Like all good college students, I was an expert in procrastination. I am reminded of the time in my astronomy class when, the night before an assignment due date, the time came to read the homework assigned the previous month. It was when I found myself in a bit of a predicament.

The assignment was to fashion a sundial-like device and precisely record the position of the sun’s shadow at a consistent time each day for 10 consecutive days. No amount of midnight oil would get me out of this.

Designing a subsoil drainage system is another endeavor that cannot effectively be accomplished with last-minute cramming. It’s a collaborative, multidisciplinary process that unfolds through communication, with each design entity contributing a piece of the puzzle over time. The design entities involved include the geotechnical engineer, architect, civil engineer, structural engineer and plumbing engineer.

Early in my career, I remember thinking, “I have never heard of a subsoil drainage system that did not work.” Then, I heard a story about water infiltration at the slab in a commercial building. An investigation found the root of the problem was that the subsoil drainage system was never installed. I have since encountered other types of performance issues that are more nuanced than this example. 

One thing that makes subsoil system design tricky is that it seems people don’t want to think too much about it, much less take responsibility for it working correctly. You should not have to think much about it when properly designed and installed. However, when it does not function properly, the remedy can be costly, and you may think a lot about it.

Every project is unique, and it is up to the project team to determine the solution that best fits the project circumstances. I hope to give you key practical elements to consider to ensure your subsoil system performs as intended. First, let’s understand its purpose.

Purpose of the Subsoil Drainage System

Initially, I thought the purpose of the subsoil drainage system was simply to prevent water from infiltrating the building. While this is certainly one of the objectives, it was not obvious that subsurface water can create tremendous hydrostatic forces on the structure. The structural engineer may not find it cost-effective to design the structure to withstand these forces. The subsoil drainage system is integral to maintaining the structure’s integrity.

Subsoil drainage accomplishes these objectives by redistributing the water table. Redistribution includes conveying water from beneath the building but can also include redistributing the water under the building. With expansive soils, more uniform water distribution can reduce stresses on the slab. Lastly, the subsoil drainage pipe is a conduit to convey water with less soil erosion than natural water currents. 

The system is multifaceted and can provide the following:

Prevent water infiltration of the building structure.

Reduce hydrostatic forces on the structure.

Redistribute the water table by more evenly distributing water under the slab and conveying water out from under the slab.

Reduce soil erosion.

Where should subsoil drainage systems be considered? Subsoil drainage systems should be considered whenever portions of the building are belowgrade, such as at retaining walls and belowgrade slabs. This is consistent with the more precisely written verbiage in Section 1805.1 of the International Building Code that states: “Walls or portions thereof that retain earth and enclose interior spaces and floors [belowgrade] shall be waterproofed and dampproofed in accordance with this section...”

An area to consider in buildings that may not otherwise include portions belowgrade is the elevator shaft. Some may argue that elevator shafts are equipped with sump pumps and waterproofing that effectively address water infiltration. However, this does not make water infiltration desirable or address the hydrostatic forces on the structure. 

Collaborate with the geotechnical engineer to understand the water table level, the structural engineer to understand the structural design criteria, and the architect to understand waterproofing measures.

What is the expected level of the water table? The level where water is found at the various bores is documented in the geotechnical report. Remember that this measurement is a single snapshot in time; during heavy rainfall, the level may vary drastically. Open discussions within the project team, involve the owner as necessary, and help evaluate the cost-effectiveness of installing a subsoil drainage system against the potential risks associated with rising water tables.

What about the location, pipe sizing and spacing? The geotechnical report typically specifies the locations where subsoil drainage is recommended. The locations may fall into several categories: perimeter, exterior to the grade beams; perimeter, interior of the grade beams; and interior laterals, creating a matrix at a set interval. 

The spacing for interior laterals is dependent on the percolation rate of the soil and should also be specified in the geotechnical report. Other report recommendations, such as an aggregate drainage course to direct water to the laterals, should be incorporated. The spacing may vary from 10 feet to 40 feet on centers. 

Often, the geotechnical report specifies the pipe size. In my experience, it is typically either 4 inches or 6 inches. Note that the International Plumbing Code requires a 4-inch minimum pipe size.

How should the pipe perforations be oriented, and should the pipe be sloped? One design for subsoil pipe is perforated PVC pipe with two rows of 1/2-inch diameter holes every 5 inches. These rows are located 120 degrees apart.

Several years ago, to better understand subsoil drainage systems, my company created a mockup to observe the effect of perforation orientation and sloping of the pipe. 

The mockup entailed a perforated pipe penetrating through three buckets. Each bucket represented a portion of the underslab where the water table varied. In each experiment, the middle bucket was filled, creating flow potential to other buckets as the water rose to the level of the perforations in the pipe. The effect of the water level in the adjacent buckets was observed. 

The results of these experiments are summarized below:

Holes on the top of the pipe versus holes on the bottom: With the holes on top, the water tended to enter the pipe and not redistribute to the other buckets. This would be analogous to water flowing directly out from under the building to the sump or point of discharge. Conversely, with the holes on the bottom, the water entering the middle bucket would equalize the water level across buckets before flowing to the discharge location. 

As previously stated, a goal of the system is to redistribute to create uniform water distribution. For this purpose, the holes-on-bottom approach was advantageous. This is consistent with most subsoil designs I have encountered.

Slope versus flat pipe: When the pipe was installed flat, the water redistributed more uniformly in both directions. When the pipe was sloped, the water would redistribute only downstream from where the water entered the pipe. 

Often, subsoil drainage is specified to be sloped. However, there may be times when installing the perforated pipe flat can have benefits for water redistribution and may decrease excavation depths, which in some instances could eliminate the need for a costly sump pump. 

What about keeping the pipe clean? The detail of the pipe typically will include gravel to increase the water entry perimeter and a geotextile filter fabric around the gravel. The goal is to keep the pipe from clogging over time. Cleanouts at intervals typically specified for drainage pipe should be considered for inspection and cleaning. These cleanouts are not always shown or specified in the design. 

Elevation is Everything

The perforated pipe must be at an elevation low enough to intercept the water table before it reaches the structure it is designed to protect. This sounds simple, but in practice, designers may be challenged to raise the elevation of the pipe to hit specific inverted elevations. 

Furthermore, the elevation that the system drains to must be understood. If the discharge location is an on-site retention pond, special care must be taken to understand how the flood rim of the pond relates to the system elevation to prevent a reverse flow situation. 

Portions of subsoil design criteria — such as variations in the water table level and the design flow rate for water removal — are uncertain. Further complicating matters is the relevant design criteria are generated by multiple disciplines. I have observed this uncertainty leading some team members to spend more effort trying to shed responsibility rather than contribute their useful knowledge toward a properly performing system. 

Like my college astronomy class experience, attempting to address this task with last-minute effort can lead to unwanted consequences. I encourage design teams to proactively consider subsoil drainage systems early in the design process.

Justin Bowker, PE, has been part of the engineering team at TDIndustries since 2001. He became the manager of this team in 2009 and vice president of engineering in 2016. Under his leadership, the team challenges itself to harness technical approaches to provide focused value to the owner on design/assist and design/build projects.