Do I need to do a Geotechnical Assessment?
Before one purchases a piece of property, he or she will do the wise act of investigating the land. It is important to conduct a walk through, either in person or by using a professional land surveyor, in order to know the site conditions and whether or not it is suitable to develop. This activity is crucial, as it could save the buyer thousands or even millions of dollars if it turns out that the site has too many adverse conditions.
To have an overall understanding of the site, it is also necessary to know the conditions below ground, this will require a geotechnical investigation or assessment. A geotechnical investigation goes beneath the surface. It obtains technical information about the subsurface conditions of the site, and guides designers and contractors in making decisions regarding the design of the foundation and the most suitable construction techniques to be used in the project. The investigation entails ascertaining the vertical soil profile, collecting soil samples and determining the engineering properties of the soil such as its load bearing capacity, shear strength, plasticity index and grain size distribution.
The soil is investigated by drilling holes, collecting samples and determining the engineering properties by way of laboratory testing procedures. Some tests are conducted on the soil in natural conditions, these are called in-situ tests.
How is the soil investigation used?
A soil investigation provides information that can be used for one or more of the following purposes:
To determine the depth and type of foundation required for a given structure.
The type and depth of the foundation is dependent on the columns loads (forces) exerted on the ground as well as the load-bearing capacity of the soil. Softer soils will have a lower load bearing capacity and harder (stiffer) soils will have a higher load bearing capacity. Therefore higher loads and softer soils will require deep foundations such as concrete piles; while lower loads and stiffer soils will require more shallow foundations such as wall footings or isolated pad footings.
Determination of the load-bearing capacity of the soil.
The bearing capacity of a soil is the pressure at the base of the foundation at which the soil fails in shear, i.e. the pressure at which the foundation settles. This information helps designers to determine the most suitable size and depth of footings or pile foundation.
To estimate the probable maximum and differential settlements.
As the foundation applies pressure to the ground, overtime the foundation will settle as the soil becomes more consolidated or compacted under the structure's load. Settlement cannot be completely avoided but it must not be too great nor vary significantly across the foundation (differential settlement). Therefore the geotechnical engineer will be able to estimate the amount of settlement that will occur in that particular soil and make recommendations for the most suitable foundation type that will mitigate excessive or differential settlement.
To establish the ground water level and properties.
A knowledge of the ground water levels will aid in the design of the sewage and stormwater water management system. A knowledge of the properties of the underground water will also help designers to determine the suitable concrete mix to prevent any adverse reaction and thus concrete deterioration.
To the predict the lateral earth pressure against retaining walls.
The lateral earth pressure is the horizontal pressure exerted on a retaining structure by the soil. Its magnitude is affected by factors such as properties of the soil, drainage conditions and flexibility of the retaining wall.
To select suitable construction techniques.
Construction involves a lot of excavation, therefore having a knowledge of the soil properties will aid contractors in knowing what is a safe slope for embankments, in order to prevent them from collapsing. For example, certain soils require a 1 (vertical) to 4 (horizontal) slope in order to maintain stability during construction.
To predict and solve potential foundation problems.
Investigating the soil will eliminate much of the uncertainties, and will assist engineers in making more detailed and cost effective designs, rather than taking an over-design approach. Foundation problems are often very costly to repair, and thus it is wise to solve all potential failure modes before construction.
Factors that affect Geotechnical Investigations
The type of structure to be built
The type of structure will determine the loads (forces) that will be exerted on the soil and the zone of influence for these loads. The zone of influence is the depth to which the force moves soil particles. A multi-storey structure will require more extensive subsurface exploration than a single-storey structure. The extent of the structure will also increase the number of sampling locations that are necessary, either in the form of bore holes or test pits. Subsurface exploration is a way to economise because it eliminates uncertainty and thus the need to over-design, which is a driver for construction cost.
Variability of the soil strata
The more variable the soil strata become is the more it drives the soil exploration programme due to the higher levels of uncertainty. However if the exploration progresses and the site is found to have uniform deposits then that would decrease the extent of the exploration.
Location of the project
The location of the project will determine the availability of information on the ground conditions. If the location is an already built up area and the project is relatively small and low risk, then the extent of the exploration will be reduced. In addition to this, according to the international building code, section 1803.2, building officials from our local Parish Councils are able to waive the geotechnical analysis requirement if satisfactory data from nearby sites are available and the proposed foundation type is considered as “shallow.”
