Geogrids: Stability Against the Odds

July 1, 2002

“Geogrids are plastics formed into a very open gridlike configuration,” writes Robert Koerner, an authority on geosynthetics. “That is to say, they have large apertures. Geogrids are stretched in one or two directions.”

Geogrids’ strength gives them the ability to be used for base stabilization, for slope reinforcement, to reinforce the soil mass behind retaining walls, and for berm reinforcement. In a typical application, geogrids are embedded horizontally behind retaining walls and between layers of soil. The geogrids are anchored at the retaining wall and extend deep into the soil behind the wall. “You tie the grid into the wall to make the soil and the wall act as a monolith,” explains Tom Collins, president of Huesker Inc., a geosynthetics manufacturer. “The wall, the grid, and the soil mass are all one unit.”

Major Wall Construction

A recent grade separation project in Auburn, WA, involves no fewer than 15 retaining walls–some 75,000 square feet of walls ranging up to 32 feet high. The project is located at the intersection of State Route 18 and Third Street SW; SR 18 also passes over a railroad track at the intersection. Several ramps are involved.

Originally the City of Auburn called for mechanically stabilized earth (MSE) retaining walls, with geotextile employed between 12-inch lifts of compacted soil, to stabilize the earth. At the wall’s face, the geotextile would be wrapped up and around each lift of soil. The final facing component would be a cast-in-place concrete fascia.

Tensar Earth Technologies Inc. (TET), however, proposed an alternative using a combination of geogrids to stabilize the earth and geotextile to wrap the soil layers at the edge. In addition, TET proposed the use of full-height, precast concrete panels. Working with a structural consultant and the contractor, Robison Construction Inc., TET developed a scheme by which the precast panels could be anchored into place. The city accepted the revised wall construction method; full-height precast panels could be used on walls up to 23 feet high. For higher walls, cast-in-place construction was used.

Typical lift construction for the wall proceeded as follows, explains Gerry Kehler, senior construction engineer with TET. First, L-shape welded-wire forms were placed along the fill’s edge, with the vertical leg sticking up. “The forms enabled the contractor to maintain a vertical soil face before it was supported by any concrete wall,” he describes. If only geotextile had been used to wrap the soil layers at the edge, a “pillowing” effect would have resulted, making the wall face irregular. To pour concrete up against such a vertical series of “pillows” would have meant using extra concrete at the bulges to achieve the required thickness.

In the alternative method used, TET used geotextile for soil retention only. Geotextile was embedded just 3 feet into the fill, and the outside edge was flipped over the wire basket at the edge. To reinforce the soil, the contractor laid Tensar Structural Geogrid on top of every 18-inch-thick layer and extended it into the fill a distance equal to 70% the height of the wall. For example, a 30-foot-high wall would require geogrid reaching 21 feet from the wall face into the fill. “Or in some cases where there were ramps on both sides of the fill, the geogrid extended the full width of the embankment,” adds Kehler.

The next step was to place a steel strut between the vertical and horizontal legs of the L-shape form baskets. The fill came next. Each lift of coarse sand was 9 inches thick, and the contractor compacted two 9-inch lifts into place before placing more geotextile and geogrid materials. When an 18-inch layer was completed, the geotextile was wrapped around the sand layer’s edge and laid back over the top.

After the fills were placed, they required a period of time to settle before the concrete walls were placed. One-piece panels, extending up to 23 feet in height, fitted into a cast-in-place leveling pad at the bottom. Near the panel’s top, a steel tie rod was attached at one end to the back of the panel; the rod extended 13 feet back into the fill and was attached to a concrete anchor block set in the fill.

Tensar’s alternative method allowed the construction team to increase the spacing between geogrids to 18 inches from 12 inches. “That decreased the number of geogrid layers, which meant less manual labor to install layers of geogrid,” points out Kehler. “The increased strength of the geogrid [over the originally specified geotextile] allowed a reduction in the required number of layers of soil used to build the mechanically stabilized earth structure.” More importantly, the contractor eliminated a large quantity of cast-in-place fascia and replaced it with the more economical precast concrete paver fascia.

