This Ag note describes the design and construction of subsurface pipe drainage to drain excess water from the profiles of wet soils. It discusses the major design factors that need considering when installing subsurface pipe systems. However, there are much more technical aspects to some of these factors and a contractor experienced and well trained in subsurface drainage should be consulted.
Introduction
Wet soils are a common problem in the high rainfall districts. Surface drainage systems can improve the situation, but in many cases the soil profile itself needs better drainage so that pastures and crops can grow to their potential and stock damage by treading (pugging) and compaction can be reduced. Subsurface drainage can be achieved by subsurface pipes, mole drains or a combination of subsurface pipes and mole drains. It is important to select the right drainage system for the particular soil type to be drained. Detailed site investigation is recommended before installing pipe or mole drainage systems.
Types of subsurface pipe drainage systems
There are two systems for removing subsurface water with subsurface pipe drainage.
Subsurface pipes
Subsurface pipes, referred to as tile drainage in the past due to use of short clay pipes, are best suited in deep permeable soils where their depth allows wider spacing and minimises cost. Subsurface pipes can also successfully drain heavy (clay), poorly drained soils but have to be installed so close together that they are uneconomic in extensive farmland systems.
Subsurface Pipe
Subsurface pipes can also be used in soils which may have an impeding (e.g. clay) layer at some depth but, if the clay type and content is suitable, mole drains can be installed above these in a ‘mole drainage over collector pipe system.
Subsurface pipes were formerly known as tile drains. Clay pipes or tiles have been superseded by slotted plastic PVC and corrugated polythene pipe and are now referred to as pipe drains. Specifically designed drainage machines dig the trench with a chain slightly wider than the pipe diameter, lay the pipe at the base of the trench, and funnel permeable backfill via a hopper and neck down onto the pipe. The amount of backfill used is governed by soil type, soil permeability, depth of backfill, trench width, etc. Backfill is a major cost of a drainage system but is crucial to the drain’s effectiveness so the quality and price of backfill should not be compromised.
Although trenches can be dug with a typical builder’s type trencher, producing a flat even trench base of constant slope is very difficult. Laying pipes manually, without knocking soil into the trench before the pipe is laid, is also very difficult. The specific drainage machine uses lasers to maintain constant depth and fall regardless of ground surface conditions.
Correctly installed and maintained, these systems can last many years. Regular inspection of outfalls is necessary, as this is the weak link in all drainage systems.
Interceptor drains
These drains are installed across the direction of water flow at or near the interface where sloping land meets the flats. Water flow on slopes not only runs off the surface but often penetrates into the permeable top layer until it reaches a much less permeable, high clay content soil or rocks.
The slotted PVC or corrugated plastic pipes are laid at the base of a trench (slightly wider than the pipe itself) and back filled with permeable material almost to the surface This permeable layer intercepts the water flowing from upslope and delivers it into the large diameter drain pipe which carries the water to an outflow. This system prevents excessive upslope water from encroaching onto the flats.
An interceptor drain
This system is also very useful for intercepting water from hillside springs or lines of springs. Stock traffic at or near the springs causes severe pugging and pasture damage in the areas below the springs as the soil is very wet and has little structural strength. This affected area usually spreads unless fenced off. Interceptor drains installed as close as possible to the spring and across its downhill flow can be effective in controlling the spread.
Hiring a backhoe to try to find the source of the spring and back filling this hole/trench with permeable backfill which leads to the interceptor drain is worth considering.
Soil suitability for subsurface pipe drainage
Soil survey the proposed drainage area by auguring to at least a metre depth, noting soil type and its permeability. This is best done during winter so that soil permeability can be readily estimated by noting how long it takes for the surrounding water to fill an auger hole. The permeability or hydraulic conductivity is important in estimating the required drain spacing and depth and therefore the cost. Generally, permeable soils that are sandy or loam based are suited to pipe drainage, but some well-structured clay loam soils can also be suitable.
Costs
Each drainage system will vary in cost depending on size of area to be drained, soil type (which determines pipe depth and therefore spacing), whether it is suited to mole drainage (with subsurface pipes or a combination), location of outfalls and proximity and availability of suitable backfill.
Current costs of a soil drained by mole drains over a pipe collector system are about $2000 to $2500 per hectare in soils where moles would be re-moled every 5 to 7 years. In a well maintained drainage system with appropriate pasture management, and depending on milk price, payback or breakeven price ranges from 3 to 5 years.
Acknowledgements
The previous version of this Agnote was developed by David Hopkins and published in October 2002.
