NCHRP MSE Walls Project

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The Challenge

NCHRP Project 24-22 aimed to expand reinforced fill options for MSE walls
Field test focused on “high fines” soils and drainage systems’ impact
Findings highlight importance of drainage for optimal wall performance

The objective of National Cooperative Highway Research Program Project 24-22 was to develop selection guidelines, soil parameters, testing methods, and construction specifications that will allow the use of a wider range of reinforced fill materials within the reinforced zone of mechanically stabilized earth (MSE) walls. A full-scale field test (4 wall sections with varying percent fines) was conducted in order to establish properties for “high fines” reinforced soils and associated design controls that give acceptable MSE wall performance. This “first of its kind” field test included provisions to demonstrate (1) the role of pore water pressure in the reinforced fill and (2) the importance of including a positive drainage system to obtaining good wall performance.

The walls were designed so that they would yield acceptable performance for the normal design conditions, but experience excessive deformation when subjected to additional surcharge loading and high pore pressures.

The Solution

Geocomp’s survey of MSE wall practice indicated that there are records of both successful and failed walls constructed using “high fines” and/ or “high plasticity” soils in the reinforced zone. Those walls with stability problems appeared to have high water pressures in the reinforced zone that led to excessive deformation or collapse. Our survey of state transportation agencies indicated that with only a few exceptions, they currently conform with AASHTO requirements regarding material type and properties of reinforced fill for MSE walls.

Geocomp concluded that the current AASHTO limit on maximum fines content of the reinforced fill for MSE structures can be increased from 15 to 25%, provided the properties of the materials are well defined; and controls are established to address the design issues and limit the development of positive pore water pressures in the reinforced fill. Recommendations to revise current AASHTO specifications to include use of backfill materials with “higher” fines were provided.