Building on Solid Ground

Building on Solid Ground: The Role of Geotechnical Engineering in Development

A strong foundation is the backbone of any successful construction project. Geotechnical engineering plays a crucial role in ensuring stability, safety, and longevity for your building project. Let’s explore the fundamentals of this field and how if might apply to your project.

What is Geotechnical Engineering?

Geotechnical engineering predicts the behaviour of soil and rock. By analysing their physical and chemical properties, we determine likely failure scenarios and then the best foundation solutions for buildings, retaining walls, and earthworks that will avoid these failure mechanisms.

Key Aspects of Geotechnical Engineering

• Soil Classification: One of the first steps in a geotechnical investigation is understanding the type of soil present and its behaviour to various conditions. In Gisborne, we frequently encounter a mix of clays and sands and also layers of ash. We use advanced testing methods like Cone Penetration Tests (CPTs) and hand auger boreholes associated with in-situ shear vane tests and scala penetrometer test to determine soil types.

• Soil Strength: Determining the shear strength of soil is essential for designing stable foundations. The clays, sands and ash can be highly susceptible to strength loss during liquefaction (from earthquakes) and saturation (from wet winter periods or storms).

• Groundwater Conditions: Given Gisborne’s coastal location, groundwater levels can be high, influencing foundation choices and construction methods.

• Flooding: Also, understanding significant flooding events (such as cyclones Bola and Gabrielle) from recent times and forecast sea level rise for coastal sites provides another set of risks to consider and mitigate.

Geotechnical Challenges in Gisborne

1. Liquefaction Risk

Sites located in a seismically active region are where liquefaction becomes a significant concern. Liquefaction occurs when saturated, loose sandy soils lose their strength due to shaking from earthquakes, causing the ground to behave like a liquid. You may remember videos of this occurring in Christchurch. This can lead to:

• Sudden ground settlement and tilting of buildings

• Cracking and damage to foundations

• Displacement of underground utilities and infrastructure

To model liquefaction risks, we conduct detailed site assessments using techniques such as:

• CPTs and shear wave velocity tests to evaluate soil stiffness

• Numerical modelling to predict ground behaviour during seismic events

Depending on the site conditions, mitigation strategies may include ground densification, deep foundations (such as piles), or soil stabilization techniques.

2. Landslide (Stability) Risk

Some soils are highly susceptible to weathering and strength loss. This combined with steep country can result in a high stability risk. So, for developing land that is on or near to hills, the stability of the slopes should be assessed. This involves developing a ground model from the testing data, including the water table, and then modelling the current and design case stability of the ground. Several design case solutions can be modelled to determine the most effective and efficient solution.

2. Settlement Issues

Settlement occurs when the soil beneath a structure compresses under load, causing uneven settlement. Some areas contain soft, compressible soils such as silts and clays, which can lead to:

• Immediate settlement, which happens quickly after loading

• Consolidation settlement, a slower process where water gradually drains from clayey soils, leading to long-term ground movement

• Differential settlement, where different parts of a structure settle at different rates, potentially causing structural damage

To manage settlement risks, we:

• Perform load testing and settlement analysis before construction

• Use ground improvement techniques such as soil compaction, preloading, or geogrids

• Design deep foundation systems, like driven piles to transfer loads to more stable soil layers

3. Coastal & River Erosion Impact

Proximity to the coast and major rivers will means that erosion is an ongoing challenge for construction projects. Erosion can be caused by:

• Coastal wave action, which gradually removes soil and weakens foundations near the shoreline

• Storm surges and sea-level rise, increasing the risk of flooding and infrastructure damage

• Riverbank erosion, which can undermine buildings, roads, and bridges, especially after heavy rainfall or flooding

To address erosion risks, we implement:

• Slope stabilization techniques, such as retaining walls, vegetation reinforcement, and geotextiles

• Shoreline protection measures, such as riprap and seawalls to reduce wave impact

• Flood and drainage management, including proper site grading and runoff control

Foundation Solutions

Based on site conditions, several foundation approaches can be recommended, including:

1. Shallow foundations for sites with stable ground

2. Ground improvement techniques to enhance weak soils

3. Pile foundations where deeper support is needed

Sustainable Geotechnical Practices

For us, sustainability is a priority. Our geotechnical work focuses on:

• Recommend lower carbon emission foundation solutions and reducing earthworks

• Using locally sourced materials where possible

• Optimizing foundation designs to minimize material use

Conclusion

So, geotechnical engineering is key to building safe, durable structures. By understanding your projects unique soil conditions and applying sound engineering principles, we ensure projects are built on solid ground — literally.