From Engineering Policy Guide
The presence of moisture or water within or beneath the pavement structure is almost always detrimental to some degree, depending upon many variables including pavement, base and soil characteristics, degree of saturation, the extent and range of moisture fluctuations over time as well as temperature variations. For example, water affects various subgrade soils in differing ways. A granular soil may consolidate or settle in the presence of water, particularly when subjected to traffic vibrations, and may lose little or even gain in subgrade support. A clay soil on the other hand will soften with increasing moisture content and, depending upon initial density and restraining load, may swell significantly resulting in both pavement heave and loss of support. Silty soils are especially prone to capillary movement of water, to frost heave, and to pumping when saturated and subjected to dynamic stresses from traffic.
Subsurface water may have a number of sources although ultimately all originates from rainfall. Subsurface water has been classified as gravitational or free water, which can be more readily drained by cuts or by interception drains, and as capillary water which is little influenced by such measures and is best controlled by a blanket of porous, free draining material which breaks the capillary path. Water may also move by vapor phase from a relatively warm subgrade to condense on the bottom of a cold pavement slab, for example. Experience has also shown that much of the water affecting the pavement structure originates as surface water which enters through joints and cracks and that such water eventually develops free-flow channels in relatively impermeable dense graded bases, particularly along and beneath the pavement-shoulder joint, leading to softening and loss of support at the edge of the pavement.
Most subgrade soils experience an increase in moisture content after paving, as do low permeability, dense graded bases. Other (mostly seasonal) variables aside, a degree of moisture equilibrium is usually reached within a few years, at which time many formerly smooth pavements may reflect both differential settlement and swell. Long-lasting, smooth pavements can be achieved, however, by conscientious efforts to provide uniform, (not necessarily high) subgrade support and by providing the maximum possible degree of drainage for the pavement and base.
605.1.2 Types of Underdrains
Pipe-Aggregate Underdrains. Pipe-aggregate underdrains consist of a geotextile lined trench, perforated plastic or metal pipe, and porous backfill. Functionally, these may be installed either as pavement edge drains to drain permeable base courses, or as pavement cross drains to intercept localized sources of water. Pay items are specified for the estimated length of underdrains and number of outlets.
French Underdrains. French underdrains consist of fabric wrapped, coarse porous backfill in a trenched installation. No pipe is used except for a short length of metal pipe as an outlet. Functionally, this is an interception type drain.
Geocomposite Pavement Edge Drains. Geocomposite pavement edge drains consist of a geotextile wrapped plastic core or drainage medium that is installed in a narrow trench along the edge of pavements being rehabilitated that meet certain criteria. See Rehabilitation Strategies for the details of using geocomposite edge drains in pavement rehabilitation projects.
605.1.3 Location and Use of Underdrains
The movement of moisture in soil and rock strata can be very complex and is influenced by the seasons, surface topography, vegetation, the configuration of subsurface soil and rock profiles and the relative degrees of permeability of the various layers, as well as animal burrows, root holes, and the activities of man (utility trenches, well, ponds, and lakes, etc.). Accordingly, it should be recognized that it is extremely difficult, if not impossible, to determine all of the possible locations requiring drainage during the design stage of roadway plans. These sources will, in some instances, be determined during the soil survey or during design field checks. Many times, however, these sources are missed entirely because the source may be small, or because the soil surveys are made during dry periods when these sources may not be evident. In many instances, only a careful inspection of conditions existing during construction will reveal sources of water which may cause detrimental effects on the roadway and which should be drained. The use of cross drains or other methods should be determined by the engineer after examination of the site and consultation with the district soils and geology technologist.
It is also entirely possible that locations for cross drains may be specified which will prove to be unnecessary. In any case, details for underdrains required for known conditions, other than those shown on the standard plans, are shown on the design plans based on the best information available at the time the plans are prepared.
