Where building substructures enclose basements, parking garages, or other usable space, groundwater must be kept out. Concrete alone is rarely adequate for this purpose. Moisture can migrate through its microscopic pores, or through other pathways created by shrinkage cracks, form tie holes, utility penetrations, and the joints between concrete pours. To ensure a substructure’s resistance to water entry, two approaches are used: drainage and waterproofing.
Drainage draws groundwater away from a foundation, reducing the volume and pressure of water acting on the foundation’s walls and slabs. Waterproofing acts as a barrier, stopping water that reaches the foundation from passing though to the interior. Drainage, consisting of some combination of drainage backfill (well-sorted crushed stone or gravel), drainage mat, and perforated drain piping, is used with almost every building substructure (Figure 1).
Drainage mat is a manufactured component that may be made of a loose mat of stiff, inert fibers, a plastic egg-crate structure, or some other very open, porous material. It is faced on the outside with a filter fabric that prevents fine soil particles from entering and clogging the drainage passages in the mat.
Any subterranean water that approaches the wall descends through the porous material of the mat to the drain pipe at the footing. Perforated drain piping is frequently laid around the outside perimeter of a building foundation. The pipes are 4 or 6 inches (100 or 150 mm) in diameter and provide an open channel in the crushed stone bed through which water can flow by gravity either “to daylight” at a lower elevation on a sloping site, to a municipal storm sewer system, or to a sump pit that can be automatically pumped dry whenever it fills.
The pipes are laid at least 6 inches (150 mm) below the top of the basement floor slab to maintain the groundwater level safely below that of the slab. Perforations in the pipes face downward so that water is drained from the lowest possible level. Where groundwater conditions are severe, rows of perforated pipe may be installed under the basement slab as well (Figure 2).
On most foundations, some form of water-repelling barrier is also used to protect against the passage of groundwater.
Dampproofing is a moisture-resistant cement plaster or asphalt compound commonly applied to residential basement walls and to other substructures where groundwater conditions are mild or waterproofing requirements are not critical. Cement plaster dampproofing, or parge coating, is light gray in color and troweled on. Asphalt or bituminous dampproofing is dark in color and is applied in liquid form by spray, roller, or trowel.
Dampproofing is less expensive and less resistant to water passage than true waterproofi ng. Waterproofing, unlike dampproofing, can prevent the passage of water even under conditions of hydrostatic pressure. It is used where groundwater conditions are severe or the need to protect subgrade space from moisture is critical.
Waterproof membranes are most commonly formulated from plastics, asphalt compounds, or synthetic rubbers and come in a great variety of forms. Liquid waterproofing is applied by spray gun, roller, or squeegee and then allowed to cure in place. It is easy to install and easy to form around complex shapes. When fully cured, the finished membrane is seamless and fully bonded to the underlying substrate.
However, because liquid membranes are formed in the field, they are subject to uneven application, and the surfaces to which they are applied must be clean, smooth, and dry to ensure reliable adhesion of the membrane. Preformed sheet membrane waterproofing may be adhered or mechanically fastened to substructure walls or laid loosely over horizontal surfaces (Figure 3).
Fabricated under controlled factory conditions, sheet membranes are reliably uniform in material quality and thickness. However, they can be more difficult to form around complex shapes, and the seams between sheets, which are sealed in the field, may be subject to lapses in quality.
Sheet membranes that are loosely laid or mechanically fastened can be used over substrates that will not bond with liquid-applied or adhered sheet membranes. They are also a good choice where substrate cracking or movement may be expected, because such movement is less likely to stress or damage the membrane. An advantage of adhered membranes (both sheet and liquid) is that in the case of a defect, water cannot travel far under the membrane, limiting the extent of water damage that may occur and simplifying the tracing of leaks.
Bentonite waterproofing is made from sodium bentonite, a naturally occurring, highly expansive clay. It is most often applied as preformed sheets consisting of dry clay sandwiched within corrugated cardboard, geotextile fabric, or plastic sheets.
When bentonite comes in contact with moisture, it swells to several times its dry volume and forms an impervious barrier to the further passage of water.
Bentonite sheets can be placed directly on the soil under a concrete slab on grade or mechanically attached to uncured, damp concrete walls. In slurry form, bentonite can be sprayed even onto highly irregular, rough stone walls. The swelling behavior of bentonite clay also allows it to adjust to cracking and movement in the substrate.
Integral waterproofing includes cementitious plaster or crystalline admixtures for concrete or mortar that react chemically to stop up the pores of these materials and render them watertight. It may be applied to the surface of existing concrete or masonry or used as an admixture in new concrete. Unlike most other waterproofing materials, many integral waterproofing materials can be applied as negative side waterproofing, that is, applied to the inner side of a concrete wall acting to resist water passage from the opposite side. Blind-side waterproofing is installed prior to the pouring of concrete walls.
This occurs most commonly when a substructure wall is built close to a property’s edge, and excavation cannot be enlarged beyond the property line to permit workers access to the outer face of the wall after its construction. Drainage matting is first applied directly to the excavation sheeting, and then any of a number of possible waterproofing membranes are applied over the drainage mat.
