Guide to Subsoil Drainage for Effective Water Management

Subsoil drainage is a crucial component in maintaining the structural integrity of buildings and preserving landscapes. This system is designed to remove excess ground water from the soil, thereby preventing damage to building foundations and landscapes.

By employing subsoil drainage systems, homeowners and property managers can protect their buildings and gardens from damage caused by excessive water. Water controlling drainage systems effectively remove excess water, preventing flooding and avoiding potential harm to properties.

In this guide to subsoil drainage, we explore how subsoil drains are used for effective water management on properties, and what you need to know.

Table of Contents

What is Subsoil Drainage?


Subsoil drainage is a system designed to remove excessive water from the soil, preventing damage to buildings and landscapes. A subsoil drain is installed beneath the ground and should ideally be planned
during the initial stage of building or landscaping to achieve the most effective results.

In certain situations, subsurface water movement can damage buildings or cause a loss of amenity due to the build-up of excess moisture or lateral water pressure. Typically, subsoil drainage systems are necessary in locations where subsurface water movement could cause damage to structures or result in the loss of amenity due to excessive moisture or lateral water pressure. 

Such locations may include the uphill side of cut and fill sites, adjacent to deep footings, behind retaining walls, and next to basement walls. The essential part of these systems is perforated or slotted pipes, which are laid underground and covered with geotextile material to prevent soil from entering them.

Causes of Subsoil Water Accumulation


When it comes to the causes of excess soil moisture or subsoil water accumulation, there are both natural and structural components that can contribute.

Natural Causes


Subsoil water accumulation can occur due to various natural factors. One common cause is the presence of
impermeable layers in the soil, such as clay or bedrock, which obstruct the natural movement of water through the soil layers. This results in water becoming trapped, leading to saturation and waterlogging of the topsoil layer. Additionally, heavy rainfall and storms can result in excess surface water that cannot be absorbed by the soil quickly enough, leading to pooling and saturation. Furthermore, the topography of the land can also contribute to subsoil water issues, particularly in areas with slopes or depressions, where water tends to accumulate and saturate the soil.

Anthropogenic Causes


Human activities can also
contribute to subsoil water accumulation. For instance, urbanisation and construction can cause soil compaction and the creation of impermeable surfaces, such as roads, buildings, and pavements. These surfaces can hinder or alter the natural drainage patterns, leading to an increase in run-off and, subsequently, accumulation of subsoil water. Moreover, inadequate drainage systems around buildings, retaining walls, and other structures can contribute to increased subsoil water levels, causing potential damage and instability to the structures. Irrigation practices in agricultural areas can also play a role in subsoil water accumulation when excess water is applied to crops or when poorly designed irrigation systems fail to drain away excess water efficiently.

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Implications of Poor Subsoil Drainage


Poor subsoil drainage can lead to a variety of issues, such as dampness within below-ground building sections, compromised foundation strength, and deterioration of pavements.

What is the Standard Subsoil Drainage in Australia?


In Australia, the
National Construction Code (NCC) 2019 Volume Two outlines the requirements for site preparation and drainage. These standards aim to ensure that subsoil drainage systems effectively alleviate groundwater pressures and prevent damage to infrastructure. Proper subsoil drainage design must also account for the specific geotechnical issues at each site. Accordingly, the standard or grade of subsoil drainage in a given area are usually determined by local councils or regional regulations.

When is a Subsoil Drainage System Required?


A subsoil drainage system is required under certain conditions, such as when groundwater pressures are likely to cause dampness in below-ground internal parts of buildings or damage to the foundations of structures and pavements. Additionally, subsoil drainage can aid in increasing soil strength by reducing the moisture content of the soil. In agriculture, for example, particular attention is paid to
soil compaction, which is directly impacted by machinery and can affect crop growth and yield.

When designing a subsoil drainage system, it is crucial to consider factors such as soil type, topography, outlet placement, and existing wetlands. Quality installation and thorough evaluation of these on-site factors will ensure the drainage system effectively performs for many years to come.

