The French Drain: Theory, Application & Practice

Mr. French

The French drain, also called a sub-drain, is so called because the person who is supposed to have invented it, an American, was named French. First used for agricultural purposes in New England before the development of perforated plastic pipe, the system invented by Mr. French was probably somewhat different from what we call a French drain today. Unlike a surface drain system which collects and removes surface moisture, this type drain system collects and removes subsurface moisture.

A sub-drain, by modern definition is basically a trench filled with gravel with a perforated pipe at the bottom. The pipe and gravel are wrapped in a protective “geotextile” fabric. Moisture accumulating in the trench percolates down and enters the pipe which transports the moisture to some point of discharge. This page will discuss when and where a French drain should be installed. I will also provide step-by-step details on how the work should proceed. Just for fun, I have added highlights from a French drain job which I found both challenging and rewarding. Before I do that, I want to talk a bit about a corollary in nature which can help explain the basic principals at work:

A Natural French Drain

A few years back I invested in a small farm in the San Simon Valley of Arizona. Originally developed back in the 1950’s, the farm used furrow irrigation. The property was carefully leveled to a uniform gradient prior to installation of the irrigation system. The USDA maps describe the soils as clayey loam. The first year, I planted wheat and oat seed on about 80 acres and pumped out over 15 million gallons from my ground-water well to the irrigation furrows. Although I harvested a crop, the yields were far below normal. Later I was told by locals that I had not irrigated enough!

After studying some maps and old aerial photos taken prior to development of the farm, I discovered that there had been three buried streambeds cutting diagonally across the trend of my irrigation furrows. Grading and leveling had covered them over but they were still there. After walking the property and observing the soil closely, I was able to locate the buried streambeds visually based upon the increased frequency of rounded cobbles in the overburden soil. The sub soil in these zones contained a high percentage of sands and gravels. It became evident to me that a significant proportion of the 15 million gallons of irrigation water had “percolated” into sands and gravels of the buried streambeds without any benefit to my thirsty crops.

Permeability

Those cobbles, sands and gravels of the streambed just below the surface of my land have a much higher “permeability” than the adjacent clayey loam. This means that they soak up water and draw it away much faster. This is exactly what a French drain system does. A French drain employs the high permeability of coarse gravel to draw moisture away from finer and less-permeable soils.

Mr. French would probably have been pleased to discover buried sands and gravels cutting across his swampy New England farmland. In the arid climate of Arizona, water is at a premium and my experience with irrigation and farming caused me to rethink my plans for the property. I am now restoring the farm to a native ecosystem.

While agricultural drainage is a subject of considerable interest, my discussion here will focus on the use of the French drain as applied to problems related to Foundation Repair, which after all is the theme of this website. In this context, the French drain serves to drain excessive moisture from the surrounding foundation soils, protecting the foundation from a variety of moisture-related problems. Another topic on this site, Yard Drainage, is concerned primarily with surface drainage problems and solutions. However, many of the moisture-related problems discussed under the topic of Yard Drainage will apply equally to the topic of the French Drain.

Hydrostatic Pressure

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.

Perched Groundwater

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!

Expansive Soils

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.

Perforated Pipe

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.

Sump Pumps

I usually try to avoid using a sump pump in conjunction with any drainage project. On reason is the added cost. Also, a sump pump system must rely on the electrical grid- which is most likely to fail during a heavy rainstorm, just when you need it the most. There are times when a sump pump cannot be avoided. Having a backup generator is recommended.

Storm water will easily overcome all but the very largest of sump pumps. If a sump pump is required for a French Drain system, it is particularly important that surface drainage be kept out of the system. This will allow the sump design to be kept at a smaller size and cost. The most reliable pump systems are those which are normally employed on sewer and septic tank systems where engineering is required. These systems, which are quite expensive, have alarms, back-up pumps and very large capacity.

Made of pre-cast or cast-in-place concrete, the minimum sump should about two feet square with two feet of depth below the inlet pipe invert. An overflow discharge pipe should be installed to prevent flooding if the pump fails. The pump should be of high quality and have a float-activated switch. Power wire in conduit should be plugged to a wall outlet high and dry. Keep enough extra wire for connecting to the generator.

