
Soil investigations play a crucial role in the success of infrastructure projects. Any structure’s stability, safety, and longevity heavily depend on the characteristics and behavior of the soil upon which it is built. Neglecting or inadequately conducting soil investigations can lead to costly failures, delays, and catastrophic consequences. Therefore, project managers, engineers, and stakeholders need to understand the significance of thorough soil investigations and how they contribute to the overall success of infrastructure projects.
The purpose of this blog post is to provide a comprehensive guide to soil investigations for infrastructure projects. We will explore the various types of infrastructure projects, the objectives of soil investigations, the stages involved, and the methods and techniques employed. By the end of this post, readers will better understand how soil investigations are conducted and how they contribute to the design and construction of safe and reliable infrastructure.
Types of Infrastructure Projects
Buildings
Residential, commercial, or industrial buildings require a stable foundation to ensure their structural integrity. Soil investigations for buildings aim to determine the soil’s bearing capacity, settlement potential, and other geotechnical parameters that influence foundation design. The type and depth of the foundation, such as shallow footings or deep piles, depend on the soil conditions and the building’s load requirements.
Roads and highways
Roads and highways are critical infrastructure that facilitate transportation and connectivity. Soil investigations for these projects focus on determining the subgrade soil’s strength, compressibility, and drainage characteristics. The pavement design, including the thickness and composition of the layers, is based on the soil properties and the expected traffic loads. Proper soil investigations ensure that roads and highways are durable and can withstand the stresses imposed by vehicles and environmental factors.
Bridges
Bridges span over water bodies, valleys, or other obstacles to provide passage for vehicles or pedestrians. Soil investigations for bridges are crucial in determining the foundation type and design, such as piers, abutments, or piles. The soil’s bearing capacity, settlement potential, and scour susceptibility are evaluated to ensure the bridge’s stability and safety. In addition, seismic and liquefaction risks are assessed in earthquake-prone areas.
Dams and reservoirs
Dams and reservoirs are constructed to store water for various purposes, such as irrigation, power generation, and flood control. Due to the high risks associated with dam failures, soil investigations for these projects are extensive and critical. The soil’s permeability, shear strength, and compressibility are assessed to ensure the dam’s stability and prevent seepage and erosion. The foundation conditions and the soil’s ability to support the dam’s weight are also evaluated.
Tunnels and underground structures
Tunnels and underground structures, such as subways and utility corridors, require detailed soil investigations to ensure safety and stability. The soil’s properties, including strength, deformability, and groundwater conditions, are assessed to determine the appropriate excavation and support methods. The risk of soil instability, such as collapses or excessive deformation, is evaluated to prevent accidents and ensure the smooth operation of these structures.
Objectives of Soil Investigations
The primary objective of soil investigations is to determine the soil’s properties and characteristics. This includes the soil’s composition, grain size distribution, plasticity, moisture content, and density. These properties influence the soil’s behavior under different loading conditions and are essential for the design of foundations, pavements, and other geotechnical structures. Soil samples are collected from various depths and locations at the project site and tested in laboratories to obtain accurate and representative data.
Soil investigations aim to assess the soil’s stability and bearing capacity, critical factors in the design of foundations and other load-bearing structures. The soil’s shear strength, compressibility, and consolidation characteristics are evaluated to determine its ability to support the imposed loads without excessive settlement or failure. In-situ tests, such as the Standard Penetration Test (SPT) and Cone Penetration Test (CPT), are conducted to obtain these parameters directly from the field.
Another important objective of soil investigations is identifying potential geotechnical hazards that may affect the project. These hazards include soil liquefaction, expansive soils, landslides, sinkholes, and soft or compressible soils. By identifying these hazards early in the project, appropriate mitigation measures can be implemented to reduce the risks and ensure the structure’s safety. Geotechnical engineers use the data obtained from soil investigations to assess the likelihood and severity of these hazards and provide recommendations for prevention or mitigation.
Soil investigations provide essential data for the design of foundations and other geotechnical structures. The data obtained, such as soil strength, compressibility, and groundwater conditions, are used to determine the most suitable foundation type, depth, and dimensions. This ensures that the foundation can adequately support the structure and withstand the imposed loads without excessive settlement or failure. The soil investigation results also influence construction planning, including selecting appropriate excavation and ground improvement techniques, dewatering methods, and safety measures.
Stages of Soil Investigations
Desktop study and site reconnaissance
The first stage of soil investigations involves a desktop study and site survey. The desktop study includes reviewing available geological and geotechnical information, such as maps, reports, and previous investigation data. This helps understand the general soil conditions and potential hazards in the project area. Site reconnaissance involves visually inspecting the project site to identify surface features that may influence the soil conditions, such as topography, drainage patterns, and existing structures. This stage helps plan the subsequent stages of soil investigations and identify potential challenges or constraints.
Preliminary soil investigation
The preliminary soil investigation stage involves collecting soil samples and performing basic tests to obtain a general understanding of the soil conditions. This stage typically includes drilling boreholes, excavating test pits, and collecting disturbed and undisturbed soil samples. The samples are visually classified and subjected to simple tests, such as moisture content and Atterberg limits, to determine the soil’s basic properties. The preliminary investigation helps identify the soil strata, groundwater conditions, and potential geotechnical issues that require further detailed investigation.
