Soils and Foundations: Construction Drainage

Reactive Soils: Construction - Drainage

(Specialized designs and construction methods to
prevent reactive soil from causing structural damage)

When a construction containing a reactive soil has been identified, and following proper soil preparation, there are several effective methods to assist in eliminating the threat of structural damage during the construction, phase by means of assuring proper drainage.
Surface Drainage - This is an important factor for houses built on reactive soils. If the surface water is not allowed to infiltrate the soil, then the reactive potential of the soil will never translate into actual swelling, collapsing, etc. The following methods can be used to maximize surface drainage

Roof Drainage: The roof drainage system is designed at keeping rainwater and melting snow from falling close to the house. A roof drainage system should be composed of gutters, downspouts, downspout extensions, and splashbacks. All runoff should be deposited at least 5 feet away from the building. The figure below shows a typical roof drainage system.

A typical roof drainage system

Slope Drainage: The main purpose of slope grading is to divert drainage away from the house to prevent sub-surface infiltration. The degree of slope depends on the type of surface and landscaping. Paved areas should have a slope of about 1 to 2 vertical inches per every 10 horizontal feet. Landscaped areas next to a house should have a 1 foot vertical fall for every 10 horizontal feet. If a building is constructed within 20 feet of another structure, the slope should be increased to 2 to 3 vertical feet per every 10 feet of horizontal distance. The runoff should then be carried away by shallow trenches. Care must also be taken that runoff water is not directed toward neighboring structures.

Typical layout of a slope drainage system

Sub-Surface Drainage: This type of drain system is used to remove excess water that moves freely beneath the soil. Although they will not completely eliminate the risk of damage caused by reactive soils, sub-surface drains can substantially reduce the severity and frequency of damage. Several varieties of sub-surface drain systems are described below:

Perimeter Drain: A type of drain that extends around the base of the foundation. This system consists of a trench that contains a drain pipe; gravel, a draining filter, and back-fill soil. The drains should be several inches below the level of the foundation wall, and should be sloped at a rate of 1/8 to 1/4 inch per foot. The drains discharge into a disposal area, either an under drain or a pumping pit.

General layout of a perimeter drain

Interceptor Drain: These drains are used to divert sub-surface water when the source is uphill from the area to be protected. Interceptor drains are typically used to protect houses from seepage from unlined ditches or irrigation canals. The drain is simple in design: a sand or gravel filled trench sloped to move water toward the outlet location.

General design of an interceptor drain system

(Noe, 41-46)

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NOTE: This information is provided for general education only. For detailed information concerning reactive soils, you should always contact a geotechnical firm.

Reactive Soils: Construction - Drainage

(Specialized designs and construction methods toprevent reactive soil from causing structural damage)

(Specialized designs and construction methods to
prevent reactive soil from causing structural damage)