The Crucial Role of Soil Testing in Geotech Services

Engineers need to know what they're working with before they begin to build. By developing an understanding of the soil, the engineer can play to a site's strengths, reinforce its weaknesses, and ensure that the project is built on the most solid footing possible.

Enter soil testing and analysis: a way to understand soil composition, which you can use to inform all manner of decisions, from foundation design to how you approach the construction process.

In geotechnical engineering, soil testing refers to a series of tests and analyses designed to determine the physical and chemical properties of soil samples taken from a construction site.

Soil testing forms a critical part of the geotechnical investigation work that takes place before any major construction project. The tests assess the suitability of soil for the proposed project, particularly in terms of the stability and safety of the structures that will be placed on top. The tests offer up important data that will inform a range of construction decisions, most notably the design of foundations.

Soil testing methods can be placed in two main categories:

Field soil tests

Field testing is the collection of soil samples for lab testing, and the documentation of other soil properties, including shear strength and strata layers. Geotechnical engineers will log and analyse other factors that could affect long-term stability, such as slope, depth to bedrock, drainage, and existing trees and structures.

A few common soil tests conducted in the field include:

Borehole investigations: A simple borehole can give a geotechnical engineer a deep understanding of the different layers of soil and rock present on a site, and how these layers vary across the site.

Penetration tests: Penetration tests see probes or samplers driven into the ground to gauge soil density, strength and stratification. Dynamic cone penetrometer (DCP) and standard penetration tests (SPT) use a hammer, while cone penetration tests (CPT) push the probe in at a constant rate.

Shear vane tests: These tests measure the shear capacity of the soil in-situ. A rod with vanes is inserted into the ground and rotated, which measures the torque required to make the soil move. The torque is then converted into a shear strength.

Nuclear density tests: These tests determine the density and moisture content of compacted soil, earthworks or pavements, to determine relative compaction.

Lab soil tests

Lab testing sees the soil samples collected in the field taken back to a laboratory, where the soil is analysed to measure its density, shear strength, load bearing capacity, moisture content, permeability, dispersion, consolidation (compaction testing), and how it acts under certain soil conditions.

A few common soil tests conducted in specialised laboratories include:

Moisture content test: Soil moisture can be measured through a range of means, including oven-drying, microwaving, hot-plating or sand bathing. Most work in the same way: the soil sample is weighed, then heated to release the moisture. Once dried, the difference in weight between the wet and dry soil reveals the weight of water that was in the soil.

Atterberg limits test: This test is used to understand how an increase in moisture affects fine soils like silt and clay. It measures the transition points between solid, semi-solid, plastic and liquid states, and how each state affects the stability and shear strength of the soil.

Compaction test: This test is used to find the optimal moisture content and maximum dry density of soil or pavement materials like asphalt and concrete, to ensure that they are compacted to a density that is appropriate for supporting loads.

Shrink-swell index: This test measures how changes in moisture content drive changes in volume - how much soil shrinks when it's dry, and how much it swells when it's wet - to gain an understanding of the potential for ground movement.

Soil testing is critical for construction. Without a deep understanding of the ground you're building on, there's no way to know what or how you can safely build on top of it. More specifically, soil testing will:

Inform the design of foundations

Soil testing offers up data on critical factors such as load bearing capacity, shear strength and the effects of moisture. Armed with these insights, a geotechnical engineer can design foundations that strengthen the soil and are capable of supporting the structures that will be built on top.

Ensure safety and stability

Soil testing is a critical component of geotechnical site surveys because it highlights the prevalence of risks such as soil liquefaction or unstable slopes. With hazards identified, engineers can then work to minimise or negate them.

Identify fill materials

Soil testing can determine the suitability of fill materials that will add the necessary strength and stability to your soil, such as using gravel and rock to guard against uneven settlements.

Optimise your construction techniques

The results of soil testing will guide decisions around excavation, soil compaction requirements and drainage solutions, mitigating risks and ensuring the longevity and durability of structures.

Meet your regulatory responsibilities

Soil testing is a key part of ensuring the safety of both the public and the environment, particularly for large civil projects. Conducting this testing will ensure that you comply with the rules and regulations, and that you won't face any legal issues or penalties.

Ultimately soil testing gives engineers the information they need to make better decisions, particularly at the beginning of the construction process. Soil testing therefore leads to infrastructure development that is safer, more efficient and more sustainable.

What does soil testing look like in the real world? It forms a regular part of our geotechnical work here at Eliot Sinclair, as demonstrated by the following case studies.

When we worked on an ambitious residential project on the former site of a clay brick quarry, we faced a number of unique challenges, including the inherent risk that comes with building on a steep hillside. Soil testing was key to giving us a clear picture of how to stabilise the slope and best utilise the site.

When we helped construct a purpose-built 1800m2 building for Mitre 10 in Hokitika, we faced a number of geotechnical complexities associated with soft soils, including the risk of liquefaction.

For most construction projects, soil testing is a non-negotiable. It's also a very specialised skill that demands deep knowledge and expensive equipment. In short, you can't do it without the help of a geotechnical engineer.

So what does the right geotechnical service provider look like? Here are a few key characteristics to look out for:

A proven track record

Has your chosen provider successfully delivered on projects before? A quality operation should be able to point to a wealth of successful projects as proof that they can deliver what you need them to, on time and on budget. At Eliot Sinclair we offer up an extensive list of case studies that you're welcome to browse, and we're always happy to provide more detail if you need it.

Relevant experience

While all geotechnical service providers will bring the relevant skills and qualifications to the table, the best will be able to point to past projects that were similar to yours, to prove that they have experience in the specific challenges you might face. Ask the provider to offer up some information about a project that they have completed that shares similarities with yours. If possible, speak to the provider's client to gain an unbiased view of what the provider is like to work with.

A local presence

By choosing a geotechnical service provider with a local presence, you're able to more easily meet in person, and this access can prove critical for more complex projects. Local service providers also bring invaluable local knowledge, from understanding a region's environmental conditions to dealing with councils and local authorities. With seven offices dotted up and down the South Island, our team at Eliot Sinclair are always nearby and ready to help.

A multidisciplinary approach

Soil testing and site surveys form a critical part of any construction project, but a small one. The best geotechnical service providers will offer a far larger range of services, and will therefore be able to take a lot more work off your plate.

At Eliot Sinclair we aim to be the ultimate land and site development consultants. We offer geotechnical and structural engineering, planning, hydrographic/land surveying, spatial services, urban design/landscaping, land development insights and more.

Soil testing is a crucial early step in the life cycle of any major construction project. It helps to determine what you're building on, to ensure that you build stronger, safer and better.

At Eliot Sinclair, we bring all the soil testing knowledge and expertise you need to get your next project off on the right foot. We use the latest tools and techniques to give you a complete picture of your site. And with experience in residential, commercial and civil projects, we've seen and done it all before.

Speak to our experts about your next development or construction project.