PPP Notes - Basement Waterproofing

BASEMENT CONSTRUCTION

 (These notes are compiled from AGS)

BASEMENT – WATERPROOFING / DAMPPROOFING / INSULATION

 

• Consult a geotechnical engineer to determine soil types and groundwater levels, as well as their effect on drainage and waterproofing methods.
• Consult a waterproofing specialist to determine a specific design approach for problem soils and conditions.
• Sites may have groundwater contamination that will degrade the durability of the waterproofing materials.
• Generally, waterproofing will be necessary if a head of water is expected against the basement wall or under the slab.
• Because groundwater levels can vary with seasons, it is important to understand these seasonal fluctuation and design for the maximum expected head.
• Foundation drainage is recommended when the groundwater level may rise above the top of the floor slab or when the foundation is subject to hydrostatic pressure after heavy rain.
• Geosynthetic drainage material conveys water to the drainage piping, thus reducing hydrostatic pressure.
• It is important to understand the hydrostatic pressures exerted on the floor slab and wall systems if the drainage system is not adequate to remove all the water.
• Special negative-side coatings on interior face of foundation wall, such as metallic oxide, are recommended only when the exterior is not accessible (such as pits and trenches, and in particular, elevator pits).

 

BASEMENT WALL VERTICAL WATERPROOFING

 

• The grading around the building is an important part of the overall water management plan.
• The backfilling operation usually results in a more porous material than the adjacent undisturbed soil, which makes it easier for water to collect next to the building.
• The finished grade should slope away from the building, and an impervious layer of soil placed on top of the backfill against the building.
• Drainage from downspouts should be diverted away from the foundations.
• Types of waterproofing include built-up bituminous, sheet, fluid-applied, cementitious and reactive, and bentonite. 
• Built-up Bituminous: Composed of alternating layers of bituminous sheets and viscous bituminous coatings.  Bituminous waterproofing includes built-up asphalt and cold-tar waterproofing systems.
• Sheet Waterproofing: Formed with sheets of elastomeric, bituminous, modified bituminous coatings or thermoplastic materials.  Sheet waterproofing may be either mechanically attached or self-adhered.  Sheet waterproofing provides an impermeable surface to water penetration.
• Fluid-applied: Applied in a hot or cold viscous state.  Includes hot fluid-applied rubberized asphalt.  As with sheet waterproofing, fluid-applied waterproofing will bridge minor cracks in a concrete surface.
• Cementitious and Reactive: Types of waterproofing that achieve waterproof qualities through chemical reaction and include polymer modified cement, crystalline, and metal-oxide waterproofing systems.  Metal oxide is recommended for use when the exterior surface is not accessible, as in the case of an elevator pit.
• Bentonite: Formed from clay into panels and composite sheets.  When moistened, the clay swells and takes on a gel-like consistency, forming an impermeable retarder when confined.  Bentonite clay works well only when moistened.  For applications where the water table fluctuates, there may be a time lag between the rising water table and when the bentonite takes effect, during which time there is the possibility of water infiltration.  Therefore, when the water table varies, caution is in order when relying on bentonite clay for waterproofing.  Proper coordination between the wall construction details and waterproofing termination is required.
• At the interface of the foundation wall and slab, waterstops are placed on top of the footing, at vertical concrete keyed wall joints.
• Most waterproofing materials require a stable, rigid, and level substrate.
• Generally, a mud slab (sub-slab that is non-reinforced and non-structural) is used when the waterproofing material is placed below the structural slab and/or when a solid working surface is needed on unstable soils.
• When waterproofing materials are placed on top of the structural slab, a protective cover, such as another concrete slab, is required.