What are the Methods of Subsoil Drainage?


The most common type of subsoil drainage system is an underground pipe made from a porous material, such as perforated PVC or flexible corrugated plastic. These pipes allow water from the surrounding soil to enter the pipe while preventing the entry of debris that could cause blockages. Subsoil drainage pipes are usually installed in trenches filled with coarse gravel or other drainage materials, which facilitate water flow into the pipe and away from the property.

Methods of subsoil drainage include:

Vertical Draining


Vertical draining refers to the method of installing wells or boreholes into the ground to remove excessive water from the soil. These wells can either be unlined or lined with casing materials, depending on the type of soil and the required drainage capacity. The process involves drilling or digging a hole into the subsoil to reach the water table, thereby allowing groundwater to be pumped out. Vertical drainage is often used where space is limited or when the soil type makes horizontal draining challenging.

Horizontal Draining


Horizontal draining involves the installation of
subsurface pipes or trenches to remove groundwater. These pipes are laid in a horizontal manner throughout the soil, allowing excess water to flow away from the affected area. This kind of draining system is often used in larger areas, such as agricultural lands or construction sites. The materials used for horizontal draining systems are typically plastic pipes or corrugated metal pipes, which are perforated to allow water to enter.

Slope Draining


Slope draining is a method used to manage subsurface water in areas with natural or artificial slopes. By designing drainage ditches or channels along the slope, excess water is directed away from the area, preventing any potential damage to structures or landscapes due to water accumulation. The water collected from slope draining systems can typically be used for irrigation purposes or discharged into a nearby body of water.

Mole Drainage


Mole drainage includes mole drains, mole drains over a collector pipe system, and gravel mole drains. The suitability of each type of
mole drainage will depend on a number of factors, including clay content and type, sand/stone profiles, gradient, and outfall location.

Interceptor Drains


Subsoil interceptor drains are a type of subsoil drainage system that functions as a barrier between two areas, often to protect buildings or landscapes from water intrusion. These drains consist of trenches filled with coarse materials like gravel or stones, which are then covered with a layer of soil. The purpose of an
interceptor drain is to collect and redirect water away from the target area, preventing it from causing damage to structures or landscapes.

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Installation Procedures

Subsoil drainage is an important part of civil construction that helps to maintain the structural integrity of roads, foundations, and other infrastructure components. The installation process involves a series of steps, including:

  • planning and preparation of the site
  • setting out and excavating trenches to fit the drainage system
  • installing bedding materials
  • installing the actual drainage system
  • trenches are backfilled with soil and aggregate, then compacted

Only qualified professionals should attempt subsoil drainage installation. A professional has the knowledge, experience and qualifications to adhere to work health, safety, and environmental requirements, as well as ensure quality assurance as set out by the relevant industry codes and guidelines, such as those provided by the Australian Building Codes Board.

Advantages & Disadvantages of Subsoil Drainage

Subsoil drainage systems offer several advantages by providing an effective means of removing excessive water from the soil and preventing potential damage to buildings and landscapes.

One significant benefit is the improvement of soil health, achieved through the increased aeration of the soil, which promotes plant growth and better responses to fertiliser use. Furthermore, subsurface drainage can help prevent or reduce soil salinity, which is beneficial for both plants and the overall soil quality.

However, there are also disadvantages to consider when implementing a subsoil drainage system. First and foremost, these systems must be carefully designed and installed to ensure they function effectively. Poorly designed drainage systems may lead to inadequate water removal or even worsen existing issues. Additionally, subsoil drainage systems can become expensive to install and maintain, particularly if there are complex landscaping requirements or challenging soil conditions. The installation process can also be time-consuming and may require ongoing maintenance to ensure optimal performance.

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Can’t Deal with Your Soggy Backyard? Contact Plumbers Express Today

Plumbers Express can help you with the planning and installation of a soil drainage system that fits your household needs and balances budget.

Our team of plumbing specialists are fully qualified and insured – contact us today for all your plumbing needs!

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