Geotech 101
Check out our commonly asked questions below and discover more about the wonderful world of geotechnical engineering!
Geotechnical reports are vital before beginning any type of construction or building. They are a prerequisite prior to construction and even before building plans are determined.
What usually happens is that a geotech consultancy will be approached by a client who is looking to build a piece of infrastructure, which could be something like an office tower, a pipeline, a mining site, or a bridge.
The client will commission the geotech consultancy to investigate and analyse the soil, rocks, bed rock properties and fault distribution of a proposed construction site. You can imagine this is critical to know if say, you wanted to build a 10,000-tonne bridge. If you don’t understand how the ground will interact with the bridge, this could lead to major structural issues after completion. Determining the quality of the soil at a building site is a key factor in the success or failure of any construction project.
In the beginning stages of planning, a geotechnical engineer will need to test the site that is proposed to be built on (or through if you’re making a tunnel or pipeline). This involves the engineer going out to site and digging a series of boreholes to conduct different soil tests (such as DCP and CPT tests) to characterise soil behaviour.
The engineer will then analyse these tests and add the findings into a detailed engineering report that specifies the physical properties of the soil on the building site. The report will usually include the chemical composition of the soil, the stability of natural slopes, possible fault distributions and other necessary details of the construction site. The report will also show on a site plan the locations where boreholes where dug.
These geotech reports have been proven to reliably reduce cost overruns and project delays (read our whitepaper about this here).
So, you can see why these reports are so crucial to any civil construction project.
A geotechnical investigation report is comprised of a site analysis that is tested by geotechs like me. These tests help us understand how reactive the soil is and ensure there aren’t any chemical or physical conditions that could damage or destroy the proposed construction.
In the beginning stages of planning, a geotechnical engineer will need to test the site that is proposed to be built on (or through if you’re making a tunnel or pipeline).
The report will specify the physical properties of the soil on your building site. It will also include the chemical composition of the soil, the stability of natural slopes, possible fault distributions and other necessary details of the construction site.
Geotechnical engineers have knowledge in geology and hydrology to provide solutions related to these sciences.
Geotechnical engineers and engineering geologists perform geotechnical investigations to obtain information on the physical properties of soil and rock underlying (and sometimes adjacent to) a site.
These professional engineers design earthworks and foundations for proposed structures, as well as repair distress caused by subsurface conditions (such as erosion from mining or landslides).
Geotechnical engineers rely on both surface exploration and subsurface exploration to arrive at a site's strength. Sometimes, a geophysical survey is necessary for pre-site investigation.
In today's world, civil and geotechnical engineers can conduct their surveying process by using advanced technologies like drones or using cloud-based software to calculate borehole findings on site.
Generally, the exploration of geotechnical engineers can be broken down into three areas.
SUBSURFACE EXPLORATION
- Exploration of the subsurface is typically done with in-situ testing (common examples include standard penetration tests and cone penetration tests). This includes site investigation, which often entails providing subsurface sampling as well as laboratory testing for soil samples. It is important for engineers to know the history of pre-existing rock beneath the Earth to develop structures that prevent future hazards.
SURFACE EXPLORATION
- The exploration steps for a surface assessment can range from geologic mapping to geophysical methods such as seismic and electromagnetic induction, or photogrammetry. Geologic mapping is typically completed in consultation with geologists or engineering geologists.
GEOPHYSICAL EXPLORATION
- Geophysical exploration is also sometimes used for civil engineering projects. Geophysical techniques for subsurface exploration include measurements of seismic waves, surface-wave methods and/or downhole methods, and electromagnetic surveying (resistivity, magnetometer, and ground-penetrating radar).
Geotechnical engineering can be applied to many fields including:
• oil and gas
• mining (including lithium mining and lithium extraction technology)
• construction
• agriculture
• water utilities
• transportation
• landfill
• coastal restoration,
• dredging, and many more.
Many aspects of construction and civil engineering rely on the expertise of geotechnical engineers to guide their decisions. No multi-million project kicks off without first consulting a geotechnical consultancy before works commence.
Primarily, geotechnical engineering is used to guide the following designs:
1- DESIGN OF BRIDGES
Geotechnical engineers create the structures that bridges are built on, such as piers and foundations. To ensure safety and longevity for a design, engineers consider the different types of loads, as well as where those loads are applied.
2- DESIGN OF TUNNELS
The kind of geotechnical input for a tunnel project will depend on whether it's an existing or proposed tunnel, and whether it is in soil or rock. It is relatively more standardized to analyze new tunnels and new liners, but modifying older tunnels can require more analysis to address the uncertainty that comes with non-standard liner geometries parametrically
3- STRUCTURAL DESIGN OF BUILDINGS
Civil engineers design, construct, operate, and maintain construction projects in various sectors including roads and buildings for the public and private sectors.
4- DESIGN OF RETAINING STRUCTURES
Retaining structures are engineered to retain soil and/or rock. They provide grade changes, right-of-way increases, and buttresses while retaining earth coverage.
A bore log refers to a record detailing the in-situ conditions and aspects of geotechnical exploration activities such as drilling boreholes and mining. The log information needed to create a bore log is obtained through visual inspection of samples obtained from the geotechnical process.
