Foundation repair engineers and contractors must often address problems unique to below-grade construction. One of these problems is the intrusion of ground water through foundation and basement walls. Moisture seepage through sub-grade concrete and masonry walls can result in chronic moisture conditions and flooding. The areas at risk include basements, parking garages, and the living space of underground and split-level buildings. Materials and methods for preventing below-grade seepage are evolving and failed systems have been the focus of considerable litigation in California and elsewhere.
It is now recognized that water acting under “hydrostatic pressure” is a formidable adversary. Hydrostatic pressure develops in “saturated” soils and increases in proportion to the depth of the “hydrostatic head”. It is not enough to apply waterproofing to the below-grade surface. Even a tiny breach in the waterproofing can result in a serious leak.
A French drain system properly designed and installed below the base of the subgrade wall, will relieve the hydrostatic pressure and is an indispensable component of any subgrade waterproofing project. I expand further on the subject of hydrostatic pressure and waterproofing on another page of this site which is entitled Basement Waterproofing.
Another common application for a French drain system is the protection of a retaining wall. Retaining walls are commonly used in property developments of limited space to increase the amount of usable (level) grade. A retaining wall provides lateral restraint for soil which is backfilled against one side of the wall while the other side of the wall is left exposed.
Ground water soaking into and saturating the backfill soils has the effect of weakening the soil and increasing the lateral load on the wall. The increased hydrostatic pressure, discussed above, often results in moisture seepage into the wall itself, causing damage to the wall materials such as concrete, masonry and rebar.
A French drain system at the base of a retaining wall will serve to reduce the lateral loads on the wall and decrease the hydrostatic pressure to mitigate damage to the wall materials. Such a system is often installed in conjunction with waterproofing as discussed above. Most liquid-applied waterproof membranes are subject to damage during backfill and an appropriate “protection board” should go against the new waterproofing prior to placement of gravel for the French drain.
Sometimes a French drain system is needed to correct drainage problems due to locally “perched” groundwater. Groundwater is often thought to exist only below a “static level” known as the water table. However, due to variations in geology and soils, groundwater can accumulate locally in pockets of perched groundwater and the subsoil in these areas will become saturated. In urban areas, high levels of yard irrigation will feed these zones of saturated subsoil. After a heavy rainfall, the ground can become completely saturated right up to the surface.
Perched groundwater from off-site sources can and will move horizontally, especially if the sub-soil has good permeability (as is the case with sands and gravels). Even if the native soils are composed of less-permeable clays and silts, large quantities of moisture can and do move quickly through loosely compacted backfill soils of underground utility trenches. In fact, utility companies often require their underground conduit and pipe trenches be backfilled with imported (and highly permeable) sand to prevent damage and facilitate compaction.
When sandy or gravely soils are being “charged” with groundwater from off site sources upslope, all the on-site surface drainage in the world is not going to keep those trenches from delivering groundwater to a down-slope destination. If that destination is the footprint of a building, a French drain system may be the only way to cut off the flow and protect the foundation soils and building materials from damaging effects.
A Wet Crawl Space
When surface drainage control is not enough, I often recommend French drain systems to dry out wet crawl space areas under pier-and-beam foundations. I have discussed the low-profile foundation under the topic of Yard Drainage. In this situation, the exterior grade is higher than the crawl space grade and there is no way to cut off the crawl space moisture by surface drainage controls alone. This is especially true when groundwater is moving in laterally as discussed above.
Some contractors may be tempted to pursue a cheap solution and install surface drains in the crawl space area, collecting water when it starts to pond in the lowest areas. These contractors do not understand the problem. The crawl space must be kept dry at all times. Waiting for water to pond in the crawl space and then drawing it down is like living in a house with no roof and then mopping the water off the floor after each rainstorm!
There are a variety of methods used to mitigate the destructive effects of expansive soils. These methods include removal of the expansive soils to a specified depth and replacement with non-expansive soils. Other methods include foundation removal and replacement with structural concrete or post-tension concrete slabs, installation of concrete cut-off walls, underpinning, subgrade irrigation, treatment of the soils with chemical admixtures, or the construction of French drain systems.