Detailed soil investigation
The detailed soil investigation stage involves a comprehensive evaluation of the soil’s properties and behaviour using advanced testing methods. This stage includes the performance of in-situ tests, such as the Standard Penetration Test (SPT), Cone Penetration Test (CPT), Vane Shear Test (VST), and Pressuremeter Test (PMT), to obtain detailed information on the soil’s strength, compressibility, and deformation characteristics. Undisturbed soil samples are collected for laboratory testing, which includes tests for shear strength, consolidation, permeability, and other relevant properties. The detailed investigation provides the necessary data for the design of foundations, pavements, and other geotechnical structures.

Supplementary soil investigation (if required)
In some cases, supplementary soil investigations may be required to address specific issues or uncertainties identified during the preliminary or detailed investigations. These investigations may include additional boreholes, specialized in-situ tests, or laboratory tests to obtain more detailed information on specific soil layers or properties. Supplementary investigations are typically conducted when the initial investigations indicate the presence of problematic soils, such as expansive clays, collapsible soils, or contaminated soils, which require further evaluation to ensure the project’s safety and success.
Methods and Techniques for Soil Investigations
Soil sampling
1. Disturbed samples
Disturbed soil samples are collected using methods that do not preserve the soil’s in-situ structure and properties. These methods include auger boring, split-spoon sampling, and bulk sampling. Disturbed samples are used for soil classification tests, such as grain size analysis and Atterberg limits, which do not require preserving the soil’s in-situ characteristics. These samples are easier and less expensive to obtain compared to undisturbed samples.
2. Undisturbed samples
Undisturbed soil samples are collected using methods that preserve the soil’s in-situ structure and properties as much as possible. These methods include thin-walled tube sampling, piston sampling, and block sampling. Undisturbed samples are used for laboratory tests requiring the soil’s in-situ characteristics, such as shear strength, consolidation, and permeability. These samples are more difficult and expensive to obtain than disturbed ones and require careful handling and preservation to maintain their integrity.
In-situ tests
1. Standard Penetration Test (SPT)
The Standard Penetration Test (SPT) is a widely used in-situ test for evaluating the soil’s strength and density. The test involves driving a standard sampler into the soil using a hammer and counting the number of blows required to achieve a specified penetration depth. The blow count, known as the SPT N-value, is correlated to the soil’s relative density, consistency, and shear strength. SPT is commonly used for cohesionless soils, such as sands and gravels, and provides valuable information for foundation design.
2. Cone Penetration Test (CPT)
The Cone Penetration Test (CPT) is an in-situ test that measures the soil’s resistance to the penetration of a cone-shaped probe. The probe is pushed into the soil at a constant rate, and the resistance is measured continuously using sensors in the probe. CPT provides detailed information on the soil’s strength, stratification, and pore water pressure, which is used to design foundations and other geotechnical structures. CPT is particularly useful for soft soils, such as clays and silts, and can provide a continuous soil profile.
3. Vane Shear Test (VST)
The Vane Shear Test (VST) is an in-situ test used to determine the undrained shear strength of cohesive soils, such as clays and silts. The test involves inserting a four-bladed vane into the soil and rotating it to measure the torque required to cause shear failure. The undrained shear strength is calculated from the measured torque and the vane dimensions. VST is commonly used to design foundations, excavations, and slope stability analyses in cohesive soils.
4. Pressuremeter Test (PMT)
The Pressuremeter Test (PMT) is an in-situ test that measures the soil’s deformation and strength properties by applying radial pressure to the soil through a cylindrical probe. The test involves expanding the probe in the soil and measuring the pressure-deformation relationship. PMT provides information on the soil’s shear strength, deformation modulus, and in-situ horizontal stress, which is used to design foundations, retaining walls, and tunnels. PMT is suitable for various soil types and can be performed in boreholes or using self-boring techniques.
Laboratory tests
1. Soil classification tests
Soil classification tests are performed on disturbed soil samples to determine the soil’s basic properties and to classify the soil according to standard classification systems, such as the Unified Soil Classification System (USCS) or the AASHTO classification system. These tests include grain size analysis, Atterberg limits (liquid limit, plastic limit, and plasticity index), and specific gravity. Soil classification is essential for understanding the soil’s behaviour and selecting appropriate design parameters and construction methods.
2. Strength and compressibility tests
Strength and compressibility tests are performed on undisturbed soil samples to determine the soil’s shear strength and deformation characteristics under different loading conditions. These tests include the unconfined compression, triaxial, direct shear, and consolidation tests. The results of these tests are used for the design of foundations, slopes, retaining walls, and other geotechnical structures and for predicting the soil’s settlement and stability behaviour.
3. Permeability tests
Permeability tests are performed on soil samples to determine the soil’s ability to transmit water. These tests include the constant head permeability test and the falling head permeability test. The results of these tests are used to design drainage systems, septic systems, and groundwater control measures and to predict the soil’s behaviour in the presence of water.