Pavement edge drains are required for new rigid or flexible pavements on medium and heavy duty routes, and permeable base courses will be provided on all heavy duty pavements with the following exceptions. Pavement edge drains and permeable base courses are not required where a minimum of 12 in. or 18 in. of daylighted rock base can be furnished for the top of the subgrade or where hydraulically placed sand fill comprises the top 4 ft. (1.2 m) of the embankment with not more than 2 ft. (0.6 m) of soil cap on the slopes. Thin courses of permeable bases cannot be relied upon to provide permanent drainage when daylighted and should never be used without pavement edge drains.
All new rigid and flexible pavements, light, medium, or heavy duty, are to be provided with 12 in. or 18 in. of rock base in the top of the subgrade whenever available in suitable quantities on or in close proximity to the job.
Pavement cross drains are designed with a minimum fall of at least one and preferably 2 percent or more and an outlet at least 6 in. (150 mm) and preferably 12 in. (300 mm) above the ditch flow line. The cross drains are skewed to obtain the necessary fall and clearance. The roadway ditch should be deepened to meet these requirements if necessary. Pavement edge drains are designed with flow lines paralleling the pavement and base grades and with outlets on their respective side of the pavement unless it is absolute necessary to carry the water beneath the pavement to an outlet. See Standard Plan 605.10 for details concerning outlet location and spacing.
605.2 Construction Inspection
605.2.1 Description (Sec 605.1)
This work shall consist of furnishing and installing underdrains and edge drains as shown on the plans or as directed by the engineer, and shall include excavating the trench, installing all required drainage media, and backfilling with material as specified or as directed by the engineer.
605.2.2 Construction Requirements (Sec 605.3)
Although probable locations of underdrains are shown on the plans, modifications may be necessary due to conditions found on the project. If pavement repair or undersealing are set up in the plans they should be completed before the edge drains are installed. If the drains are installed first they will get filled with grout. The Standard Plans should be reviewed before inspection is performed. The details are important and the general notes provide valuable information about correct installation. Care should be taken not to damage or plug drain outlets during finishing operations.
605.3 Materials Inspection
See related materials requirements in Aggregate for Drainage, Geotextile, Geocomposite Edge Drain, Outlet Pipes, Pipe Aggregate Pavement Edge Drain, Corrugated Metallic-Coated Steel Pipe Underdrain, or Corrugated Aluminum Alloy Pipe Underdrain.
605.4 Maintenance Activities
Underdrains are placed for base drainage, to remove excessive ground water and as interception drains in unstable soil to prevent slides. Underdrains placed at the time of construction are marked at the outlet ends with "Drain" markers on steel posts and any placed by maintenance should be marked in the same manner. It may be necessary for maintenance forces to place underdrains but these must be approved by the district maintenance and traffic engineer. It is recommended that the district geologist be consulted in exceptional cases. Underdrain outlets shall be cleaned out as necessary to provide drainage away from the ends of the pipe.
605.4.2 Underdrain Installation
The depth of underdrains should be so that all seepage is intercepted. In some cases, this will require advice from the district geologist and core drillings to determine water level or seepage veins. The excavated trench should allow for a minimum of 4 inches of granular backfill on each side of the pipe and 2 inches below the pipe. Highly unstable soils such as in slide areas, may require a V-shaped trench or a shored up trench depending on the depth of excavation. Outlets of drains in cuts should be a minimum of 6 inches above the bottom of the roadside ditch.
The size of underdrain pipe depends on the drainage capacity required. Perforated pipe is available in 6, 8, 10, 12, 15 and 18 inch diameters.
All perforated underdrain pipe within granular backfill shall be laid with the perforations down. Outside the limits of granular backfill the perforations should be up and covered with two layers of tar paper or be pipe that is not perforated. If perforated vitrified clay pipe is used all joints must be filled with mortar.
Backfill material for perforated pipe should be a coarse graded sand such as concrete sand to at least one foot above the top of the pipe. Creek gravel with a high percentage of fines may be used above this height to fill the remainder of the trench. The trench should then be capped with clay or other impervious material. Backfill material must permit ready flow of water and keep fine soil from entering and clogging the subdrain. Bell Joint vitrified clay pipe without perforations is not recommended but if used a coarse backfill material must surround the pipe to prevent the fine filter material from entering and clogging the system.
Standard design and construction specifications will provide additional information regarding underdrain installations.