Paving Over Peat

A Tensar welded-wire retaining wall, approximately 12 ft. high, during construction. The precast panels were installed after the embankment settled.

Sometimes it takes more than a simple geogrid to stabilize the soil. In Kent County, MI, officials have paved several highway projects in recent years over unstable subgrades with the help of a geocell–the Geoweb Cellular Confinement System from Presto Products Company. Made of high-density polyethylene, the Geoweb product resembles a honeycomb in structure. The three-dimensional network of interconnected, perforated cells is filled with select infill materials such as topsoil, aggregates, concrete, or a combination of those materials.

Sections of Geoweb come in various sizes, cell sizes, and cell depths. Cell depths, for example, are 3, 4, 6, and 8 inches. By confining the base material within the cell walls, the Geoweb system decreases the rate of the infill material’s lateral movement and creates passive resistance between adjacent cells. The product helps create a stiff base and distributes loads laterally.

In Kent County, a 1,000-foot section of 20-Mile Road had presented problems for years. Over time, three culvert pipes had settled into the peat at one place, one above the other. Construction began by widening the road on each side with a 6-foot-wide section of sand. “Then we spread separator fabric full width over the original road,” recalls Tom Byle, assistant engineer for the Kent County Road Commission. For the fabric, the county specified an 8-oz., nonwoven geotextile.

Next, 8-foot-wide sections of 8-inch-thick Geoweb were laid down along the sides of the road, parallel to the centerline, with a 4-ft. width over the existing road and 4 feet over the widened sand section. “We filled the geocell with blast furnace slag,” continues Byle. “That way we stiffened the edge between the old base and the synthetic base.”

The remainder of the road base consisted of another 4 in. of slag, followed by another layer of nonwoven geotextile, which was topped by 6 in. of blast furnace slag. The road was paved with a 1.5-inch mat of flexible hot-mix asphalt. “We knew 20-Mile Road would settle, but we used the fabric and the geocell to get it to settle uniformly,” explains Byle. It has been several years since the geocell project was built. “We’ve never been back,” he concludes. “The road is in relatively good shape for its age. It was built over 30 feet of peat. Using that technique, we were able to pave a road that we otherwise couldn’t have paved.”

For the 20-Mile Road project, Kent County also used an 8-inch-thick section of Geoweb to stabilize the base under a culvert. The Geoweb was laid into the culvert’s trench and filled with blast furnace slag. Culvert pipe was placed on top of the filled geocell, and the trench was backfilled with slag. “Then we brought fabric over the whole road grade, over the slag at the trench,” states Byle. “The purpose of the fabric was to keep the pipe from pushing down under load. And the purpose of the geocell was to distribute the load of the pipe so that it wouldn’t push down.”

Stabilizing Soft Subgrade

The Geoweb system was specified to provide needed load support over organic deposit.
Geoweb sections were placed over needle-punched, nonwoven geotextiles; the system was then filled with an open-graded, expanded slag.

An unstable subgrade also was the problem facing excavating contractor Raye Vest Corporation in building a parking lot for a Bennigan’s Restaurant in Waldorf, MD. In excavating the site, notes Vice President Raymond Vest, “We came to a soft, silty clay. No matter what you did to it, it wouldn’t hold together. It would not compact.”

Vest began construction by using dozers and loaders to strip away 6-10 inches of topsoil from the site. A proof-rolling revealed that the subsoil was unsuitable, he recalls. So the contractor used an excavator to undercut about 2,000 cubic yards from the site. In terms of depth, the subgrade excavation reached another 3-5 feet down. Still, the bearing capacity was inadequate. “After we undercut it, it would still pump again under truck wheels,” says Vest.

The solution was to place some 6,000 square yards of Webtec TerraGrid 100 geogrid over the unstable subgrade. TerraGrid 100 is a two-layer grid, and the layers are held together with polypropylene stitching, describes Marketing Manager David Snyder. When the two layers overlap, the result is a pattern of varied aperture sizes that conform to a number of different fill materials. The product is used for tensile reinforcement of the soil.