4. Chemical tests
Chemical tests are performed on soil samples to determine the presence and concentration of harmful substances, such as sulphates, chlorides, and organic matter. These tests are important for assessing the soil’s corrosivity and potential impact on the durability of concrete and steel structures. The results of these tests are used to select appropriate construction materials and design protective measures against soil-induced corrosion.
Interpretation and Reporting of Soil Investigation Results
The soil profile and stratigraphy are derived from the results of the soil investigations and provide a visual representation of the soil layers and their properties with depth. The soil profile includes information on the soil types, layer thicknesses, colour, texture, and consistency. The stratigraphy describes the soil layers’ sequence and arrangement and geologic origin. The soil profile and stratigraphy are essential for understanding the soil conditions and selecting appropriate foundation types and depths.
Geotechnical parameters and design values are derived from the results of the in-situ tests and laboratory tests performed on the soil samples. These parameters include the soil’s shear strength, compressibility, permeability, and other relevant properties. The design values are selected based on a statistical analysis of the test results and the application of appropriate safety factors. These values are used for the design of foundations, slopes, retaining walls, and other geotechnical structures and for predicting the soil’s behaviour under different loading conditions.

Based on the soil investigation results and the geotechnical parameters and design values, recommendations are provided for the design and construction of foundations and other geotechnical structures. These recommendations include selecting appropriate foundation types (e.g., shallow footings, deep foundations, raft foundations), sizing and reinforcement of the foundations, selecting construction methods and equipment, and implementing ground improvement techniques, if necessary. The recommendations also address potential geotechnical hazards and guide mitigation measures.
The soil investigation report should also discuss the limitations and uncertainties associated with the investigation results and the derived geotechnical parameters and design values. These limitations may include the spatial variability of the soil conditions, the testing methods and equipment limitations, and the assumptions and simplifications made in interpreting the results. The uncertainties may arise from the inherent variability of the soil properties, anomalies or heterogeneities, and the potential changes in the soil conditions over time. Discussing limitations and uncertainties is important for understanding the reliability and applicability of the investigation results and for making informed decisions about the design and construction of the infrastructure project.
Best Practices for Soil Investigations
Engaging qualified and experienced geotechnical professionals is essential for ensuring the quality and reliability of soil investigations. These professionals should have the necessary education, training, and certification in geotechnical engineering and soil mechanics. They should also have relevant experience conducting soil investigations for similar infrastructure projects and be familiar with the local soil conditions and regulations. Engaging qualified professionals helps obtain accurate and reliable soil investigation results and avoid costly mistakes and delays.
Ensuring adequate scope and coverage of investigations
Ensuring adequate scope and coverage of soil investigations is important for obtaining a comprehensive understanding of the soil conditions and for minimizing the risk of unexpected soil-related issues during construction. The scope of the investigations should be based on the type, size, and complexity of the infrastructure project, as well as the anticipated soil conditions and potential geotechnical hazards. The investigations should cover the entire project area and include sufficient numbers and depths of boreholes, test pits, and in-situ tests to capture the spatial variability of the soil conditions. Adequate coverage helps avoid data gaps and obtain representative soil parameters for design.
Adhering to relevant standards and guidelines
Adhering to relevant standards and guidelines is important for ensuring soil investigations’ quality and consistency and complying with regulatory requirements. These standards and guidelines include national and international soil sampling, testing, and reporting standards, such as ASTM, ISO, and Eurocode standards. They also include local and regional guidelines and regulations for geotechnical investigations and design, such as building codes and environmental regulations. Adhering to these standards and guidelines helps ensure the investigation results’ reliability and acceptability and avoid legal and financial liabilities.
Communicating effectively with project stakeholders
Effective communication with project stakeholders is essential for ensuring the successful planning, execution, and interpretation of soil investigations. The stakeholders may include the project owner, designers, contractors, regulators, and the public. Effective communication involves:
- Clearly defining the objectives and scope of the investigations.
- Providing regular updates on the progress and findings.
- Explaining the implications of the investigation results for the project design and construction.
- Addressing any concerns or questions raised by the stakeholders.
Effective communication helps build trust and confidence in the soil investigation process and facilitates informed decision-making.
Soil investigations play a critical role in the success of infrastructure projects by providing essential information on the soil conditions and properties that influence the structures’ design, construction, and performance. Neglecting or inadequately conducting soil investigations can lead to costly failures, delays, and safety hazards. Therefore, it is important for project managers, engineers, and stakeholders to understand the importance of soil investigations and to allocate sufficient resources and attention to their planning and execution.
To ensure effective soil investigations in infrastructure projects, it is recommended to engage qualified geotechnical professionals, ensure adequate scope and coverage of investigations, adhere to relevant standards and guidelines, and communicate effectively with project stakeholders. It is also recommended that appropriate methods and techniques be used for soil sampling, in-situ testing, and laboratory testing, and the investigation results should be interpreted and reported clearly and comprehensively. By following these recommendations, project managers and engineers can obtain reliable and actionable information on the soil conditions and properties and make informed decisions about the design and construction of the infrastructure project.