Soils Permeability Testing

Permeability testing is crucial for geotechnical and environmental projects because it provides essential data on the ability of soils and rocks to transmit fluids, which influences a wide range of design and management considerations, such as: 

• Assessing groundwater flow and seepage rates for dewatering, slope stability, and foundation design 

• Designing drainage systems, including filters, drains, and wells, to control groundwater and prevent flooding or erosion 

• Evaluating contaminant transport and migration in soils and groundwater for environmental site assessments and remediation planning 

• Selecting and designing clay liners and covers for waste containment facilities, such as landfills, tailings dams, and leachate ponds 

• Assessing the suitability of soils for various construction applications, such as road subgrades, embankments, and backfills 

• Determining the feasibility and design requirements for groundwater recharge, extraction, and storage systems 

By accurately measuring and characterising permeability, project teams can make informed decisions, optimise designs, and mitigate risks related to groundwater, seepage, and contaminant transport. 

The permeability of soils and rocks is influenced by several factors, including: 

• Particle size distribution: Coarser-grained soils, such as sands and gravels, generally have higher permeability than finer-grained soils, such as silts and clays, due to larger pore spaces between particles. 

• Soil structure and fabric: The arrangement and orientation of soil particles, as well as the presence of macropores, fractures, or other preferential flow paths, can significantly affect permeability. 

• Porosity: Higher porosity, or the volume of voids relative to the total volume of the soil or rock, generally correlates with higher permeability. 

• Degree of saturation: Permeability is typically measured under saturated conditions, but partially saturated soils may have lower permeability due to the presence of air in the pore spaces. 

• Fluid properties: The density and viscosity of the permeating fluid can influence the measured permeability, with higher viscosity fluids resulting in lower permeability values. 

• Confining stress: Increasing confining stress can reduce permeability by compressing the soil or rock matrix and closing pore spaces or fractures. 

• Chemical interactions: Certain minerals, such as montmorillonite clays, can swell or disperse in the presence of water, reducing permeability over time. 

Understanding these factors and how they vary across a site is essential for accurately characterising permeability and developing appropriate design and management strategies. 

The selection of an appropriate permeability testing method depends on several factors, including: 

Soil or rock type 

Different methods are suitable for different soil or rock types, such as triaxial permeability testing for cohesive soils, falling head testing for fine-grained soils, and packer testing for rock formations. 

Project requirements 

The required accuracy, precision, and turnaround time for permeability data may influence the choice of testing method, with some methods providing more rapid or cost-effective results than others. 

Sample availability and quality 

The availability and quality of soil or rock samples may dictate the feasibility of certain testing methods, with some methods requiring undisturbed or intact samples, while others can accommodate disturbed or reconstituted samples. 

In-situ or laboratory testing 

Permeability can be measured in the field using methods like borehole permeability tests or infiltration tests, or in the laboratory using methods like triaxial or falling head permeameters. The choice between in-situ and laboratory testing depends on factors such as site accessibility, sample disturbance, and the need for controlled boundary conditions. 

Regulatory or client requirements 

Certain projects may have specific regulatory or client requirements for permeability testing methods, such as using a particular standard or method for compliance or quality assurance purposes. 

Consulting with experienced geotechnical professionals, such as those at Lucion, can help you navigate these factors and select the most appropriate testing method for your project’s needs and constraints. 

Sample disturbance can significantly affect permeability test results, especially for fine-grained or structured soils. Disturbance can occur during sampling, transport, storage, or preparation of soil specimens, and can alter the soil structure, fabric, and pore spaces, leading to changes in permeability. Common effects of sample disturbance on permeability include: 

Overestimation of permeability 

Disturbance can create artificial macropores or preferential flow paths, leading to higher measured permeability than in-situ conditions. 

Underestimation of permeability 

Disturbance can also compress or smear soil samples, reducing pore spaces and connectivity, leading to lower measured permeability than in-situ conditions. 

Increased variability 

Disturbed samples may exhibit greater variability in permeability test results, making it more difficult to characterise the true permeability of the soil or rock formation. 

To minimise the effects of sample disturbance on permeability test results, it is important to: 

• Use appropriate sampling techniques, such as thin-walled push tubes or rotary core barrels, to obtain high-quality, undisturbed samples. 