“Once we laid down the geogrid, we started putting down 8- to 10-inch lifts of granular bank-run gravel,” relates Vest. “It was a sandy material with traces of clay. We put from nine to 12 lifts of granular fill material over the geogrid and rolled each lift with a smooth drum. They took density tests on each lift.” Vest notes that the dozers stayed up on the fill as they pushed the first lift out over the geogrid, to avoid tangling the grid into the dozer tracks.

Did the geogrid stabilize the subgrade and base? “Yes, that parking lot soil was hard,” relates Vest. “The geogrid is a very fine product. The subgrade didn’t pump at all. The grades for all the lifts held true up to the top of subgrade.”

Nonproprietary Specification

Some 6,000 yd.2 of Terragrid 100 geogrid was placed on the subgrade at this Maryland parking lot to stabilize the soil. Lifts of granular bank-run gravel, each 8-10 in. thick, were compacted on top of the geogrid to form a stable base for paving.

Geogrids also were used recently to stabilize the soil behind a block retaining wall on State Highway 1 in Carmel, CA. But this project brought with it an interesting lesson: a way to write nonproprietary specifications for block-wall-and-geogrid systems.

“There haven’t been that many block walls built by Caltrans [California Department of Transportation] because, up to now, no one has written a nonproprietary spec that would produce a project with the kind of standards Caltrans could be comfortable with,” says Phil Gregory, principal engineer with Cal Engineering & Geology Inc. in Walnut Creek, CA. Gregory’s firm was brought onto the Carmel project by Whitson Engineers in Monterey, CA, the project’s civil engineer.

For the project in Carmel, Gregory demonstrated to Caltrans that a nonproprietary specification in fact can be written for a block-wall system with geogrid reinforcement. “We wrote a technical specification with regard to three main elements,” explains Gregory. “We specified the size and strength of the block; we specified the strength of the geogrid; and third, we wrote the block-grid connection characteristics. If you put out the effort, you can write a spec that is nonproprietary and allows multiple systems to compete.”

Gregory’s spec worked. Three combinations of geogrid and block could meet the specification he wrote for the Carmel project. The winning geogrid selected was Huesker Fortrac 55/30-20, a coated polyester geogrid. And the winning block was Anchor Vertica Pro. Construction started in November 2001; by April 2002, work was complete.

The project consisted of widening the embankment to carry the highway and building 8,000 square feet of retaining wall to contain the widened section. At the start of construction, Whitaker Contractors Inc. excavated a 6- to 8-foot-deep trench along the toe of the embankment’s slope. The contractor next placed a leveling pad of aggregate base into the trench. Perforated plastic pipe, 150 mm in diameter, was installed for drainage along the slope side of the leveling pad.

Next came the segmental retaining wall units. Each block was 18 inches wide by 8 inches high, and extended 20 inches deep into the embankment. “The first course of block got geogrid on top, the third course got grid, then the fifth, and so forth,” says Gregory. The geogrid was placed to extend 11.5 feet from the wall into the embankment. “You cut 11.5-foot-long pieces from the roll and lay them perpendicular to the face,” he continues. “That way the strong dimension is perpendicular to the wall.” Each course of blocks was backfilled with crushed stone. After the stone was placed against the wall, the contractor compacted 8-in. lifts of soil into place.

“The spacing of the geogrid is controlled by seismic design,” points out Gregory. “We placed a grid every 16 inches. If it were not for seismic design, we could have spaced the grids every 24 inches.” He says the wall face was battered at a 1:8 slope–for each 8 inches of vertical height the wall was set back 1 inch, mainly for aesthetic reasons.

“The geogrid creates an internally stable block of soil,” explains Gregory. “And that block of soil acts as a gravity structure to retain the rest of the embankment. Once you make the wall and the geogrid-reinforced soil into one unit, that unit acts as if it were a gravity retaining structure.”