Below Ground Waterproofing

The foundation wall of a building may be a cast-in-place concrete retaining or basement wall or a structural wall complete with load-bearing pilasters. Materials used may be concrete or reinforced masonry. The foundation wall system may include an earth retention system of soldier piles and wood lagging or shotcreted rock requiring consideration of waterproofing applied to the earth retention system.

For most portions of the foundation wall, water removal and control is of prime importance. However, water removal measures around foundation walls below the water table may be impractical and expensive over the long-term and the waterproofing strategy becomes critical. In the upper areas of the foundation wall thermal loading considerations must be addressed.

Below ground waterproofing systems include drainage materials, including aggregate drainage layer and prefabricated synthetic drainage layers, or drain boards. They also include waterproofing membranes, typically post-applied to the exterior side of the foundation wall. PVC drainage pipes, typically 4″ or 6″ in diameter, carry water away that accumulates in the drainage materials away from the home or building.

Drainage Materials

Drainage Materials for below grade enclosures include aggregate drainage layer and prefabricated synthetic drainage layers, or drain boards.

Aggregate drainage layers include graded pea-gravel aggregate or coarse sands. Graded pea gravel is naturally rounded stone between 3/16 inch and 3/8 inch in diameter. Coarse sands varying from No. 30 to No. 8 sieve are suitable. Gap-grading the sand provides uniform grain size, which accelerates drainage flow rates.  Hayward sells bags of pea-gravel as a convenience item.

Drain boards consist of a combination of plastic composite drainage cores with adhered geotextile fabrics. The plastic composite “dimpled” drainage cores are available in various configurations and are typically constructed using polypropylene, polystyrene and polyethylene. The geotextile fabrics retain sand, soil, concrete, or grout allowing water to migrate into the free draining core. The fabrics are available in various forms including non-woven for clay type soils and woven or small opening geotextiles for sandy or high-silt type soils. Many drainage mats also include a polyethylene sheet backer to uniformly disperse the loads imposed on the membrane and reduce the potential for damage caused by non-uniform profiles (dimples) in the composite core. Hayward carries a selection of drain boards and drainage mats.

Hayward recommends that a full system approach be used in applications where water leakage is not tolerable; a full system approach should include both a synthetic drainage layer and granular drainage layer.  Design considerations include selecting an appropriate design to achieve the required flow rate. These systems are advantageous in their lightweight design and cost effectiveness.

Geotextile filter fabrics offered by Hayward are also used for separating differing soil types in below grade enclosure applications. This separation of differing soil types maintains flow rates of soils used as drainage layers and minimizes settlement from finer materials filling in more coarse materials. Geotextile fabrics are typically constructed using polypropylene, polyester, or nylon and are available in either woven or non-woven designs. Woven products are constructed using individual threads or filaments and have good strength and stiffness; however, the material can be penetrated by angular aggregate reducing the ability to properly filter or separate fine elements. Non-woven products are typically continuously extruded and spun and then needle-punched to create uniform openings that can be selected depending on the design. In general, when properly designed, non-woven products have good filtration and separation properties.

Waterproofing Membranes

Positive-side waterproofing membrane systems are post-applied to the surface of the element that is directly exposed to moisture, typically the exterior side of the foundation wall. Negative-side waterproofing systems are post-applied to the surface of the element opposite the surface exposed to moisture, typically the interior of the foundation wall. Blind-side waterproofing systems are pre-applied to the area where the concrete element will be placed that is directly exposed to moisture.

Waterproofing membranes can be categorized as either sheet membrane systems, fluid applied systems, bentonite clay systems, or cementitious systems.  Hayward stocks a sheet membrane system.

A sheet membrane system used in foundation wall applications can be made of thermoplastics, vulcanized rubbers, or rubberized asphalts. Hayward recommends having a continuously bonded and adhered waterproofing layer to reduce the potential for lateral moisture migration beneath the membrane.  If heat-welded seaming is employed and loose-hung membranes are tough and protected from damage by protection board, they may be effective waterproofing materials, but if a leak occurs, the leak will be difficult to locate and correct due to the loose application of the waterproofing layer in those cases.

Fluid-Applied Systems systems include urethanes, rubbers, plastics and modified asphalts. Fluid-applied membranes are applied in liquid form and cure to form one monolithic seamless membrane. For foundation wall applications, these fluid applied systems can be cold applied or hot applied rubberizes asphalt. Some systems include reinforcing mesh embedded into the liquid.

Bentonite Clays systems include composite sodium bentonite systems with HDPE liners and geotextile fabrics, which are more common and more effective than the traditional systems. Bentonite clays act as waterproofing by swelling when exposed to moisture thus becoming impervious to water. Bentonite is, therefore, most effective when properly confined so the product can swell to fill voids and so that it cannot be washed away.  Clay panels and geotextile sheets are popular for use in blind-side waterproofing applications such as on retaining earth systems and elevator and sump pits.

Cementitious Systems systems contain Portland cement and sand combined with an active waterproofing agent. These systems include metallic (metal oxide), crystalline, chemical additive, and acrylic modified systems. These systems should only be considered for use as a secondary (back-up) waterproofing to a positive-side waterproofing system, unless they are used with special details provided by a waterproofing expert.

Other Materials

Protection Boards shield waterproofing membranes from construction damage, damage from backfill materials in service, and ultraviolet radiation. The most commonly used protection board is a semi-flexible sheet containing an asphalt core placed between asphalt impregnated glass fiber mats. The material may have polyethylene film on one side and a glass mat surface on the other side.

Drainage pipes, typically 4″ or 6″ in diameter, used in below grade systems are primarily made of corrugated PVC or polyethylene and in some cases of porous concrete. PVC and polyethylene pipes are available in smooth or corrugated configurations and are slotted on the bottom half of their cross-section to allow water infiltration. Corrugated PVC drain tile piping can collapse under the weight of backfill, and the preference is to use stiffer PVC pipe if possible.

All drain tile piping should be laid onto large, river-washed aggregate stones, which is laid onto a filter fabric, which should be wrapped around and over the drain tile to try to prevent fine soils from filling the drain tile. Regarding slope to drain, drain tile is intended to be installed with some slope to ensure that the water moves toward the sump collector. The outlet must be the lowest point in the system at each juncture.

Ask an Expert Get a Quote