Trench Safety

The minimum width of a French drain trench is a bit wider than the width of a shovel or about ten inches. Backhoe trenches dug with a twelve-inch bucket will end up about thirteen to fifteen inches wide. I have seen French drain trenches specified to be eighteen inches wide and for deeper trenches up to 24 inches. Deep trenches must be sloped back or shored. Now is a good time to talk about safety:

Trench safety must be paramount during construction. People can die in a heartbeat! Get an OSHA permit where-ever required and follow all rules and regulations. Never permit workers to go into un-shored trenches. Expect wet weather which will weaken the trench walls. Keep heavy trucks and equipment away from the sides of trenches. Keep trenches covered. To reduce exposure, backfill trenches as soon as possible. Fence off the job and post keep-out signs. Use common sense.

Gravel and Geotextile Filter

Backfill gravel for a French drain should be washed three-quarter-inch up to one-and-one-half-inch crushed rock. The cross-sectional area of rock will be the area of the trench less the area of the upper 12 inches which should be backfilled with native soil. If washed rock is not available, the pipe should be fitted with a filter “sock” to keep the fine material from getting into the pipe.

The entire volume of crushed rock must be wrapped with a “geo-textile” filter fabric which is overlapped across the top of the trench. Filter socks and fabrics are commonly available at irrigation and drainage outlets. Geotextile fabric is a durable synthetic blanket which can be rolled out and easily cut with a box cutter.

Errors to Avoid

The French drain is never to be used for collection of surface drainage. A separate system of surface area drains should be installed along the trend of the French drain to minimize percolation of surface water into the gravel. The surface drain system will then connect to a separate “tightline” pipe which may occupy the same trench as the perforated pipe. Make sure there are no leaks by gluing and/or taping at all pipe connections.

Roof drainage or surface drainage pipes should never be connected to the perforated pipe of a French drain. This will cause storm water to be charged into the sub-soils which you are trying to drain. At the lower end of the French drain, the perforated pipe will connect to a separate tightline which will then carry the collected water to “daylight”. It is best to keep the French drain tightline isolated as well. If a clog were to develop toward the end, any other connecting drain lines could cause the system to backup and saturate the subgrade.

Cleanouts

Cleanouts should always be installed for French drain as well as for surface drain systems. Install cleanouts at the high point of the system. Cleanouts should also be installed below any sharp changes in the elevation of the perforated pipe. The cleanouts should connect into the pipe with a “sweep” fitting so that a snake or water jet can be run down the pipe to clear any clogs. Mark each cleanout as to whether it is for a tightline or a perforated pipe. You don’t want a future contractor installing new surface area drains and connecting them to your French drain system.

Design and Drawings

A good French drain design will take into consideration the guidelines from our discussion so far. The system should be illustrated on a scaled drawing with plan view showing proposed route of the pipe together with cross section details. The plan drawing should also include elevations including the elevation at the discharge point and target elevations for the perforated pipe invert along the proposed route.

Most of the specification will normally be provided on a cross section which will indicate the depth and width of the trench as well as the size, type, orientation and slope of the perforated pipe. The cross section should also illustrate the relationship between the perforated pipe and any foundation or structural elements such as a footings, slabs, stem-walls, building walls, and retaining walls.

The cross section will also provide a specification and illustrate the placement of gravel, geotextile filter fabric, backfill, and any surface improvements such as area drains and hardscapes. Specifications for any waterproofing and protection board are also commonly provided on the cross section.

Subsurface Conditions

It should be understood that any plan for a French drain system is subject to modification as work proceeds and more is learned about subsurface conditions. The shape of foundation elements and the specific location of underground utilities can often interfere with the desired gradient of the perforated pipe. For example, what happens when the main sewer drain ends up crossing the scheduled path of the perforated pipe?

On sloping jobsites, step-down concrete footings present a unique challenge. The installer does not know where the step-downs occur until the trench is opened up. In dealing with a step-down footing, I will make the drop with a short tightline on a 45 degree angle or I will put in vertical tightline with a sweep at the bottom and a cleanout access at the top.

Ideally, all trench excavation would be completed prior to installation of the pipe and gravel. All gradients could then be checked and compensation could be made for any obstructions. This approach is usually possible for new construction or on waterproofing jobs where total access for sandblasting is needed and where a seamless waterproof membrane is required.

