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Why do we need a soakaway test and what is involved

July 10, 20243 min read

In the realm of civil engineering and landscape architecture, the design and implementation of efficient drainage systems are paramount. A cornerstone of this process is the soakaway test, a crucial procedure that measures the soil's infiltration rate. Understanding soil infiltration rates is fundamental for developing systems that manage stormwater effectively, mitigate flooding risks, and prevent soil erosion. This article delves into the importance of soakaway tests, outlining their role in designing robust drainage solutions.

Understanding Soakaway Tests

A soakaway test, sometimes referred to as a percolation test, evaluates how quickly water infiltrates the soil. The test typically involves excavating a pit in the proposed location of the drainage system, filling it with water, and measuring the rate at which the water seeps into the ground. This infiltration rate is then used to calculate the size and design of the soakaway system needed to manage the expected volume of water.

The Necessity of Soakaway Tests

  1. Accurate Drainage System Design: One of the primary reasons for conducting a soakaway test is to ensure the drainage system is appropriately designed. Soakaways are underground structures that allow water to percolate into the soil, thereby preventing surface water accumulation. Without accurate infiltration rates, it is challenging to design a system that can handle the volume of water from rainfall or other sources. Incorrect estimates can lead to undersized systems that fail during heavy rain, causing waterlogging and potential property damage.

  2. Preventing Flooding: In regions prone to heavy rainfall, efficient drainage systems are essential to prevent flooding. A properly designed soakaway system, informed by reliable infiltration data, can mitigate the risk of flooding by ensuring that water is absorbed into the ground at a manageable rate. This not only protects buildings and infrastructure but also safeguards the environment by preventing the erosion and degradation of landscapes.

  3. Sustainable Water Management: Soakaway tests contribute to sustainable water management practices. By facilitating natural groundwater recharge, soakaways help maintain the water table and reduce the need for artificial drainage solutions. This is particularly important in urban areas where impermeable surfaces, such as roads and pavements, prevent natural infiltration, leading to increased surface runoff and pressure on municipal drainage systems.

  4. Complying with Regulations: Many local authorities and regulatory bodies require soakaway tests as part of the planning and development process. These regulations ensure that new developments are equipped with adequate drainage systems to handle stormwater. Compliance with these standards is not only a legal obligation but also a responsible practice to prevent adverse environmental impacts.

  5. Identifying Soil Suitability: Different soil types have varying infiltration capacities. For instance, sandy soils typically allow water to infiltrate quickly, while clay soils have much slower infiltration rates. Conducting a soakaway test helps identify the suitability of the soil for drainage purposes. In cases where the soil is not conducive to natural infiltration, alternative drainage solutions, such as retention ponds or engineered drainage channels, may be necessary.

The Process of Conducting a Soakaway Test

Conducting a soakaway test involves several steps:

  1. Site Selection and Preparation: Choose a representative location for the test within the area intended for the drainage system. Dig a test pit, usually about 300mm square and at least 300mm deep, to mimic the conditions of the proposed soakaway.

  2. Filling and Measuring: Fill the pit with water and allow it to drain completely. Refill the pit with water, then measure the drop in water level at regular intervals, typically every 30 minutes, until the water level stabilises. This provides data on the infiltration rate.

  3. Calculating Infiltration Rate: The infiltration rate is calculated by dividing the change in water level by the time taken. This rate is then used to design the soakaway system, ensuring it is capable of handling the anticipated water load.


Soakaway tests are an indispensable part of designing effective drainage systems. By providing accurate measurements of soil infiltration rates, these tests enable engineers to develop solutions that manage stormwater efficiently, prevent flooding, and promote sustainable water management. In an era where climate change is increasing the frequency and intensity of rainfall, the importance of reliable and effective drainage systems cannot be overstated. Thus, conducting thorough soakaway tests remains a fundamental practice in civil engineering and landscape management.

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