Southwest Environmental Limited

Percolation Testing and Drainage Field Design

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Percolation testing is a fundamental field assessment used to determine the rate at which water infiltrates the soil. This data is essential for designing effective drainage fields, soakaways, and other subsurface infiltration systems. Proper assessment ensures that treated effluent is dispersed safely into the ground without causing localized flooding, environmental pollution, or public health risks.

Why Percolation Tests are Required

When developing a site that relies on non-mains drainage, such as a septic tank or a package treatment plant, understanding soil porosity is a strict regulatory requirement. These tests are mandated to satisfy Building Regulations Part H and are often required by local authorities to support broader Planning Statements. The results directly inform the size and viability of the proposed drainage infrastructure.

• Drainage Field Sizing: Establishing the exact dimensions and required trench lengths for subsurface infiltration pipes based on the soil's V-value.
• Regulatory Compliance: Proving that the chosen discharge method satisfies Environment Agency binding rules and local building control standards.
• System Feasibility: Determining whether a site is genuinely suitable for ground infiltration or if an alternative off-site drainage strategy is required.

Methodology and Complementary Assessments

The testing procedure typically involves excavating trial pits to the proposed invert depth of the drainage field, pre-soaking the ground, and measuring the precise time it takes for water to drop between specific levels. This controlled methodology yields an accurate soil infiltration rate, forming the basis of all subsequent engineering calculations.

While standard percolation testing focuses on foul water, similar principles are applied on a larger scale for surface water management. We frequently conduct BRE 365 Soakaway Tests to assess ground suitability for rainwater attenuation features. Integrating these findings provides a holistic approach to site-wide water management, ensuring both surface and foul water systems operate effectively and sustainably.

Case Study: Private Foul Drainage Assessment for Agricultural Development

Southwest Environmental Limited was commissioned to conduct a private foul drainage assessment for a Site in Somerset. The project assessed the site for a new agricultural development comprising two static caravans, which will be used as full-time accommodation for agricultural workers The primary objective was to acquire accurate infiltration data and produce a drainage field design that strictly complies with Part H of the Building Regulations.

Site Assessment and Geological Context

The development is positioned in a level, rural location where surface drainage naturally falls to the south. Desktop mapping indicated that the site's underlying bedrock is dominantly composed of red mudstones, while the superficial geology features tidal flat deposits consisting primarily of unconsolidated mud and sand. During the physical site investigation, the observed soil profile consisted of a medium brown topsoil layer, transitioning into grey-brown sandy clay, and finally into a grey-brown clayey gravel containing frequent mudstone cobbles.

Percolation Testing Methodology

To accurately gauge the ground's suitability for wastewater disposal, three trial pits were excavated across the site. Regulated infiltration tests were then carried out within each pit. The testing revealed a noticeably slow infiltration rate, with each individual test taking approximately 1.5 hours to complete[cite: 122]. Following the completion of the tests, the average percolation value (Vp) for the site was calculated to be 60.09 seconds per millimetre.

Drainage Strategy and Recommendations

Although the calculated Vp value of 60.09 fell within the acceptable regulatory limits for a standard septic tank system, it was recommended that the site utilize a more advanced treatment solution. A 12-person capacity Klargester Bioair 4 Sewage Treatment Plant was specified to comfortably accommodate the site's capped maximum population of 10 workers. Upgrading to a package treatment plant is highly recommended to prolong the operational lifespan of the drainage field and to provide significantly better environmental outcomes. Based on the percolation data and the intended population, the required floor area for the accompanying drainage field was calculated at 120.2 square meters. Furthermore, because mudstone inherently has low permeability and relies on pervious flow via fractures, it was emphasized that the drainage layer must be carefully sited within the fractured mudstone strata.

On this ocaasion the builder undertook the tests themsleves and we created the associated drawings and did the clacuations to size the drainage field. We are happy to split the service in this way if desired