Technical Notes


Thrace-LINQ TECH NOTE #19
SUBGRADE STABILIZATION/SEPARATION

The Federal Highway Administration's Geotextile Design & Construction Guidelines states "A major cause of failure of roadways constructed over soft foundations is contamination of the aggregate base course with the underlying soft subgrade soils. Contamination occurs both due to:

  1. Penetration of the aggregate into the weak subgrade due to localized bearing capacity failure under stresses exerted by the wheel loads, and
  2. Intrusion of fine-grained soils into the aggregate because of pumping or subgrade weakening due to excess pore water pressures.

The associated subgrade weakening and loss in aggregate thickness result in inadequate structural support which often leads to premature failure of the system."

A geotextile should be placed between the aggregate and the subgrade to act as a separator, which prevents the subgrade and aggregate base course from mixing and thus maintains the desired strength of the aggregate base. The primary function of a geotextile in roadway application is separation, along with filtration and drainage. A geotextile increases the bearing capacity of the system by interfering with the bearing capacity failure surface. Also, a high modulus geotextile, can provide additional reinforcement through lateral restraint of the base and subgrade through friction between the aggregate, soil and geotextile. These reinforcement mechanisms are also applicable to geogrids, but the Federal Highway manual states that "grids are not able to provide the separation and filtration function, and therefore they must be used together with a geotextile in roadway applications."

Penetration of the aggregate into the weak subgrade due to localized bearing capacity failure.



The bearing capacity (q) of undrained soils is defined by Terzaghi (1943) as: q=NcC

Where:
q = soil bearing capacity (lb/ft2)
Nc = soil bearing capacity factor
C = soil undrained shear strength (lb/ft2)
Based on research conducted by Kinney and Barenberg the following bearing capacity factors are recommended for construction of the stabilization lift:
Nc = 3.1 roadways constructed without a geotextile
Nc = 6.2 roadways constructed with a geotextile
The required depth (Z) of the stabilization lift can be calculated using the following Boussinesq equation:
Z = R / [(( 1 / (1-q/p) - 1)0.67)]0.5
R = [ .5P/3.14p] 0.5
Where:
p = tire pressure
P = design axle load
Based on a tire pressure of 80 psi and an axle load of 18,000 pounds the following design chart can be developed:



Cohesionless base materials have no tensile resistance and generally depend on the subgrade to provide lateral restraint. In weak subgrades, very little lateral restraint is provided. Thus the aggregate at the bottom of the base tends to move apart, allowing intrusion of the soft subgrade. Geotextiles restrain aggregate movement by providing tensile strength to the base course much like steel provides tensile strength to a concrete beam. The net effect is a change in the magnitude of stress imposed on the subgrade. This spreading of the stresses over a larger area improves the load carrying capability of the pavement structure. Giroud (1981) indicates that geotextiles which possess high modulus will provide more load spreading ability for the same rut depth. The geotextile therefore provides tensioned-membrane support (Christopher and Holtz 1985) through the soil-geosynthetic stress strain characteristics and frictional resistance.

Intrusion of fine-grained soils into the aggregate because of pumping or subgrade weakening due to excess pore water pressures.

The separation function prevents contamination of the stone base course by creating a permeable barrier between the subgrade soil and aggregate, thus preserving the structural integrity and drainage capacity of the base course. Utilization of a separation geosynthetic minimizes the potential for aggregate being forced down into the subgrade by the action of the applied loads or by the migration of the subgrade up into the base course. As little as 8 percent (Jorenby and Hicks 1987) intermixing of subgrade fines can completely destroy the strength of the base course. The solution to contamination of a stone base course by subgrade fines is the use of a geosynthetic functioning as a separator between the soil subgrade and the stone base course (Koerner and Koerner 1994).



The contamination from the subgrade into the aggregate base lowers the resilient modulus of the base course. This, in turn reduces the AASHTO Layer Coefficient and the number of 18 kip Equivalent Single Axle Loads (ESAL) needed for failure of the system.