soil samples, especially for small scale, geo-environmental purposes. In favorable soil conditions, such as clay or sand, a person can dig 10 x 1 meter deep holes in a day using a small spade, digging bar, and scissor shovels. This method is suitable for sites where only the top 1 meter of soil needs to be gathered, such as back gardens of terrace houses. The main advantage of spade digging is that it can be done anywhere and is very neat, leaving little mess. However, it may not be feasible for large-scale projects.
Digger Excavators
Digger Excavators of 2.5 tons or more can also be used to obtain soil samples, especially in clays, sands, and loose fill. These machines can reach a depth of 3.5 meters, making it possible to dig out large chunks of intact soil for testing. A large digger or excavator can be useful when dealing with cobbles, which are challenging for other drilling methods. However, the use of an excavator comes with several drawbacks, including a high level of mess and potential damage to landscaping and surfaces. The holes required can also be large, making it unsuitable for small sites.
Handheld Window Sampling
Handheld window sampling is the most basic form of drilling or boring, using a hollow tube with gaps in the side, which is bashed into the ground using a pneumatic or hydraulic breaker. This method is great for hard to reach places and confined spaces where fumes are a concern. The equipment can be carried anywhere, and a depth of 5 meters can be reached in clays and about 3 meters in sands and gravels. The main limitation of handheld window sampling is the lack of casing, which can lead to hole collapse in granular soils when the sample tube is removed.
Window Sample Rig
Window sample rigs are a popular option for soil sampling, especially for in-situ strength measurement of granular soils. This type of rig has the ability to case the hole using steel tubes, which helps keep the hole open and prevent soil collapse. Window sample rigs are also capable of conducting Standard Penetration Tests (SPT), which provides valuable information on soil strength, shear angle, and other parameters. However, window sample rigs have limitations in terms of depth, with a maximum of 3 meters, and are not suitable for sampling rocks. A windowless sample rig can drill 20 x 1 meter deep holes in a day or 2 x 5 meter holes, but progress is slow in deeper holes.
Hollow-stem Auger Boring
This is a power-operated method where a hollow stem serves as a casing. It is widely used for general-purpose applications in soils and other locations that require a cased hole. It is commonly used for disturbed and undisturbed soil sampling, in situ testing, foundation and landslide investigations.
Rotary Drilling
This method utilizes power rotation of a drilling bit to cut through materials, with a circulating fluid removing the cuttings from the hole. Progress is indicated by the rate of progress, the action of the drilling tools, and examination of the cuttings in the drilling fluid. Casing is usually not required except near the surface. It is a versatile method used to advance borings through various materials, including large boulders and broken rock. It is commonly used for obtaining rock cores, drilling probes, installing horizontal drains and instruments.
Wire-line Drilling
This rotary-type drilling method involves using a coring device that is an integral part of the drill rod string, which also serves as a casing. Core samples are obtained by removing the inner barrel assembly from the core barrel part of the drill rod. The inner barrel is released using a retriever that is lowered through the drilling rod using a wire-line. This is an efficient method for recovering core samples of rock and is commonly used for foundation investigations, material source assessments, rock cut examinations, and general rock coring.