Soft subgrade remedies under heavy axle loads - 2.5-year TTCI test results - Case Study

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Courtesy of Presto Geosystems

Rapid track geometry degradation can occur under heavy axle loads. Several proposed remedies for this deterioration have been evaluated on the Heavy Tonnage Loop at the Transportation Technology Center, Inc., Pueblo, Colo.

A 700-foot-long, soft-subgrade test track, installed in the HTL, provides a nominal track modulus of 2,000-to-2,500 lb/in./in. This section is referred to as the low track modulus section.

Extensive tests and investigations provided valuable insights into the rapid trackgeometry deterioration that occurred in the LTM section under 39-ton heavy axle loads.

With an l8-inch granular layer construction, the surfacing cycles required to maintain an acceptable track geometry averaged only 15 mgt, ranging from about 60 mgt to less than one mgt. The variance was mainly due to the effects of water on the clay subgrade surface.

To date, several remedy methods with GEOWEB®¹ have been used for correcting the soft-subgrade failure. These include an increased granular-layer thickness and reinforcement of granular layer. Use of a 27-inch granular-layer thickness improved track performance, but did not prevent failure following a heavy rainfall.

Use of the granular layer with GEOWEB (24 inches of total thickness) greatly improved track performance. 

In the Summer of 1999, upon completion of the GEOWEB test, a hot-mixasphalt underlayment was to be applied over the soft subgrade to measure its effectiveness under heavy axle loads.

The investigations and tests of potential soft-subgrade remedies under heavy axle loads are a cooperative effort between TTCI, a subsidiary of the Association of American Railroads; the Federal Railroad Administration; the railroad industry and various suppliers.

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