Why Geotechnical Lab Testing Matters in Construction?

There are many different types of soil encountered across New Zealand, deposited by rivers, oceans, volcanic eruptions and wind, among others. Soil Mechanics is the science behind how soil behaves under different conditions (e.g. under loading from buildings, during excavation or after the installation of drainage etc.). It plays a crucial role in Geotechnical Engineering, which helps us to design and safely construct various types of infrastructure, such as buildings, retaining walls, roads, bridges, tunnels, and dams.

Geology is also essential in construction projects. Geologists study the earth's materials and help engineers understand the ground conditions. While geology provides a general (qualitative) understanding of soil and rock, geotechnical engineers must rely on testing (quantitative) methods to measure soil properties and responses to changes in loading or environmental conditions.

Before any construction begins, engineers need to study the soil on-site. This process starts with collecting soil samples through drilling or augering. The samples are visually inspected and described based on colour, texture, and plasticity (how easily the soil can be moulded or reshaped without cracking or breaking). This step is called Soil Description, and it requires training and experience to ensure accurate assessments.

However, visual descriptions are subjective and can sometimes be misleading. That’s why a laboratory testing method known as Soil Classification is also used. Laboratory tests follow standardized systems like USCS (Unified Soil Classification System) to determine how a soil will behave in engineering applications. These tests help assess factors like:

• Soil strength and stability
• How well the soil can support foundations
• Drainage and compaction properties

In some projects, especially small-scale residential ones, soil description is often used instead of laboratory testing due to budget limitations. However, skipping the laboratory testing can lead to unexpected soil behaviour, causing costly delays during construction, repairs in the future or litigation if nearby property is damaged.

Soil Description vs. Soil Classification

• Soil Description: Based on visual inspection and simple field tests (e.g., moulding the soil by hand). It provides a general idea of soil type but may not always be accurate or repeatable to the extent needed by the geotechnical engineer. There may be some disagreement between two experts on the correct soil description.
• Soil Classification: Involves laboratory testing to measure key properties like water content, particle size distribution, and plasticity. These tests give more reliable data for engineering decisions. The process is objective, the same result would be repeated by two different experts regardless of their experience or judgement.

In many cases, early visual soil descriptions do not match the soil classification from laboratory test results. If the soil description is incorrect, it can lead to poor engineering decisions, affecting the performance of foundations, retaining walls, drainage and earthworks. Ultimately, this can impact project costs and long-term sustainability.

Real-World Example: Discrepancies in Soil Type

A residential project in Wellington highlighted the risks of relying on visual soil descriptions. Engineers initially described a sample as sand, which would have been considered suitable for bulk fill use. However, laboratory tests showed that it was in fact a sandy clay, which behaves very differently. If this misclassification had gone unchecked, the soil conditions could have caused stability issues for the project.

Sustainable and Cost-Effective Engineering

For large projects, such as land developments, social housing, schools, railways, and ports, relying only on visual soil descriptions is not sufficient and may lead to unacceptable risks during construction or operation of the facility. Laboratory testing ensures:

• Durability: The project will not require constant repairs or unexpected maintenance.
• Cost Savings: Prevents construction delays or future failures that could lead to expensive fixes.
• Environmental Benefits: Reduces unnecessary use of materials, emissions, and waste.

Conclusion: The Value of Lab Testing and Soil Classification

While laboratory testing adds a small upfront cost, it significantly improves the reliability and safety of a project. It eliminates the potential for subjective assessment or mistakes from impacting the geotechnical assessment. Good foundations and stable soil are crucial for any structure, drainage characteristics are also crucial for many engineering applications. As the saying goes, what you can’t see matters the most—and in construction, that means ensuring the ground beneath a structure is properly understood to inform good decisions during design and construction.