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Drainage and Backfill for Below-Grade Waterproofing

Waterproofing is the formation of an impervious barrier that is designed to prevent water from entering or exiting from various sections of the building structure.

The waterproofing process integrates several building components and different trades, and designers must focus on drainage and backfill operations as well as the waterproofing membrane. Photo courtesy of Carlisle Coatings & Waterproofing Inc.

Waterproofing is the formation of an impervious barrier that is designed to prevent water from entering or exiting from various sections of the building structure. The waterproofing system is a series of integral components that function in unison to prevent moisture intrusion into the facility. The system configuration is generally similar in all waterproofing applications. The material components that are common to all types of waterproofing applications are structural substrate, flashing, membrane and insulation.

The typical system configuration for below-grade vertical applications is:

Structural substrate

Flashing

Membrane

Insulation

Drainage course

Backfill

Many designers focus on the waterproofing membrane and limit design of two critical components: the drainage course and backfill operations. These components are key elements in successful waterproofing system design and application. In fact, a significant percentage of waterproofing failures are attributed to improper design and application of these components.

Preparation requirements vary by the type of material and application methods used. It is important that the manufacturer’s requirements for substrate preparation are followed. Photo courtesy of Carlisle Coatings & Waterproofing Inc.

Drainage Design: Vertical Applications

Below-grade waterproofing is subjected to water transmission from two sources - surface water and groundwater. Proper control and drainage of these water sources is required for successful waterproofing system performance. Sources of surface water include rain, melting snow and sprinklers. The most effective form of controlling surface water is by directing it away from the structure. This can be achieved through sloping of the landscape and by installing roof gutters and downspouts that divert water away from the structure. Sprinklers should also be adjusted so they point away from the structure. Moisture infiltration in above-grade components (masonry, siding, etc.) could work its way down and create leaks in below-grade areas.

The control of groundwater is a little more complicated due to the fact that groundwater levels fluctuate throughout the year. In the Northern Hemisphere, groundwater levels are typically at their highest levels in the spring after winter thaws and at their lowest levels in dry summer conditions. Groundwater levels typically rise from heavy rain accumulations and from natural capillary action of the soil. The waterproofing membrane must be designed and applied to accommodate groundwater when it is at the highest level, even if this is a temporary or infrequent condition.

Proper below-grade waterproofing design must include a system for collecting, draining, and discharging groundwater away from the structure. The most effective way to properly collect and discharge groundwater is through the use of foundation drains. Foundation drains can be field-constructed drainage systems or prefabricated soil drainage systems.

Field-constructed drainage systems consist of a perforated pipe (typically PVC) that is set in a bed of gravel at the bottom of the foundation. The perforation in the pipe is applied downward to allow the water to flow into the gravel bed. A drainpipe is installed next to the structure slightly above the bottom of the foundation to prevent the soil under the foundation from washing away. The pipe is set to slope the water towards drain fields, bare soil or sump pits. A layer of coarse gravel is set around the drainage pipe for additional water accumulation. In some cases, meshes and/or mats can be applied over the top gravel layer to prevent soil build-up from interfering with water flow to the drainage system. The biggest disadvantage with these systems is that they rely on proper field construction, and they are not always completed properly. Over time, they may become clogged with dirt, soil and contaminants.

Due to unreliability of the older field-constructed methods, manufacturers have developed prefabricated drainage systems that are inexpensive and effective in controlling groundwater on all types of construction projects. Prefabricated drainage systems are made of a variety of plastic composite formulations (polypropylene, polystyrene, and polyethylene) that combine specially designed drainage cores with attached geotextile fabrics. Installation requirements include trenching, setting the pipe to the desired slope and backfilling. The materials come in widths up to 36 inches and lengths up to 500 feet. These products are puncture resistant and are typically not damaged in backfilling operations. Most of the systems have elongation capabilities and can accommodate movement after installation.

Ideally the backfill process should be completed immediately after waterproofing is applied, as the backfill helps hold the membrane in place and protects it from UV rays. Photo courtesy of Carlisle Coatings & Waterproofing Inc.

Backfill Design

The backfilling process is a critical component to the success of a waterproofing system and it requires the appropriate attention in the specifications. Damage created by improper backfilling is the most common cause of premature waterproofing failures. Damage typically results from the use of improper fill materials, such as rocks, frozen soil and miscellaneous debris. It also occurs from punctures created by backfill equipment including loaders, bulldozers and shovels. In some cases, backfilling operations have been completed prior to the waterproofing application, leaving the area totally exposed to moisture infiltration. This is often the result of a carless subcontractor.

Proper backfill requirements listed in the specifications can eliminate these errors by providing an enforceable document. If it is determined that incorrect application materials and methods were applied, then corrective remedies would be required. The most important backfill requirement is that the process is completed immediately after waterproofing is applied. This is critical for two reasons: (1) in vertical applications, the backfill holds the membrane in place; and (2) it provides ultraviolet protection for the waterproofing materials, especially materials that are not ultraviolet resistant.

An important requirement of the backfilling operation is that the backfill material is compacted in layers. Typically, this is best completed with the proper mechanical equipment designed for this purpose. The type of geotextile material that is used is based on the soil conditions. The geotextile materials required for soil conditions are as follows:

High clay content: nonwoven, needle-punched geotextile.

Sandy soils: woven materials with high permeability.

High silt content: small-opening geotextiles.

Some of the other backfill requirements that should be provided in the specifications include the following items:

Require that the compaction of the backfill is in conducted in accordance with ASTM D 1557, “Test Method for Laboratory Compaction Characteristics of Soil Using Modified Effort.”