However, I have completed a number of French drain installations on foundation repair jobs where it is not feasible or desirable to open up the entire excavation at one time. This may be for safety reasons or because certain sections must be backfilled to provide needed space for access or stockpiling of “spoils” (excavated earth) and other materials. For these projects a contingency should be included in the estimate for relocation of obstructions.

Start at the Point of Discharge

When constructing a French drain system, I prefer to begin at the point of discharge. If a rainstorm hits, storm water filling up my trenches will then flow away so that I can get back to work as soon as possible. Also, it is easier to avoid excavation errors when starting at the bottom of the system. If it becomes necessary to raise the pipe level slightly for some reason, trench depths uphill can be adjusted on paper before any over-excavation has occurred.

The point of discharge for any drainage system should be part of an established natural or constructed storm drain such as the street gutter. If necessary, a structure to dissipate energy and absorb the water should be constructed. The property owner is responsible for damage to adjacent property which has been caused by actions which divert the natural drainage regime. Refer to the Yard Drainage page for a discussion of curb coring and drainage discharge.

Tools and Equipment on a French Drain Project

In the initial demolition phase, it may be necessary to use a concrete-cutting saw to make a clean joint between hardscape which is to remain and the alignment of the proposed French drain. The concrete saw will require water to cool the blade so that water control during the process will be required.

Once the saw-cuts are made, an air compressor and ninety-pound pneumatic jackhammer may be required to break and remove existing hardscapes. Where large sections of concrete must be removed, a skid-steer loader switching to a hydraulic breaker will be significantly faster than jackhammer and wheelbarrow. The skid-steer can also be used to load the broken concrete into the haul truck for prompt removal from the jobsite.

Where space will allow, French drain trenches can be dug with a backhoe-loader or in tighter spaces, a mini rubber-track excavator. If the mini-excavator is used, spoils and gravel can be moved about with a skid-steer loader. Even small excavating equipment will tear up existing improvements and repair costs may exceed the savings over hand excavation. Hand excavation is often the only alternative on French drain jobs around existing houses.

Spoils to be removed from the jobsite can be loaded into a low-boy dumpster or stockpiled for removal by a dump truck. If the volume of earth is large and being excavated quickly, it will be best to haul it off in dump trucks. For smaller volumes and where there is a lot of hand excavation, a dumpster will usually be the better approach. The decision will also depend upon logistics and costs.

All French drain jobs will require plenty of wheelbarrows and shovels-both pointed- and square-nosed. Picks, digging bars and several sizes of sledge hammer will also be needed. One tool which I have found indispensable on any hand-excavation job is the hand-held electric jackhammer/clay spade- both the thirty-pound and sixty-pound version will usually be required.

A variety of small tools and supplies will be required on most French drain jobs. These will include axes and saws for cutting roots; plumbing tools (and fittings) for repairing broken pipe; box-cutter knives for cutting filter fabric; plywood and two-by-fours for protecting existing walls and covering trenches; plastic sheeting, jute rolls and flexible pipe for storm protection; glue and tape for joining tightlines and perforated pipe; carpenters level and plumbers level for checking pipe grade; circular saw, hand saw, hammers, nails, screws, duct tape, plumbers tape, string, rope, wire, trash bags, buckets, trash cans, hazard tape, barricades, garden hoses, rakes, and brooms will also probably be needed before the job is complete.

Installation Guidelines

During trench excavation for a French drain, I use a builder’s level to set offset reference stakes- usually on ten foot centers. I shoot the top of each stake and then calculate the distance from the top of the stake to the desired grade at the bottom of the trench. It is best to get the trench grade just right so that installation of the pipe is simplified and the amount of water in the trench is kept to a minimum.

If using a machine excavator, a spotter should be close at hand to provide depth readings to the operator. A finish grading at the bottom of the trench should be done by hand. If any backfill is required prior to pipe placement, it should be done with native soil and not gravel. A final check of the trench bottom should be done with a builder’s level.

Filter fabric is placed in the trench ahead of the pipe. I use plenty of fabric to provide a double layer across the top of the trench once the gravel has been installed. Take care to keep native soil out while placing the perforated pipe on top of the fabric. I use the builder’s level again to shoot the top of the pipe, assuring that I have the required gradient. I double check to make sure the top line is straight up so the perforations are in their proper orientation. When satisfied with the pipe placement, I carefully backfill with clean gravel to secure the pipe into position.