• Handle, transport, and store samples carefully to avoid excessive disturbance, using appropriate containers, cushioning, and temperature controls. 

• Prepare test specimens using methods that minimise further disturbance, such as trimming or cutting rather than remoulding or compacting. 

• Consider the use of in-situ testing methods, such as borehole permeability tests, to avoid the need for sampling and laboratory testing altogether. 

Discussing your project’s sampling and testing requirements with Lucion’s experienced geotechnical team can help ensure that sample disturbance is minimised and permeability test results are as representative as possible. 

Permeability test results are typically reported as a coefficient of permeability or hydraulic conductivity, expressed in units of length per time (e.g., m/s or cm/s). These values are used in various geotechnical and environmental design calculations and models, such as: 

• Seepage analysis: Permeability values are used in seepage analysis to calculate flow rates, pore water pressures, and exit gradients for earth dams, levees, and other water-retaining structures. 

• Dewatering design: Permeability values are used to estimate groundwater inflow rates and design dewatering systems for excavations, tunnels, and mines. 

• Contaminant transport modeling: Permeability values are used in contaminant transport models to predict the migration and fate of pollutants in soils and groundwater, and to design remediation strategies such as pump-and-treat systems or permeable reactive barriers. 

• Landfill and waste containment design: Permeability values are used to design clay liners, covers, and leachate collection systems for landfills and other waste containment facilities, ensuring that the required hydraulic performance and containment criteria are met. 

• Stormwater infiltration and drainage design: Permeability values are used to design stormwater infiltration basins, permeable pavements, and other sustainable drainage systems (SuDS), ensuring that the required infiltration rates and storage capacities are achieved. 

When interpreting and applying permeability test results, it is important to consider factors such as the representativeness of the tested samples, the spatial variability of permeability across the site, and the potential effects of scale, anisotropy, and heterogeneity. It is also important to use appropriate factors of safety or conservative design assumptions to account for uncertainties in permeability data and the potential for long-term changes in permeability due to factors such as clogging, dissolution, or geochemical reactions. 

Lucion’s geotechnical experts can provide guidance on the interpretation and application of permeability test results for your specific project, ensuring that the data is used appropriately and effectively in your design and decision-making process. 

Ensuring the quality and reliability of permeability testing for your project involves several key considerations: 

• Selecting a reputable and accredited testing laboratory, such as Lucion, with experience in permeability testing for geotechnical and environmental projects. Look for laboratories with UKAS accreditation (e.g., No. 8180) and ISO 9001 certification, which demonstrate a commitment to quality assurance and best practices. 
• Developing a clear and comprehensive testing specification, outlining the project requirements, soil or rock types, testing methods, sample preparation procedures, quality control measures, and reporting formats. Lucion’s team can help you develop a testing specification that meets your project’s specific needs and constraints. 
• Ensuring that samples are collected, handled, transported, and stored in accordance with best practices to minimise disturbance and maintain sample integrity. This may involve using appropriate sampling techniques, containers, and preservation methods, and documenting the sample chain of custody. 
• Reviewing and assessing the testing laboratory’s quality assurance and quality control (QA/QC) procedures, including equipment calibration, test method validation, operator training and certification, and participation in proficiency testing programs. Lucion’s commitment to quality is demonstrated through their rigorous QA/QC procedures and accreditations. 
• Communicating regularly with the testing laboratory throughout the project, providing feedback on testing results and requirements, and addressing any issues or concerns that arise. Lucion’s team is committed to maintaining open and proactive communication with clients to ensure that testing meets their project’s needs and expectations. 
• Comparing permeability test results with published data or correlations for similar soil or rock types, and considering the potential effects of sample disturbance, scale, anisotropy, and heterogeneity on the measured values. Lucion’s geotechnical experts can provide context and interpretation of permeability test results based on their extensive experience and knowledge of soil and rock behaviour. 

By partnering with a reputable and accredited testing laboratory like Lucion, developing a clear testing specification, ensuring proper sample handling and QA/QC procedures, and maintaining open communication and feedback, you can have confidence in the quality and reliability of permeability testing for your geotechnical and environmental projects.