It is also obtained through geophysical measurements made along the borehole axis by using special logging instruments that collect and record geotechnical data. A standardised log spreadsheet is used in the documentation of the data. A good bore log should have all sections completed with the appropriate data.
The data contained in a bore log include project details (name, number, place), type of drill rig, boring depth, drilling method used, sample recovery, soil classification, soil density, soil consistency, colour, grain size, particle shape, sedimentary structure, blow counts, layer boundaries and thickness, plasticity, topsoil thickness, mineral composition, texture, strength, weathering and altercation, water content/moisture, and photographs. The data covers both soil and rocks. The units used depends on the specific aspect being measured; for instance, boring depth can be presented in meters while grain size is in millimetres.
The information provided in the bore log is useful as it gives information on soil types, consistency of soil, and water tables. Such information is applied in assessments to ascertain the desirability of formations identified during drilling, depending on the well type. It is mainly used by geotechnical engineers, geologists, and geoscientists, among other drilling specialists. Consequently, it helps determine the nature of water from boreholes; for instance, it needs further treatment before use.
A drillers log is a record of the geologic materials encountered during drilling.
The data for a driller's log is collected by obtaining samples of the formations during the drilling process and noting where and at what depth(s) the samples were collected and how and where the samples change with depth. The thickness of geologic formations and the depth to water are also noted. An ideal driller's log will also have a detailed and accurate description of the surface location of the well, borehole, or piezometer that is drilled.
While geotechnical investigation report and “geo” report are often used synonymously, they refer to different yet related services.
A geotechnical investigation report is prepared by a geotechnical engineer. These reports are typically composed of a text portion, site plan, boring logs, some figures, and engineering calculations. Geotechnical engineers analyse field conditions and soil laboratory data, perform calculations, and provide recommendations for building foundations.
A geology report is prepared by a geologist or engineering geologist. Geologists discuss and analyse natural conditions and seismic hazards, such as earthquakes, landslides, high groundwater, or mudflow and debris flows, in greater detail than say engineers. In many geographical areas, the expertise of both an engineer and geologist are required prior to site development, such as hillside areas, fault zones, and areas subject to high groundwater. As such, most reports include both a soil engineering and geologic portion, and as such are referred to as geotechnical investigation/geology reports.
The term “geotechnical report” can have many meanings. In the construction industry, a geotechnical report typically refers to a report of the physical and engineering characteristics of a soil which will be used as building material.
A soil contamination report is a separate service and requires testing of soil samples for contaminants such as solvents, hydrocarbons, and pesticides. Such a report often follows an environmental investigation and is prepared by a professional geologist or engineer with experience in environmental consulting.
An environmental consultant helps private and public sector clients address environmental issues and lower environmental impact in areas such as water pollution, waste, management, air quality, and soil contamination.
Environmental consultants have extensive knowledge on environmental regulations and can advise clients in private industry or public government institutions on how to steer clear of possible fines, legal action, or misguided transactions.
An environmental consultant has the scientific knowledge and technical expertise to conduct thorough environmental assessments. This can prove to be very beneficial to a public or private company. For example, if the company is involved in a court case, the environmental consultant can testify on their behalf. Or if a company is considering buying a piece of land for development, the consultant can fully assess the land before the purchase, and research any previous investigations of that particular site. They may also conduct field surveys, and collect data to establish a baseline condition for levels of pollution or contamination for the area of consideration.
An environmental consultant conducts both field and desk-based research, and will hand over completed and detailed scientific reports, written in a manner that can be understood by non-technical people. Their research will identify whether water, air, or land contamination will have an adverse impact on people or groundwater, for example. They interpret data, which includes a detailed assessment of the data, sometimes using software-modelling packages to see whether contamination exists in accordance with current legislation.
An SPT test, which stands for Standard Penetration Test, is a common geotechnical engineering field test used to obtain information about the subsurface soil properties at a specific location. It's a widely used method for characterizing the soil's resistance to penetration by a standard sampler driven by a hammer dropped from a standard height.
Here's how the SPT test typically works:
1. Setup: A borehole is drilled to the desired depth using a drilling rig or equipment suitable for the site conditions.
2. Sampler: A standard split-spoon sampler is attached to a drill rod, and this assembly is lowered into the borehole.
3. Testing: The sampler is driven into the soil at the bottom of the borehole using a standard hammer weighing 140 pounds (known as the SPT hammer). The hammer is typically dropped from a height of 30 inches, and the number of blows required to drive the sampler the next 12 inches into the soil is recorded. This number of blows is known as the "blow count" or "N-value."
4. Sampling: The split-spoon sampler captures a soil sample as it is driven into the ground. This sample can be extracted and analyzed later to determine the soil's composition and characteristics.
5. Interpretation: The N-value obtained from the SPT test provides valuable information about the subsurface soil's relative density, stiffness, and resistance to penetration. Geotechnical engineers use this data to assess foundation design, liquefaction potential, and other geotechnical considerations for construction projects.
The SPT test is relatively simple, cost-effective, and provides a quick estimate of soil behavior. However, it has limitations, and the interpretation of results requires experience and consideration of other site-specific factors. In some cases, additional tests (such as laboratory tests on soil samples) may be conducted to refine the geotechnical analysis.
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