Some engineered French drain designs call for the use of plastic sheeting as a moisture barrier. The sheeting is placed either on top or on one side of the gravel trench. If plastic sheeting is to be used, I recommend a 10 ml or double thicknesses of 4 or 6 ml plastic to improve durability. Care should be taken to avoid punctures during installation. I question the use of plastic sheeting under loose gravel. The plastic will degrade with exposure to the elements and foot traffic on the gravel will cause punctures.

During backfill, be sure to keep the top of the gravel well below adjacent grade. It is not necessary to compact the gravel. After the top of the gravel has been covered with fabric, cover with fine grained soil such as silt or topsoil. Avoid placing sand on top of a French drain unless the sand will be covered with some type of impervious material such as concrete or brickwork.

A French Drain Job to Remember

A recent California French drain job is worth mentioning here to illustrate how each job can present its own set of challenges which in turn can provide the backdrop for some lively brainstorming and work-place theater. This particular job had me recruiting a pair of brothers from Ohio- Mark and Paul- to assist me.

A banjo player recently back from an extended visit to a rural French village, Paul was a natural French drain man with carpentry skills and a love of plants. His main job, as he understood it, was to protect the landscaping and charm my customer which he did quite successfully. As a result, she treated us daily to a variety of snacks and refreshing beverages.

Brother Mark was a bass player, a watch collector, and a diesel mechanic with an unquenchable need to solve technical problems. The two Hispanic laborers who lent us a hand (both named Mario) called him Marko. Marko’s job, as he saw it, was to come up with amazing French drain pipe connections including one which he named his “manifold destiny”.

When we began our French drain project, the front yard was a swamp, inundated by a maze of large tree roots. We set out to drain the saturated ground while trying to determine to best way to route our pipes through the maze. The young man I hired to drill the curb holes had ambition but he was a tad short on experience.

When one of his holes was found to plunge at a down angle from the street gutter, I told him it was a bad hole and would plug with silt. He was loath to make it right so I had to serve him some choice words which raised his ire to a fever pitch. After a nights rest and thoughtful consideration, he relented and gave me a reasonably level hole. Marko was obviously embarrassed by the scene but the customer found it amusing and later presented me with a framed photo of the lad at the helm of his coring rig.

After lots of chopping and sawing of roots, our route revealed itself and I assigned Paul to remove and save all lawn and vegetation for replanting. He chopped the turf into squares and carefully dug up the plants and roots. These were all protected with plastic, placed in the shade and regularly watered until the trenches were backfilled and the turf could be replaced.

Instead of opening up the whole French drain job at once, we completed the work in sections. Since there was a local ordinance against putting dumpsters in the street, I had ours delivered and placed on the driveway which we protected with plywood. We build a ramp and used our “wheelies” to get the dirt up through the open doors. At the point where the doors had to be closed, we leveled out the dirt and threw all the roots on top.

After a couple of dumpster loads had been removed, we started installing the tightline pipes, working up from the curb at the discharge end. We dug potholes along the route of our French drain, which would go completely around the backyard and behind a retaining wall. We used my electronic level to make the measurements and then I calculated the minimum trench depth we would need to keep the flow at one inch per ten feet on the new French drain. I drove stakes along the route and marked the trench depth on each stake. Each of the “Mario brothers” had his own tape measure so I could turn them loose to do the digging.

With all of this information, we could now continue with confidence, installing and backfilling the downhill pipes. We trenched and connected the front downspouts and new area drains. As we continued excavating toward the rear, the “spoils” could be wheeled a short distance and put right back down on the new pipes behind. Paul retrieved the squares of turf which he had been tending and planted them on top of the fresh backfill.

Before we even started installing the perforated pipes for the French drain itself, the front yard was virtually complete and the dumpster, no longer needed, was removed for a final time. This opened up the driveway for placement of our gravel which was dumped out of the back of a ten-wheeler. We used heavy lumber to spread out the load and were pleased that there were no new cracks in the sidewalk or driveway as the truck pulled away.