Later, the concrete wall is poured against the membrane. The sheeting remains permanently in place (Figure 4). Joints in construction require special attention to ensure watertightness. Preformed waterstops made of plastic, synthetic rubber, or metal can be cast into the mating concrete edges of both moving and nonmoving joints to block the passage of water (Figures 5).
Waterstops for nonmoving joints such as between concrete pours of a wall or slab can also be made of strips of bentonite or mastic that are temporarily adhered to the edge of one pour. After the adjacent pour is complete, these stops remain embedded in the joint, where they form a watertight barrier.
Most waterproofing systems are inaccessible once building construction is complete; they are expected to perform for the life of the building, and even small defects in installation can allow the passage of large volumes of water.
For these reasons, waterproofing membranes are inspected carefully during construction and horizontal membranes are often flood tested (submerged for an extended period time while leak-checking is performed) to detect the presence of defects while repairs can still be easily made.
Once inspection and testing are complete, membranes are covered with a protection board, insulation board, or drainage matting to shield the membrane from prolonged exposure to sunlight and to prevent physical damage during soil backfilling or subsequent construction operations.
Basemet Insulation: Comfort requirements, heating fuel efficiency, and building codes often require that basement walls be thermally insulated to limit the loss of heat from basements to the soil outside. Thermal insulation may be applied either inside or outside the basement wall. Inside the wall, mineral batt or plastic foam insulation may be installed between wood or steel furring strips.
Alternatively, polystyrene foam or glass fiber insulation boards, typically 2 to 4 inches (50 to 100 mm) thick may be placed on the outside of the wall, held by either adhesive, fasteners, or the pressure of the soil. Proprietary products are available that combine insulation board and drainage mat in a single assembly.
Shallow Frost-Protected Foundations: In shallow frost-protected foundations, extruded polystyrene foam insulation boards can be used in cold climates to construct footings that lie above the normal frost line in the soil, resulting in lower excavation costs.
Continuous layers of insulation board are placed around the perimeter of the building in such a way that heat fl owing into the soil in winter from the interior of the building maintains the soil beneath the footings at a temperature above freezing (Figure 6). Even beneath unheated buildings, properly installed thermal insulation can trap enough geothermal heat around shallow foundations to prevent freezing.
Backfilling: After the basement walls have been waterproofed or dampproofed, insulating boards or protection boards have been applied, drainage features have been installed, and internal constructions that support the basement walls, such as interior walls and floors, have been completed, the area around a substructure is backfilled to restore the level of the ground. (A substructure built tightly against sheeted walls of an excavation needs little or no backfilling.)
The backfilling operation involves placing soil back against the outside of the basement walls and compacting it there in layers, taking care not to damage drainage or waterproofi ng components or to exert excessive soil pressure against the walls.
An open, fast-draining soil such as gravel or sand is preferred for backfilling because it allows the perimeter drainage system around the basement to do its work. Compaction must be suffi cient to minimize subsequent settling of the backfilled area.
In some situations, especially in backfilling utility trenches under roadways and floor slabs, settling can be virtually eliminated by backfilling with controlled low-strength material (CLSM), which is made from portland cement and/or fly ash (a byproduct of coalburning power plants), sand, and water. CLSM, sometimes called “flowable fill,” is brought in concrete mixer trucks and poured into the excavation, where it compacts and levels itself, then hardens into soil-like material.
The strength of CLSM is matched to the situation: For a utility trench, CLSM is formulated so that it is weak enough to be excavated easily by ordinary digging equipment when the pipe needs servicing, yet as strong as a good-quality compacted backfill.
CLSM has many other uses in and around foundations. It is often used to pour mud slabs, which are weak concrete slabs used to create a level, dry base in an irregular, often wet excavation.
The mud slab serves as a working surface for the reinforcing and pouring of a foundation mat or basement floor slab and is often the surface to which a waterproofing membrane is applied. CLSM is also used to replace pockets of unstable soil that may be encountered beneath a substructure or to create a stable volume of backfill around a basement wall.
Up–Down Construction: Normally, the substructure of a building is completed before work begins on its superstructure. If the building has several levels of basements, however, substructure work can take many months or even years.
In such a case, up–down construction is sometimes an economical option, even if its first cost is somewhat more than that of the normal procedure, because it can save considerable construction time. As diagrammed in Figure 7, up–down construction begins with installation of a perimeter slurry wall.
Internal steel columns for the substructure are lowered into drilled, slurry-filled holes, and concrete footings are tremied beneath them. After the ground floor slab is in place and connected to the substructure columns, erection of the superstructure may begin.
Construction continues simultaneously on the substructure, largely by means of mining machinery: A story of soil is excavated from beneath the ground floor slab and a level mud slab of CLSM is poured. Working on the mud slab, workers reinforce and pour a concrete structural slab for the floor of the topmost basement level and connect this floor to the columns. When the slab is sufficiently strong, another story of soil is removed from beneath it, along with the mud slab. The process is repeated until the substructure is complete, by which time the superstructure has been built many stories into the air.