For many situations, I believe that a French drain system, beneath a perimeter hardscape apron, is the most practical and cost effective solution to the problem of expansive soils. The French drain prevents excessive ground water saturation while the perimeter apron prevents excessive surface water saturation. The apron has the added function of preventing excessive shrinkage of the foundation soils during the dry season.
Depth of a French Drain
Installation of a French drain system can add considerable cost to a drainage correction project and careful consideration of the scope of the project is required for the most cost-effective result. Understanding that groundwater movement is controlled by gravity, the installation of a “cutoff” French drain on the uphill side of a building may be adequate to provide the needed protection. Soils and building materials soak up groundwater like a sponge. The French drain must be deep enough so that the soaking and wicking action is disrupted and overcome by the force of gravity drawing the moisture down and away from the structure.
The proper depth of a French drain system depends upon the application. When protecting a below-grade living space, the “invert” (bottom) of the perforated pipe must always be kept well below the level of the finish floor. For purposes of protecting a crawl space or treating expansive soils, the depth of the French drain may become more a matter of judgment. For the best result, an experienced expert should be consulted.
Another important constraint on the depth of a French drain is the proximity and depth of foundations. An over-deep trench will undermine the supporting soil of an adjacent concrete footing and could cause settlement of the building or structure. To overcome this problem, the French drain could be installed in sections so that only a short length of the footing would be undermined at any one time. If another accommodation cannot be made, the adjacent footing may need to be extended by “underpinning”.
If feasible, a more cost-effective solution would be to move the location of the French drain trench away from the footing. As a rule of thumb, the bottom of the trench should be no deeper than the depth of an imaginary line drawn from the bottom of the footing on a downward projection of forty-five degrees. In these situations where the integrity of the existing foundation may be at risk, consultation with an experienced foundation engineer is recommended.
Pipe and fittings normally specified for commercial and engineered French drainage jobs is four-inch or six-inch PVC or ABS: SDR-35 perforated. Occasionally you will see schedule 40 specified but I have had difficulty finding schedule 40 perforated. On most residential jobs and small commercial jobs, I will use three-inch or four-inch smooth-wall “poly” drainage pipe, perforated with belled ends.
I use only smooth wall pipe for drainage. I never use the popular corrugated plastic pipe for French drains or tightlines. The corrugated pipe is easily destroyed by a roto-rooter and I have replaced too many of them which were clogged with silt and roots. The corrugated pipes are fine for temporary or on-surface installations such as a run down the face of a slope.
The perforated pipe used for French drains is typically manufactured with two parallel rows of perforations (round holes) on the down side of the pipe about 120 degrees apart. The perforated pipe should always be installed in the very bottom of the trench with the perforations down. The goal is to collect the maximum amount of water and keep the water level in the trench as low as possible. This can be accomplished by carefully grading the bottom of the trench to match the desired ‘fall” of the perforated pipe.
The Invert between Two Rows of Holes
Water collecting in the gravel trench will flow into the pipe through the perforations. Providing the pipe has been installed properly, collected water will flow along the pipe “invert”- the invert is the bottom of the pipe and between the two rows of holes. To aid the installer, there should be a single reference line running along the top of the pipe. When that line is straight up, the perforations on either side of the bottom are kept an equal distance from the pipe invert where the water will flow.
If the perforations are mistakenly installed in the up position, a great deal more water will collect in the bottom of the trench before any can be collected in the pipe. If one row of perforations is mistakenly placed along the bottom at the pipe invert, water collected uphill will be lost back into the trench. In both cases, the goal of collecting the maximum amount of water is being defeated.
Slope of a French Drain
The usual recommended slope of the perforated pipe for a French drain system is one inch drop per ten feet horizontal run. Any specified depth of the perforated pipe is subject the need for a continuous downward gradient to the point of discharge. If gravity flow is to be maintained, the depth of the perforated pipe will be constrained by the elevation of the discharge point. I have used pipe gradients as flat as 0.5 inch per ten feet in order to keep my drain as deep as possible while still maintaining a gravity-flow system.