Is it better to place cobbles or sand directly on a weak subgrade?

Grader
Image source: Wikimedia Commons

In grading for a road improvement, about 200 feet of extremely wet clayey silt was found, which was so unstable that it was difficult to move the grader across it. One of the crew suggested laying in several inches of coarse sand before putting in the base. The grader operator proposed using several loads of 4-6 inch cobbles as a real "bottom" for the base. Would you recommend placing cobbles or sand
directly on the wet soil? Why?

Answer to "Why?"

A layer of coarse sand (or a clean, graded gravel-sand mixture) will accomplish more than the cobbles.

The trouble with cobbles is that they have large voids between the stones. Virgil LaVeck ([former] Town of Richmond) advises, " ... after removing as much of the wet soil as possible, place a layer of sand because large stone will not stop the clayey silt from coming up through the voids ...

If the "coming up" of fine soil is difficult to visualize, picture a basket of cabbage-size cobbles and a similar basket containing a coarse sand. If we pour a bucket of soupy wet clayey soil over both the cobbles and the sand, we can bet the soupy soil would reach the bottom of the basket with the cobbles first. The sand tends to serve as a filter of the clayey silt particles. If soupy soil moves downward through large voids in cobbles, it will also move upward in the roadbed under the action of traffic. The intrusion of wet soil reduces the effective thickness of the road foundation.

Answered by Jim Spencer in the September & October 1959 issues of Highway Topics

Update to Answer

Since Jim answered the question the use of geotextiles have replaced the use of sand as a filter. Both woven and non-woven geotextiles can be used to separate the subgrade from the base gravel. The choice geotextile or sand will depend upon costs as both methods will work to separate the layers. If the subgrade is very soft and weak a geocell or other confinement technique may  be needed. Please contact the Cornell Local Roads Program for more information.

Resources

Cover of the CLRP workshop manual, Basics of a good road

Basics of a Good Road (pdf)
CLRP workshop manual

 

 

 

 

 

Geosynthetics Classification (pdf)

Cover of the Roadway and Roadside Drainage

Roadway and Roadside Drainage (pdf)
CLRP workshop manual

CLRP Tech Assistance logo

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