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Gravel Drainage Specifications

A subsurface drainage system will be installed into the subsoil fill. The pattern of drainage pipes should be designed so that the main drain(s) is placed along the sidelines, and laterals are installed at an angle across the slope of the field, allowing a natural fall to the main. Lateral drains shall be spaced not more than 20 feet apart and extended to the center of the field. At the low end of the gradient, where the main drain exits the field, drainage pipe should be connected to the 'subgravel drainage'.

Drainage pipe shall be perforated plastic, minimally conforming to ASTM 2729 or ASTM F 405, with a minimum diameter of 4 inches (100 mm). Waffle drains or any tubing encased in geotextile sleeves are not recommended. Drainage trenches minimally 6 inches wide and 6 inches deep shall be cut into a thoroughly compacted subsoil so that drainage pipes maintain a consistent slope to the outlet of at least 0.5%. Spoil from the trenches should be removed from the subgrade cavity, and the floor of the trench should be smooth and clean. If a geotextile fabric is to be used as a barrier between unstable subsoil and the gravel drainage blanket, it should be installed at this time. Under no circumstances should the fabric cover the drainage pipes or trenches. A layer of gravel (see below for size recommendations) should be placed in the trench to a minimum depth of 1 inch. It may be deeper, as necessary, to ensure minimal slope requirements. All drainage pipes should be placed on the gravel bed in the trench. PVC drainpipe, if used, should be placed in the trench with the holes facing down. Pipe connections shall not impair the overall function of the pipeline. The trenches should then be backfilled with additional gravel, taking care not to displace any of the drainage pipes.

As an alternative to round pipe placed in a trench, flat pipe placed directly on the prepared subgrade may be employed, provided the flat pipe conforms to ASTM D 7001 (provisional), is a minimum of 12 inches (300 mm) in width, and is not covered by a geotextile sleeve. The flat pipe shall be stapled to the subgrade, or otherwise held in place to prevent shifting during construction. Rational combinations of round and flat pipe may be employed within the fields drainage system. All other guidelines for drainage system installation shall apply for this alternative construction method.

Gravel Placement

Place grade stakes at frequent intervals over the subgrade and mark them for the gravel drainage blanket layer, and intermediate layer.

The entire subgrade then shall be covered with a layer of clean, washed, crushed stone or pea gravel to a minimum thickness of four inches (100 mm), conforming to the proposed final surface grade to a tolerance of ± 1 inch.

Soft limestones, sandstones, or shales are not acceptable. Questionable materials should be tested for weathering stability using the sulfate soundness test (ASTM C-88). A loss of material greater than a 12% by weight is unacceptable.

The LA Abrasion test (ASTM C-131) should be performed on any materials suspected of having insufficient mechanical stability to withstand ordinary construction traffic. The value obtained using this procedure should not exceed 40.

Size Recommendations for Intermediate Layer
Performance Factors
Not more than 10% larger than 4 mm
At least 50% of the particles between 1.0 and 4.0 mm
Not more than 30% smaller than 0.25 mm
Not more than 10% smaller than 0.05 mm
Compacted percolation rate will exceed 20 in/hr.
(ASTM F1815)
Size Recommendations for Gravel
Performance Factors Recommendation
Bridging Factor D15 (gravel) less than or equal to 8 X D85 (intermediate layer)
Permeability Factor D15 (gravel) greater than or equal to 5 X D15 (intermediate layer)
Uniformity Factors
  • D90 (gravel) / D15 (gravel) is less than or equal to 3.0
  • No particles greater than 12 mm
  • Not more than 10% less than 2 mm
  • Not more than 5% less than 1 mm

Selection and Placement of Materials When the Intermediate Layer Is Used

The tables above describe the particle size requirements of the gravel and the intermediate layer material.

The intermediate layer shall be spread to a uniform thickness of two to four inches (50 to 100 mm) over the gravel drainage blanket (e.g., if a 3-inch depth is selected, the material shall be kept at that depth across the entire area), and the surface shall conform to the contours of the proposed finished grade.

Selection of Gravel

Selection of this gravel is based on the particle size distribution of the intermediate layer material. The construction superintendent must work closely with the soil testing laboratory in selecting the appropriate gravel. Either of the following two methods may be used:

Send samples of different gravel materials to the lab when submitting samples of components for the intermediate layer material. As a general guideline, look for gravel in the 2 mm to 9.5 mm range. The lab first will determine the best intermediate layer material, and then will test the gravel samples to determine if any meet the guidelines outlined below.

Submit samples of the components for intermediate layer material, and ask the laboratory to provide a description, based on the intermediate layer material tests, of the particle size distribution required of the gravel. Use the description to locate one or more appropriate gravel materials, and submit them to the laboratory for confirmation.

It is not necessary to understand the details of these recommendations; the key is to work closely with the soil testing laboratory in selecting the gravel. Strict adherence to these criteria is imperative; failure to follow these guidelines could result in drainage failure.

The criteria are based on engineering principles which rely on the largest 15% of the root zone particles “bridging” with the smallest 15% of the gravel particles. Smaller voids are produced, and they prevent migration of root zone particles into the gravel yet maintain adequate permeability. The D85 (root zone) is defined as the particle diameter below which 85% of the soil particles (by weight) are smaller. The D15 (gravel) is defined as the particle diameter below which 15% of the gravel particles (by weight) are smaller.

  • For bridging to occur, the D15 (gravel) must be less than or equal to eight times the D85 (root zone).
  • To maintain adequate permeability across the root zone/gravel interface, the D15 (gravel) shall be greater than or equal to five times the D15 (root zone).
  • The gravel shall have a uniformity coefficient (Gravel D90/Gravel D15) of less than or equal to 3.0.

Furthermore, any gravel selected shall have 100% passing a ½" (12 mm) sieve and not more than 10% passing a No. 10 (2 mm) sieve, including not more than 5% passing a No. 18 (1 mm) sieve.