How to interpret borehole data for foundation design?

30th June 2025

Understanding how to interpret borehole data is essential when planning the design and construction of a reliable foundation. Borehole drilling plays a critical role in this process, as it provides engineers with first-hand information about the physical properties of subsurface materials. The quality and detail of data obtained from borehole drilling services allow for accurate decisions about site conditions, soil stability, groundwater presence, and the suitability of different foundation types.

What is Borehole Drilling, and Why is It Conducted?

Borehole drilling is a method used to explore underground soil, rock, and groundwater conditions. It is commonly conducted before the start of major construction projects to assess ground stability and determine whether the site can safely support a structure. The process involves using a rotating drill bit to penetrate the earth’s surface and reach different depths, depending on the intended use of the borehole and the underlying geology. Drilling may be carried out using common methods such as auger drilling for softer ground or rotary drilling when penetrating harder rock formations.

How Borehole Logs Are Used in Foundation Design

Borehole logs, which document observations and data collected during drilling, provide a detailed record of subsurface conditions. These logs typically include information such as soil type and consistency, rock core recovery and quality, groundwater levels, and the presence of fractured zones or anomalies. When geophysical surveys are performed within the borehole, additional data—such as gamma ray, resistivity, or sonic profiles—may also be included. This comprehensive information is essential for assessing how the ground will respond to structural loads and determining the most suitable type of foundation. For example, in locations with deep clay layers or highly variable geology, additional boreholes or deeper drilling may be required to assess changes in soil strength and behaviour with depth. In contrast, competent rock layers often provide a strong bearing stratum but may require specialised drilling techniques and equipment to access. Borehole logs can also reveal the presence of loose sands, fill materials, or soft sediments that could impact ground stability and influence foundation design. Engineers use this data to develop accurate ground models and determine the bearing capacity of the soil or rock. This analysis informs the selection of appropriate foundation types—whether shallow footings, deep piles, or raft foundations—and guides construction methods, particularly in sites with high groundwater levels or poor drainage conditions.

What Can Borehole Data Reveal About Ground Conditions?

Borehole data analysis provides critical insight into the composition, structure, and strength of subsurface materials. It can identify soft or compressible soils that may lead to settlement issues, as well as dense or hard rock layers that require specialised construction approaches. The analysis also helps assess groundwater conditions, including depth to the water table and potential for seepage or uplift pressures. Soil and rock samples collected during drilling are often tested in geotechnical laboratories to determine properties such as grain size distribution, moisture content, Atterberg limits, compaction characteristics, permeability, and shear strength. These results are used to classify the materials according to standards (such as AS 1726 in Australia or ASTM/USCS elsewhere) and to assess whether further investigation is needed. In addition to geotechnical properties, borehole investigations can inform environmental risk assessments. This is particularly important when working near sensitive areas such as watercourses, wetlands, or known contaminated sites, where subsurface flow patterns and groundwater movement may affect or be affected by construction activities. By integrating borehole data with surface observations and hydrological context, engineers gain a more complete understanding of how ground conditions will influence foundation performance over time. This is especially important in coastal or flood-prone areas, where fluctuating groundwater levels and soil saturation can significantly impact stability and long-term durability.

Why Location, Depth, and Technique Matter

The success of borehole drilling for foundation design depends heavily on the selection of appropriate locations, depths, and drilling techniques. Borehole locations must be strategically positioned to reflect anticipated structural loads, proposed building footprints, and areas of geotechnical interest. Drilling depth must be sufficient to investigate all soil and rock layers that may influence the performance of the foundation system. If boreholes are too shallow, critical strata or groundwater conditions may be missed; if excessively deep without engineering justification, the investigation may become inefficient and unnecessarily costly. Different drilling methods are selected based on ground conditions. Auger drilling is typically used in soft to medium soils such as sands or clays, while rotary drilling is more suitable for hard or cemented layers. In stratified ground conditions, a combination of methods may be employed to adapt to varying materials encountered at depth. Steel casing is often used to support the borehole wall, particularly where loose soils or groundwater are present, to ensure borehole stability and maintain sample integrity. An experienced geotechnical team can determine the optimal combination of drilling techniques, depths, and locations based on site-specific conditions and project requirements. This ensures that the subsurface data collected is both representative and reliable, forming the basis for safe and cost-effective foundation design.

How Borehole Data Supports Long-Term Foundation Performance

Interpreting borehole data is not only critical during the early stages of design and construction, it also plays a vital role in predicting the long-term performance of a foundation. Information obtained during the drilling process, including geotechnical samples and geophysical logs, provides a comprehensive understanding of geological conditions that influence settlement behaviour, groundwater variability, and structural stress over time. Boreholes may also be drilled specifically to monitor water table fluctuations or assess groundwater flow and quality. These investigations give engineers valuable insight into seasonal changes and how they may affect subsurface stability. Such monitoring is particularly important for projects located in areas prone to rising groundwater levels, perched water, or underlying aquifers. A clear understanding of subsurface conditions, including the presence of fractured rock, compressible soils, or layered sediments, enables engineers to design foundations that minimise differential settlement, resist erosion, and reduce the risk of long-term structural issues. The consequences of overlooking these factors can range from inefficient foundation systems to costly maintenance and remediation. Beyond conventional sampling and laboratory testing, geophysical logs obtained through downhole surveys provide additional validation of material consistency, stratification, and anomalies. These non-destructive techniques add depth to the borehole data, supporting more resilient and adaptable foundation designs.

Integrating Geophysical Testing for Deeper Insight

At Douglas Partners, we complement borehole data interpretation with advanced downhole geophysical surveys to ensure foundation designs are based on the most accurate ground models possible. These methods are particularly valuable when working with deep boreholes, where direct sampling may be limited and continuous profiling is needed to understand complex subsurface conditions. Two key techniques we often employ are sonic integrity testing and downhole magnetic resonance. Sonic integrity testing is used to evaluate the quality of cast-in-place concrete piles, helping detect internal defects or discontinuities that could compromise structural performance. Downhole magnetic resonance provides in-situ measurements of key soil and rock properties such as porosity, permeability, and moisture content, giving our geotechnical engineers valuable insight into how the ground will behave under load. These non-destructive methods enhance the reliability of our recommendations, particularly in deeper investigations, supporting safer and more efficient foundation solutions.