A subsurface drainage system is used to intercept water before it can infiltrate the subgrade under a paved surface or to remove it once it is there. A saturated subbase becomes unstable and no longer provides a reliable foundation for the pavement. The importance of drainage is documented in a recent FHWA survey that reveals a strong correlation between the quality of pavement drainage and the life expectancy of that pavement:
The following discussion assumes that attention has been given to grading considerations as well as to the strategic placement of surface inlets. It will address only the subsurface drainage system.
The subsurface drainage system consists of two components:
Collectors are placed in a pattern beneath the surface to collect water from saturated soils under the pavement. The collector system in this discussion is assumed to be a Multi-Flow water collection system.
Once water has been collected, it is transferred to the transport system. The transport system has the sole purpose of delivering water to the exit point(s).
1. Determine the outlet capacity.
In ideal situations, there are no restrictions on the amount of water that can be discharged and the rate at which it can be discharged. However, this is frequently not the case. Consequently, it is essential to discover the outlet capacity before designing the system. Outlet capacity will significantly determine the structure of the collection system.
Determine the amount of water that should be evacuated.
The site must be studied for the source and volume of water. Unwanted water in the base and subbase will originate from one or more of these three sources: infiltration, encroachment, and/or water table elevation.
A surprising amount of water infiltrates from the surface.
Engineer for 30% – 50% infiltration. Attempt to evacuate all surplus subsurface water within 12 hours of a rain event.
A second source of water is from lateral movement. This results when the paved surface is adjacent to an area with a higher elevation and an impermeable layer that prevents the natural downward movement of the water. Under some circumstances, water can travel long distances laterally. Borings can help determine the amount of water moving laterally.
c) Water table elevation
Drainage planning must take into account the possibility that water may invade from below. Soil structures and rainfall amounts will dictate the rate at which water table levels change.
1. Design a transport system capable of carrying water from the Multi-Flow collector system to the outlet. The capacity of the transport system at the outlet should be:
2. In most cases, Schedule # 40 PVC pipe should be specified for the transport system. Under certain conditions PVC sewer pipe or dual wall HDPE pipe is adequate if engineered properly. In all cases, the transport system must have load bearing capabilities equal to or greater than that of the collector system.
3. A minimum of .5% slope must be maintained on the transport system.
4. For purposes of calculating the correct size PVC for transport system assume these nominal flow rates:
These rates do not take into account the effect of head, which may be substantial. Head pressure will vary depending on the amount of rainfall and the depth of the transport system relative to the depth of the collector system.
Before selecting the size and location of collector lines, the use of the site and/or the load-bearing demands placed on the site need to be accounted for. It is also essential to identify the areas of the site that have greater potential for accumulating excess water. For this reason two parameters must be established:
While establishing the parameters for designing the collector layout, consider these two categories in the broadest sense.
Hydrologic features include the presence, location, amount, source, and movement of water. Examples of hydrologic features might be:
Water present under the pavement due to a high water table or underground spring.
Will the system avoid future failure and blockage?
Multi-Flow addresses these concerns by providing systems that are:
Each feature will require a unique response.
Physical factors include both the intended use and the load bearing capacity of the pavement. Both of these factors will contribute to collector system design decisions. Saturated bases and subbases exhibit dramatically reduced load bearing capabilities and do not provide an acceptable foundation for the pavement.
Physical factors can be divided into two parts: a) Usage factors and b) Structural factors.
Furthermore, it need not be trenched in but is simply rolled out over the subgrade where it lies out of reach of cup cutters and coring equipment. (See Applications: Golf Courses)
a. Usage factors The use of a paved surface may range from light pedestrian traffic to heavy truck traffic. The heavier the anticipated usage, the closer the collector lines will need to be located to each other.
b. Structural factors The condition and makeup of the pavement, base, and subbase will further determine the line spacings. Perhaps the largest single factor is the hydraulic conductivity of the base material. Densely packed clay will require closer line spacings than free draining fill.
Typical effective line spacings range from 10 to 50 feet. In calculating appropriate line spacings for any given area, it is crucial to assess and rate both categories of physical factors. Hydrologic features will have less impact on line spacing. They will more likely affect pattern, positioning, and product size selection.
A lightly used pavement with an excellent structure might be adequately drained using 45 foot spacings. A heavily used pavement with a poor structure might require 10 foot spacings. The attached charts give examples of suggested spacings in light water volume situations and in heavy water volume situations.
By accounting for hydrologic features and physical factors, drainage collector lines can be placed where they are most needed and with the concentration that is required. This provides affordable and effective drainage.
If the outlet is insufficient for rapid evacuation, spacing the lines further apart and deeper will widen the drawdown gradient and slow down the desaturation process.
The drainage pattern will be dictated by the surface profile.
The product size (6, 12, or 18-inch) will be determined by three factors:
If the outlet capacity is sufficient, use larger versions of Multi-Flow in runs that are longer or deeper.
For purposes of planning assume that:
each line of 6-inch Multi-Flow can deliver 17 gpm
each line of 12-inch Multi-Flow can deliver 29 gpm
each line of 18-inch Multi-Flow can deliver 45 gpm
Overall system capacity
Select product size that is consistent with the overall outlet capacity of the system. Do not deliver water to the outlet faster than it can leave the site.
Volume of water to be removed
In some situations where there are large amounts of water to be removed such as near an underground spring or when intercepting large amounts of laterally moving water, a larger product size is justified.
6. A perimeter drain or a curtain drain should be specified wherever there is an intrusion of water under the pavement from an adjacent slope and wherever large amounts of water are anticipated to rush off of the pavement.
Collector pipes must maintain a minimum slope of .5%, 1% is preferred
8. Backfill material should be clean, very coarse sand. Predominantly #10 to #30 sieve size is optimal. (See Selecting Backfill)
Do not design the trench to be lined or covered with filter fabric. These fabrics are prone to blinding. The fabric on the Multi-Flow is protected by the sand backfill but trench liners are exposed to surrounding soil without protection.
D. Plot the collector system, transport system, and outlet(s) onto the site plan. Require excavation lines be painted on to existing paved surface if it exists.
E. The illustration below demonstrates several design considerations. Collector lines are placed at three different spacings reflecting three intended uses. They are placed such that they will intercept the water flow direction. Curtain drains are placed to intercept water invading from the hillside to the right as well as from the heavily irrigated islands near the top. Transport pipes running along the top and bottom from right to left and along the left side from the bottom to the top take water from the collectors and removes it from the site.
II. Installation considerations
Excavation must take place in a manner that results in a 4-6 inch wide trench with a clean, sharp edge. The bottom of the trench will be free from loose material. All excavated spoil is to be removed from the site.
If the elevation of the outlets allows for it, locating the transport system below the collector system has several benefits. This method increases flow performance and reduces the risk of rehydration of low-lying areas.
When the transport system is located below the collector system, Multi-Flow multi-purpose connectors are used to outlet the Multi-Flow collectors.
(Connectors ending in 09 and 0M are most commonly utilized)
If the site includes elevation constraints the transport system can be located level with the bottom of the collector system.
When the transport system is located at the same level as the bottom of the collector system, end outlets or side outlets are used to outlet the Multi-Flow collectors.
This method should not be used unless it is dictated by the elevations.
1. Backfill will be accomplished using select, washed, very coarse sand. See Selecting Backfill Material for more information on this topic.
2. Thorough and effective consolidation is required.
Replacement pavement should take place after consolidation is complete. Replacement pavement should exceed the thickness of surrounding pavement.