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Selecting reinforced fill soil for MSE retaining walls

June 1st, 2006 / By: / Feature, Geotextiles

A full-scale field test is currently being conducted to establish properties for “high fines” reinforced soils and associated design controls that give acceptable MSE wall performance.

Mechanically Stabilized Earth (MSE) retaining walls on public sector transportation projects are generally conservatively designed with “low fines” reinforced soils. Private MSE walls are less conservatively designed, and use a variety of reinforced soils (NCMA recommends 35% < 0.075mm or greater). It is also clear from the literature that the combination of reinforced soil consisting of fine-grained soils (either “high” fines or “high” plasticity) and water in the reinforced zone were the principal reasons for serviceability problems (excessive deformation) or failure (collapse).

However, a higher quantity of fines can be safely allowed in the reinforced fill, provided the properties of the materials are well-defined and controls are established to address the design issues. The potential savings from replacing AASHTO reinforced fill materials with marginal reinforced fill materials could be in the range of 20% to 30% of current MSE wall costs.

A full-scale field test is currently being conducted in order to establish properties for “high fines” reinforced soils and associated design controls that give acceptable MSE wall performance. The field test includes provisions to demonstrate the role of porewater pressure in the reinforced fill and the importance of including a positive drainage system to obtain good wall performance. Based on the survey of the literature, to date, full-scale test or experimental MSE walls have not rigorously evaluated this important aspect.

The full-scale field test is primarily funded by the Transportation Research Board, under the National Cooperative Highway Research Project (NCHRP) 24-22, with a portion funded by the National Concrete Masonry Association (NCMA). The objective of NCHRP Project 24-22 is 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.

NCHRP Project 24-22 includes four sections:

1) One section with an AASHTO A-1-a reinforced fill to provide a baseline of performance for current AASHTO and FHWA standards.

2) A second section with an AASHTO A-2-4 reinforced fill to demonstrate that non-plastic, silty sand materials with up to 35% fines (of no plasticity) can provide suitable reinforced fill for MSE walls.

3 & 4) The third and fourth sections with an AASHTO A-4 material to demonstrate that silty soils (50% fines) of low to moderate plasticity can provide suitable reinforced fill for MSE walls.

Welded wire was used for the wall face system. NCMA sponsored two additional sections utilizing dry-cast concrete modular block for the wall face. The two NCMA sections consist of the AASHTO A-1-a and A-4 reinforced fill soils used in the NCHRP sections.

Polyester geogrid is being used for the reinforcement in all sections, with the exception of one NCHRP section where nonwoven geotextile reinforcement with in-plane drainage capability was used.

An essential component of an MSE retaining wall that uses reinforced fill with “high fines” soil is aggressive drainage to prevent the buildup of porewater pressure in the reinforced zone. Porewater pressure produces an additional outward force that the wall must resist, and it reduces the strength of the soil that holds the wall in place. Therefore, the field test includes provisions to demonstrate the role of porewater pressure in the reinforced fill and the importance of including a positive drainage system to obtain good wall performance.

Figure 1 shows how this will be accomplished. A geocomposite drainage material has been placed at the back of the reinforcement in each test section. It was wrapped around a slotted drain pipe at the bottom of the reinforced fill that will remove water from the drain.

To simulate groundwater, water is pumped to a feed line at the top of the test sections. A system of valves will control the introduction of water from the feed line into the individual drainage soil zones of each test section via slotted, vertical fill pipes. This will initiate horizontal flow towards the wall and into the geocomposite drain for the wall. By controlling the head in the drainage soil (with the drain pipe open), the effect of rising groundwater level on the performance of the wall can be simulated. We would expect little, if any, effect on the test sections as long as the geocomposite drains function as designed.

This phase of the test is intended to demonstrate that various reinforced fill materials will provide suitable performance, even in areas with high groundwater conditions, as long as they are properly drained.

By closing a valve on the drain pipe and spraying water on top of the reinforced fill, the effects of poor drainage and heavy rainfall on the performance of MSE walls with the various reinforced fills can be simulated. The porewater pressure in the reinforced fill can be increased until the wall experiences noticeable distress. This phase will provide valuable information to evaluate the ability of the numerical models to consider the effects of pore pressure.

Finally, the test areas can be drained, a surcharge added and the test sequence repeated to measure the effects of groundwater and rainfall. The walls have been designed so that they should experience considerable distress when subjected to a surcharge and high porewater pressures (i.e., the factor of safety is essentially 1.0, based on numerical model and limit equilibrium analyses).

Figure 2 illustrates the proposed test sequence:

Each test section is fully instrumented to record data that will be used to evaluate a number of technical questions. Instrumentation consists of strain gages mounted on the geosynthetic reinforcement; piezometers, thermistors, multiple position horizontal extensometers, and vertical extensometers positioned throughout the reinforced fill; vertical inclinometers; and an array of high-precision prisms mounted on the face of the test walls where optical survey readings are being obtained using automated robotic total station technology.

Most instruments are electronic and connected to automatic data logging equipment using the iSite™ system. This system has been programmed for each instrument to have a warning level at which an electronic notice is sent to key personnel indicating that some activity is occurring at that instrument. Instruments are being read four times each day and stored in the on-site data loggers. These data loggers are connected by cell phone modem to our Web server, which periodically contacts the site and updates its database with the latest readings on all instruments.

The database is accessible with a Web browser and provides up-to-date process readings plotted in engineering units at any time from any location with Web access. This allows the field tests to be carried out with far more extensive monitoring than typically possible. The benefit of this extensive monitoring is to identify the effects of environmental changes, such as temperature and rainfall on the performance of the wall to a degree of detail not previously possible. Figure 3 presents a “sample screen capture” from real-time processed strain gage data for Test Section A, for a one-week period in October 2005.

Construction of the full-scale test walls began in summer 2005 and was completed in November 2005 (Figure 4). The test sequence indicated in Figure 3 is under way and monitoring will continue through spring 2007. Guidelines for selecting MSE reinforced fill soils, representative soil parameters, appropriate testing methods, and construction specifications for a wide variety of reinforced fill soils will then be prepared.

Dick Stulgis, P.E., is currently a senior consultant at Geocomp Corp. in Boxborough, Mass.

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