Along with our pipe delivery, we got a big roll of geotextile fabric which Marko would cut and lay down in the trench. He then began installing the perforated pipe for our French drain, placing it on top of the fabric (holes down and reference line up). There was also a tightline pipe with a separate elevation schedule which we placed right down in the gravel but above the perforated pipe.

One of my jobs was to double check all pipe elevations with my nifty electronic level. Marko made double sure that the down drains and area drains would not leak into the French drain by gluing and taping all the connections. Once everything checked out, the Mario brothers brought the gravel up from the driveway and dumped it down on the pipe.

We pushed back on both sides of the house simultaneously. It was a good thing we started in front because near the left-rear corner of the house, our trench started to fill with water which was seeping down from above the back yard. The hill was saturated and the owners were pleased at that point because they could see how important their new French drain was. They gave us a thumbs-up as they watched the seep water come running out of the new discharge holes at the curb face.

In the backyard, we had to deal with an existing patio with a substandard drain system. Since the patio was to remain, we had to deal with the existing drain pipe- if we could find it. After a lot of testing, we figured out where the pipe should be and we dug down and found it. It was badly damaged and non-functional. Marco figured out a way to make the tricky connection to our tightline- somewhere between a rock and a hard place- and this is where he found his manifold destiny.

As the two sides started to converge behind the retaining wall, we had to stockpile dirt. Fortunately by this time, we had finished up along the right side of the house and, by protecting the stucco with plywood, we were able to pile the extra dirt there with no problem. Access behind the retaining wall was really tight and we had used up most of the gravel. I made the decision to put the perforated pipe in with only a shallow covering of gravel.

I bought a roll of plastic drainage sheet and ran it behind the wall and buried the bottom of the sheeting in the gravel just above the perforated pipe. (Plastic drain sheet is a “gravel substitute” consisting of filter fabric attached to plastic which has been molded to provide a complex of drain channels. A full discussion of these sheets is provided on the Basement Waterproofing page of this site.) Now we didn’t need any more gravel and we had a place to put the most of our remaining spoil. The final pile of spoils consisted of topsoil which we spread around the landscaping plants so that there was no dirt to haul off the job when we were done with the project.

A final touch to the job came as a small extra. While digging along the left side, we had removed a wood fence in order to do our trenching. Unfortunately, the wood was rotten and the thing wasn’t worth saving. Paul came to the rescue and made a fine replacement fence which only went to enhance his stature with the owner who, as I think back, was more like a hostess. This job is an example of work as play. But without bragging too much I must admit that it could have turned into a nightmare had we not all worked so hard to give the customer the very best job we had in us.

French Drains for Basement Waterproofing and Retaining Walls

As has been mentioned, hydrostatic pressure is a very important consideration when dealing with a basement waterproofing job. This is why I always insist on putting in French drain system on these projects. In order to keep the moisture level below the vulnerable joint between the base of the wall and the footing, the perforated pipe must run entirely below the level of the basement floor.

A French drain is also recommended to relieve hydrostatic pressure behind a free-standing retaining wall. Retaining walls are often neglected in this regard and significant damage to the wall materials, as well as structural failure of the wall, can be the result.

The installation of a French drain to protect an existing retaining wall or basement wall is somewhat different from the “stand-alone” type French drain I have been discussing. For these below-grade wall applications, I have seen specifications that gravel be placed only around the perforated pipe at the base of the wall with drain sheets placed in direct contact with the wall. With filter fabric attached to the outside surface, these plastic sheets are designed to work with gravity, channeling water down to the bottom of the wall where it is collected and removed.

Because of the excavation depth which is often required to expose basement walls and retaining walls which required new waterproofing and French drain systems, these projects often require significantly more excavation than other French drain systems which I have been discussing on this page. Important questions arise about excavation methods, stockpiling or hauling of spoils, the use of shoring, and decisions about backfill, compaction and soil testing.

For a discussion of these issues and others which are closely connected with a retaining wall or a basement waterproofing project, I make reference to the Basement Waterproofing topic on this website.

Final Thoughts

A French drain system can be of great benefit in solving a variety of foundation repair problems. However, many such systems do not succeed because of inexperience and inadequate study at the jobsite. A basic understanding of soil and groundwater behavior is important. Knowing and applying a few installation guidelines as presented here can make the difference between success and failure.