A Reduction in Excavation and Backfill for a Tennessee Road Widening Project

A Reduction in Excavation and Backfill for a Tennessee Road Widening Project


First row of MSE wall panels are set in front of the excavated slope, and on top of a foundation undercut.
Reinforced Earth MSE Wall
Location: 
Nashville, TN
Owner: 
Tennessee Department of Transportation
Contractor: 
Highways, Inc.

The bottom line is safety, and that was especially true for approximately 12 miles of SR 16/US 41A between Shelbyville and Tullahoma, about 80 miles southeast of Nashville. This corridor was studied in the mid-1990s by Tennessee Department of Transportation (TDOT) for possible operating deficiencies. TDOT determined that the projected 60% traffic growth by the mid-2010s would lead to more accidents and lower overall safety on what was, essentially, a 2-lane country highway.

The recommended solution was “… increasing the number of lanes; increasing lane structure and shoulder widths …,” to be accomplished through a series of 4 contracts.  The recently completed third contract, for 2.2 miles from SR 276 to Rippy Road, was constructed by Highways, Inc. of Cookeville, TN.

As it heads northwest from Tullahoma, SR 16/US 41A passes through rolling and hilly terrain typical of central Tennessee.  In many areas the highway winds along the side of a hill, with the terrain sloping up from the edge of one shoulder and down from the other. Toward the midpoint of the project, Shipman Creek closely parallels 41A, with Shipman Creek Road running between the creek below and 16/41A high above.

Addressing this narrow right-of-way, TDOT’s contract plans called for a contractor-designed retaining wall to support the widened highway above Shipman Creek Road. Special Provision 624, which governed this wall, allowed an MSE wall. The general contractor selected RECo to provide the design. While averaging 35 feet tall, almost two thirds of the wall’s 2,007-foot length is 45 to 50 feet high – a structurally significant wall to be sure.

A typical MSE wall has uniform-length reinforcements from top to bottom and a 70% aspect ratio (aspect ratio = B/H; where B = soil reinforcement length and H = wall height). TDOT’s overall stability analysis resulted in a minimum 80% aspect ratio, accounting for a 3:1 slope rising above the wall and the (TDOT-specified) assumed friction angle of 25° for the retained fill behind the MSE structure. The combination of the 3:1 slope and the 25° retained  fill increased the lateral earth pressure on the MSE structure, pushing the aspect ratio to 90% to achieve sliding stability (Fig. 1a).

Long stretches of the wall required a strip length between 40 and 44 feet, which caused two major problems. First, the cost of the quantity of excavation and backfill, and second, the proximity of the excavation to the existing road, which needed to carry traffic during construction. The issue was further exacerbated by TDOT’s requirement for major undercut and replacement to improve bearing capacity and minimize the settlement due to the weight of the wall.

Not surprisingly, even the lowest bid significantly exceeded TDOT’s budget, so the department requested cost-reducing proposals to avoid rebidding. Retaining wall excavation was clearly a major cost item, so Highways, Inc. asked RECo for help in reducing those quantities. The physical and economic results were dramatic.

In preparing his bid, the contractor had elected to use the same granular material (38° friction angle) for both the reinforced backfill and the retained material behind the MSE structure (Fig. 1b). Using the same material (and, therefore, the same design properties) for both the retained and the select fill is allowed by TDOT if the retained fill extends behind the MSE reinforcements to a line rising at a 1:1 slope from the back end of the lowest reinforcements. Making this change created a significant design benefit because using the MSE select fill instead of weaker material in the retained zone lowered the lateral earth pressure to be resisted by the MSE volume. The result was the cost-saving opportunity of redesigning the Reinforced Earth wall using uneven reinforcement lengths – shorter at the bottom and longer toward the top (Fig. 1c).

Uneven reinforcement length, long part of RECo’s “special situation” design practice, is included in the Federal Highway Administration’s (FHWA) FHWA-NHI-10-024, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes – Volume I (November 2009).  FHWA’s recommendations for the geometry of uneven reinforcement length design state, in part:

  • The soil reinforcement length at the bottom of the wall shall be at least 40% of the wall height, or 8 feet, whichever is greater.
  • The difference in soil reinforcement   length between adjacent reinforcement layers shall be less than or equal to 15% of the wall height.

For example, a 40-foot high wall would need a minimum of 16-foot long soil reinforcements at the base, and the strip length difference between any two layers would not exceed 6 feet. All design rules for sliding, overturning, bearing and internal stability must also be satisfied. Implementing FHWA’s uneven strip length design for this MSE wall did, in fact, yield significantly reduced excavation quantities, as described below.

TDOT specified foundation undercutting and replacement with “graded solid rock”. The required undercut depth varied between 4 feet and 15 feet along the length of the wall. The bottom of the undercut extended a distance of half of the depth both in front of and behind the base width of the MSE wall, with 1:1 side slopes. As explained by Highways, Inc.’s Project Manager, Lance Roach, the excavation width (front to back) required by the undercut pushed farther back into the hillside than did even the excavation to achieve a 1:1 slope behind the as-bid reinforcing strips.

Comparing as-bid (uniform) and as-redesigned (uneven) strip lengths at a typical cross section where the wall is 47.5 feet high, the as-bid bottom strips are 42 feet long while the uneven-design bottom strips are only 23 feet – a 45% reduction. Rising from the base of the wall, uneven strip lengths increase approximately every 10 feet vertically in the following increments:  23 feet, 26 feet, 30 feet, 35 feet, and 41 ft. Always remaining shorter than the as-bid uniform length of 42 feet.

As a result, the 1:1 slope from the back of the strips (to accommodate the 38° granular material in the retained zone) required substantially less cutting and filling than would have been required for the backslope to accommodate the as-bid design. A tremendous cost savings resulted from reducing the strip length by nearly half at the bottom of the wall, and the longer strip lengths rising up the wall easily fit within the 1:1 slope. Wall construction became more economical and faster, and the project was saved from rebidding.

This was Highways, Inc.’s third project with a Reinforced Earth MSE wall. Facing the risk of losing the job to rebidding, The Reinforced Earth Company and the contractor found a solution that saved money, saved time and, literally, saved the project. As Mr. Roach put it, “The wall portion of the project couldn’t have gone better … we would use RECo again in a heartbeat.”