As stated in the International Building Code section 1803.2 -“Geotechnical investigations shall be conducted in accordance with Sections 1803.3 through 1803.5. - Exception: The building official shall be permitted to waive the requirement for a geotechnical investigation where satisfactory data from adjacent areas is available that demonstrates an investigation is not necessary for any of the conditions in Sections 1803.5.1 through 1803.5.6 and Sections 1803.5.10 and 1803.5.11.”
If the proposed location is a newly developed area, then a detailed investigation is required to know the different soil strata and their physical properties.
What to do before you conduct a site investigation:
Before conducting a site investigation, here are a few things to consider that you may not miss any crucial information:
Collect information on the general topography of the site and any existing drainage features.
Check nearby structures to see if there are any settlement cracks.
Visually analyse slopes to see if there is any evidence of landslides, creep of slopes or shrinkage cracks.
Identify any flood marks on nearby buildings.
Ascertain the depth of ground water by way of existing wells if any.
Identify the location of springs, swamps etc at the site.
Document the type of vegetation existing on site.
Locate any underground water mains, power conduit, internet etc.
Information to provide to your Geotechnical Engineer:
Information on the type of structure to be built and its proposed use.
If the structure to be built is a multi-storey structure then the geotechnical engineer should get information on column design loads and their approximate locations. This information is helpful in determining a suitable soil exploration programme.
What types of soil can you expect on your property how will it affect your structure in the long term?
The soil on your property will determine the different safety procedures you take during construction; as well as it could affect your structure in the long term. The soil has a unique profile, which goes from the surface deeper through the various layers. The primary types of soils are clay, sand, silt and loam. The soil on site is usually a combination of some or all of the types with a predominant characteristic.
Clayey soil
Clay soils are fine grained, with low permeability and high plasticity. This means that they retain water well i.e. not much passes through. Clay soils tend to expand in the rainy seasons and shrink in dry seasons. As a result the foundation of your structure will be at risk for movement producing cracks in the walls and floors. Therefore these soils require careful consideration when designing your home or office. They may need to be replaced under the foundation or stabilised by the introduction of a stabilising agent such as cement or quicklime etc.
Silty Soil
Silty soil is composed of fine particles, however these particles are larger than those of clay. Silt, like clay, has relatively poor drainage making it soft during rainy seasons. Therefore foundations in this soil are prone to differential settlement (sinking of the foundation to different depths around the structure), which can lead to major cracks throughout your home or business. To significantly reduce the risk of structural failure on this soil, it may require deep foundations such as concrete piles or soil replacement.
Sandy Soil
Sand is a granular soil made up from weathered rock, (typically limestone, granite and quartz). It has a very poor ability to hold water, therefore it has very good drainage. When properly compacted and confined it can offer decent stability, however if it is loose or unconfined it could shift or collapse leading to a failure of the structure, especially when it is in a saturated state.
Loam
Loam is a combination of the three types of soil. It offers a decent balance of the properties of each and thus has relatively good drainage and strength. Due to the presence of organic matter in this soil (it is not as dense to prevent roots from expanding), it will require some amount of evaluation to ensure that it is suitable to carry building loads.
Gravel and Rock
The very best support to foundation loads is in the form of gravels or rock. Rocks do not expand nor contract with moisture and rarely settle under loads. When one is building on rock there will be less need for reinforcement in the foundation as compared to other types of soil. The drawback with this kind of soil is that excavation is quite expensive and time-consuming, therefore it raises the construction cost. However the performance of the foundation is usually excellent over the long term.
Conclusion
All structures will apply pressure to the existing soil. Pressure is a measure of the load or force applied per unit area. For example, for every square metre a structure may apply 100 kN of force. The design engineer will take into consideration all the forces that the structure will carry to soil. These loads could be live loads (loads that are not permanent), dead loads (loads that are permanent and the building's self-weight), or lateral (earthquake or wind loads). By careful arrangement of the columns, beams or walls, the engineer properly distribute the loads, and select the most suitable foundation so that the structure’s pressure on the soil will not exceed what the soil can safely manage. This is referred to as the 'allowable bearing capacity' and it is the actual bearing capacity divided by a safety factor, for example 2 or 3.
If the structure applies a certain pressure and the soil is too soft or has a lower bearing capacity, then it will become unstable and may collapse. Therefore the nature or characteristics of the soil is an important factor in the design process and it should not be overlooked, in order to prevent any failures or material wastage in over designing.
July 14, 2026
Conchana O. Pinnock, PE
