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Paved Surface Drainage Design Assistance

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:

Quality of drainage          Average life span*
        Excellent                         38 years
        Good                              20 years
        Poor                               8 years
*Christopher, Zhao, and Hayden (FHWA)

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:

1. The collector system
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.

2. The transport 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).

I. Design

A. Capacity

1. Determine the outlet capacity.

  • Is the outlet in a single fixed location?
  • Does the outlet have a limited capacity?
  • What is its 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.

2. 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) Infiltration

A surprising amount of water infiltrates from the surface.

FHWA studies show typical infiltration rates:

• Asphalt           33% -- 50%
• Concrete        50% -- 67%

Infiltration rates can be reduced by:

  • reducing the amount of time water stands on the surface by ensuring proper grading and the strategic placement of surface inlets.
  • maintaining a uniform, non-porous surface through seal coating, etc.

Engineer for 30% - 50% infiltration. Attempt to evacuate all surplus subsurface water within 12 hours of a rain event.

b) Encroachment
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.

B. Transport system

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:

    • Less than or equal to the capacity of the outlet
    • If possible, greater than or equal to the capacity of the collector system.

    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:

    PVC Size           .5% grade           1% grade
    4” PVC                  82 gpm               119 gpm
    6” PVC                 240 gpm              349 gpm
    8” PVC                 494 gpm              718 gpm
    10” PVC               902 gpm             1,279 gpm

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.

C. Collection 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:

1. Hydrologic features
2. Physical factors

While establishing the parameters for designing the collector layout, consider these two categories in the broadest sense.

1. Hydrologic features

Hydrologic features include the presence, location, amount, source, and movement of water. Examples of hydrologic features might be:

  • Water moving under the paved surface from an adjacent area with a higher elevation. e.g. a hillside
  • Water encroachment from a heavily irrigated area. e.g. a boulevard or an island
  • Water present under the pavement due to a high water table or underground spring.
  • The pavement, and/or the subbase, slope to a “bowl” concentrating the water beneath the pavement in a given area.

Each feature will require a unique response.

  • Where water is moving to the site laterally, it can be intercepted.
  • When water is concentrated in restricted areas, these spots can be targeted with intensive drainage.
  • When the saturation is widespread, a more systematic comprehensive approach may be required.

2. Physical factors
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.

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.

3. Line spacing guidelines

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.

Usage factors can be rated:

Very Light (e.g. pedestrian and bike traffic)
Light (e.g. car parking)
Heavy (e.g. car traffic)
Very Heavy (e.g. truck traffic)

Structural factors can be rated:

Excellent (e.g. free draining soil)
Very Good
Poor (e.g. compacted clay)

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.

Line spacing in light water volume situations
Usage Factors
Structural Factors Very Light Light Heavy Very Heavy
Excellent 50 ft 40 ft 30 ft 20 ft
Very Good 40 ft 32 ft 24 ft 16 ft
Good 35 ft 28 ft 21 ft 14 ft
Fair 30 ft 24 ft 18 ft 12 ft
Poor 25 ft 20 ft 15 ft 10 ft

Line spacings in heavy water volume situations
Usage Factors
Structural Factors Very Light Light Heavy Very Heavy
Excellent 45 ft 36 ft 27 ft 18 ft
Very Good 35 ft 28 ft 21 ft 14 ft
Good 30 ft 24 ft 18 ft 12 ft
Fair 25 ft 20 ft 15 ft 10 ft
Poor 20 ft 16 ft 12 ft 8 ft

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.

4. Layout pattern
The drainage pattern will be dictated by the surface profile.

  • Relatively flat surfaces are most effectively drained using a grid pattern.
  • Sloping surfaces are best drained by orienting the collectors at a 45 degree angle to the slope. This will place the collectors in position to intercept the natural flow of the water and minimize the need to increase the trench depth to maintain grade.

5. Product size
The product size (6, 12, or 18-inch) will be determined by three factors:

• Length of run

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.

7. Grade
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

A. Excavation
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.

B. Connection of collection system to transport system

1. Preferred method
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)
e.g. see 1200M

2. Alternate method
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.

e.g. see 12004.

This method should not be used unless it is dictated by the elevations.

C. Backfill and consolidation

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.

D. Resurfacing
Replacement pavement should take place after consolidation is complete. Replacement pavement should exceed the thickness of surrounding pavement.

Paved Surfaces Installation
Paved Surfaces Applications
How Artificial Drainage Works