When properly installed, sub-surface or French drains can help to reduce the effects of rainfall on your property. French drains were first introduced in 1859 by Henry Flagg French in his book Farm Drainage. These early drains used roofing tile, which is why sub-surface drainage is still referred to as “drainage tile”. The fundamental construction of these drainage systems has not changed much, but the invention of better HDPE (High-density polyethylene) pipe and geotextile filter fabrics has helped to increase the service life of these systems. In this post, we will discuss why proper spacing of drains is important, how drainage contributes to plant health, and different types of pipe. We will also cover backfill material, filter media, and approximately how much a French drain system will cost to install.
How proper spacing of French drains contributes to plant health:
In Northeastern Ohio, most of the soils that we encounter below the topsoil layer have a high clay content, with a few exceptions. Soils that are high in clay content are not very permeable, and lateral movement of water within them is very slow. If sub-surface drainage pipes are spaced too far apart, only the water in the immediate area of the pipes will be drained and the water table will remain unchanged in the area between the pipes. We recommend that drain pipes be installed not more than 30 feet apart to provide for optimal sub-surface drainage.
Landscape drainage on a residential or commercial property lowers the water table, which provides better soil aeration. This in turn allows the soil to dry and warm faster after rainfall, and creates increased storage capacity for water to be absorbed into the soil. Lowering the water table also helps to promote deeper root zone growth for plants and can help them to sustain drought conditions better than if a high water table is frequently present. The utilization of French drains also helps the water table to remain constant during seasonal changes and maintain available water for plants.
What type of pipe should be selected for sub-surface drainage?
For modern lawn and landscape drainage, there are a few types of drainage pipe available. These consist of single and double wall corrugated HDPE, triple wall smooth HDPE, and PVC. For the drainage installation services that we provide, we only install double wall corrugated for large diameter applications, and triple wall smooth pipe for smaller diameter ( < 6” ) applications. We do this to help increase the service life of the drain and protect the investment you’ve made in your property.
Single wall corrugated pipe crushes very easily which increases the likelihood of collapse during its service life, making it prone to damage from tree roots and vehicles. The corrugations within the pipe slow down the flow of water, allowing silt and soil particles to be deposited inside the pipe which will cause the pipe to plug and be rendered useless. Perforated corrugated pipe with slots is more likely to have problems with failure from soil particles plugging the slots and blocking the flow of water into the pipe. This type of pipe also cannot be snaked easily, and attempting to do so will typically result in the pipe being damaged. PVC is stronger than the single wall pipe, but this strength comes at the cost of the pipe being very brittle and prone to cracking and fracturing if subjected to loads so we only recommend this for pressurized applications.
To provide the best value for our customers, we choose to install either double wall corrugated pipe, which has a smooth inner wall or triple wall smooth pipe, depending upon the application. These types of pipe have some distinct advantages over single wall and PVC. First, both are stronger than single wall pipe, which makes them much less likely to be crushed or damaged. While they are stronger than single wall pipe, they are also more flexible than PVC pipe and therefore less likely to be damaged due to cracking. Double and triple wall pipes are smooth inside, increasing water velocity which promotes the scouring, or self-cleaning of the pipe. This prevents buildup of sediment in the pipe and helps to dramatically increase its service life. It can also be snaked in the event of a blockage without risking damage to the pipe.
Which backfill materials are best for French drains?
There are two primary types of backfill that should be used for French or sub-surface drains; these consist of different types of open-graded aggregate, and ASTM C33 Concrete Sand. Either of these backfill materials are acceptable, however each one requires different installation procedures to filter soil particles before they reach the drain pipe. Without proper filtering of the soil particles the drain will cease to function properly much sooner than expected. In order to keep soil or other debris from building up inside the pipe, the drain should be pitched at least 1% to allow high enough water velocity to scour the pipe.
Why aren’t more sub-surface drainage systems using concrete sand?
This is a question many people ask when deciding between using aggregate or concrete sand as trench backfill. French drains were developed using gravel, and this is in large part why they are still installed in the same manner today. While this method has been implemented for over 100 years with widespread success, it is more labor intensive than using concrete sand for backfill. Installing the filter fabric along the sides of the trench requires more labor and care to ensure that it does not come out of place. Use of concrete sand results in a less labor-intensive installation process, the material is cheaper than gravel, and it eliminates the additional cost and labor of using filter fabric.
There is some concern over the difference in permeability between concrete sand and gravel and what effects this may have on functionality of the drainage system. Using average permeability coefficients obtained from http://www.geotechdata.info, we are able to calculate that gravel is on average approximately 54% more permeable than concrete sand. However, when sand or gravel is used for this application the increased permeability does not provide any additional benefit. Most sub-surface drainage relies upon lateral water movement through the soil, which at a typical depth of 18 to 24 inches is mostly inorganic clay with a few exceptions. Concrete sand is approximately 99% more permeable than inorganic clay soils therefore it does not restrict lateral water movement into the drainage system any more than the use of gravel with a filter fabric envelope.
How to install French drains using open-graded aggregate:
When using open-graded aggregate for drainage backfill, an aggregate should be selected that is uniformly graded and is approximately ¾” to 1” in size. Either gravel or crushed rock such as limestone can be used, but it should be washed to prevent adding unnecessary fine material to the drainage system. The trench should be lined with a geotextile filter fabric to prevent mitigation of soil particles into the gravel. Fine soil particles will cause the voids in the gravel to become plugged as well as creating settling issues in the immediate trench area. The filter fabric should be left long enough to be wrapped over the top of the aggregate before soil is placed on top of the trench. Approximately 3” of aggregate should be placed in the bottom of the trench followed by perforated pipe with the holes facing down. The trench should then be backfilled the rest of the way with the open-graded aggregate to within 3 or 4 inches of the final grade. Finally, the filter fabric should be wrapped over the aggregate then covered with topsoil for re-seeding or planting.
How to install French drains using concrete sand:
French drains which utilize concrete sand for backfill have a different installation procedure from aggregate to help protect the drainage system from soil particles. Instead of having a filter fabric to separate the soil from the aggregate, the pipe is wrapped in fabric otherwise known as a sock. The use of sand eliminates the need for geotextile filter fabric because the sand acts as a filter for the fine soil particles. One benefit of using sand as a filter media is that it provides a much greater filtering area than filter fabric will. Also, it is easier to cover the pipe with the sock and pour the sand into the trench as opposed to fastening the geotextile to the side of the trench and filling it with aggregate. As with the aggregate filled trench, approximately 3” of sand should be placed below the pipe, then the trench is filled to within 2” of the final grade. For the final 2”, topsoil and sand are mixed together to create a permeable layer which will also allow for grass or other plants to grow.
How much will my French drain system cost to install?
At Precision Excavating, we pride ourselves in the quality of our work, and we believe that it is wise for our customers to invest in long-term solutions that are installed by a professional, knowledgeable contractor. A typical French drain system will start out around $12 per linear foot installed, depending upon drainage system length, ease of access for equipment and local availability for materials. When you contact us to install your drainage system, you can be confident that it is the proper solution for storm water management on your site, and that it is being installed using only the highest quality materials available.
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