The Many Types of Concrete Retaining Walls_tcm45-343588

Precast walls are one of the many new ways of con-s t ructing concrete retaining walls. In some in-

stances, they cost less, take less time to build, and causeless traffic disruption than traditional cast-in-placewalls. Some systems have been in use for more than 40years; others are being introduced now.

W h e re are they used? Where ver retaining walls are re-q u i red: They support highway fills or cuts where there’snot enough room alongside the highway for an unsup-p o rted slope. In cities, they keep highway expansionswithin existing ri g h t s - o f - w a y. In wilderness are a s, theym i n i m i ze the impact of a highway on the enviro n m e n t .Some systems are used to construct bridge abutments,s e a w a l l s, and building foundation walls. Others cre a t ea t t ra c t i ve facades for landscaping around houses, store s,and office buildings.

Crib walls

At least 40 years old, crib walls are the oldest pre c a s tretaining wall system in use. They are assembled like alog cabin. First, workers set precast logs called headerson the pre p a red subgrade at equal interva l s, each per-pendicular to the wall face. Then, they set two rows ofp recast logs called stretchers across the tops of the head-e r s, each parallel with the wall face. The front row ofs t retchers forms the face of the retaining wall. The cel-

lular spaces between the stretchers and headers arebackfilled with gra vel, sand, stone, or compacted eart h .To g e t h e r, the backfill and the precast logs form a com-posite gravity wall.

One nationwide supplier says crib walls cost 20 to 50p e rcent less than conventional cast-in-place walls. Ac-c o rding to this supplier, they can be built economically 6to 40 feet high. High walls are made possible by buildingdeeper cribs or by building one or more cribs behindthe front cri b. Wo rkers connect these lower back cribs tothe front crib by interlocking the headers. They also re s tthe cribs on a back slope to further increase the wall’sresistance to ove rt u rn i n g .

The size and style of the precast logs va ry, dependingon the pre c a s t e r. One company makes 5-foot-long, 125-pound stretchers that can be placed by hand. Anothermakes 12 1⁄2 -foot-long stretchers that can be placed by asmall cra n e, forklift, or backhoe.

Spaces can be left between the stretchers for plantingv i n e s, shru b s, or gra s s. The stretchers also can be in-stalled without spaces to create an attra c t i ve concrete fa-cade with no large plain surf a c e s.

The many types of concrete retaining wallsChoose from more than nine precast wall systems

Crib walls are built like log cabins. Workers overlap the endsof precast “logs” to create hollow cells, then they fill the cellswith granular soil.

For high crib walls, workers assemble two or more interlockingcribs behind the front crib. Space may be left between thestretchers for planting vines, shrubs, or grass.

Bin walls

Interlocking precast bins stacked on top of each otherand filled with granular soil also retain earth. The gra n-ular fill and the bins act together as a gravity wall.

One pro p ri e t a ry system uses bins (or modules) with a4x8-foot face (see photo on page 697). Wo rkers stack thebins in a running bond like building blocks. They lay,fill, and backfill one course at a time. The blocks inter-lock so no mechanical or grout connections are neces-s a ry. With a special lifting device, a crane can unload onebin from a truck and set it into place within 2 minutes. Af o u r-person crew can install up to 2,000 square feet ofwall in a day. Aside from the foreman, all that are need-ed are two workers to guide the bins and a crane opera-t o r.

Another system now being introduced uses 5-foot-high bins with a 71⁄2-foot-long face. Wo rkers set the binsat 41⁄2 -foot intervals and then insert front and back dro p -in panels between them. When they place the nextc o u r s e, they offset the bins so they sit on the dro p - i npanels in the course below. In this newer system, the sidewalls of the bins step down from front to back so thatthey carry the lateral earth loads to the ground. By va ry-ing the widths of the front and back panels, the contra c-tor can produce curved walls with a radius as small as 9feet. Both systems are erected on concrete footings.

Bin walls can support both cuts and fills. The Fe d e ra lHighway Ad m i n i s t ration (FHWA) says they can cost 30to 50 percent less than conventional cast-in-place walls( Ref. 1). Because bins can easily be dismantled andreused elsewhere, they’re also practical for tempora ryc o n s t ruction walls. When used as seawalls, sometimesthey are filled with concre t e.

Precast T-shaped units

The building block for this system is a T-shaped pre-cast unit. The top of each T forms the face of the re t a i n-ing wall, and the stem acts as a soil anchor. The face ofeach unit is 5x21⁄2 feet. Stems va ry from 4 to 20 feet long,depending on the height of the wall. Ac c o rding to the de-veloper of the system, the T-shaped units can be stackedup to 27 feet high.

To increase the friction resistance with the backfill, thestems are precast with ribs on the sides and notches onthe top and bottom. A 12-inch-long precast block is setin the back notch of eve ry unit as a shear key. Be c a u s ethe top and bottom of each unit are identical, it doesn’tmatter which end is set up or down. This simplifies stor-age and erection. A small ride-on roller or walk-behindtamper can be used to compact the backfill in the 5-foot-wide space between stems.

The developer introduced this retaining wall system inDecember 1986. He plans to provide engineering, mar-keting, form s, and technical assistance to local pre c a s t-e r s.

Stacked bottomless troughs

In this system, the stretchers and headers of a crib walla re precast together as one piece. The resulting modulelooks like a trough with no bottom. Wo rkers stack onemodule on top of another and then backfill them to cre-ate a composite gravity wall. The shelves created by thes t retchers are planted with vegetation that eve n t u a l l yc overs the entire wall. Less than 50 percent of the ex-posed face is concre t e. This discourages graffiti, incre a s-

A licensed precaster of a new bin wall system built this 18-foot-high wall in mid-June. It’s the first wall built with this kindof bin. Workers set the bins at intervals and then filled thespaces between them with front and back drop-in panels. Togive visiting transportation engineers a better view, theydidn’t backfill the bins until the entire wall was erected.

T-shaped precast units stackup to 27 feet high. Notchesand ribs in the stemsincrease friction betweenstems and backfill.

es noise absorption, and permits plant grow t h .In the past 11 ye a r s, more than 300 of these walls have

been built in Sw i t zerland and surrounding countri e s.Sixteen walls we re built in the United States in 1986, thefirst year they we re available here. The tallest is 54 feeth i g h .

The trough-like modules are available in a va riety ofwidths and lengths for use in a va riety of situations:stepped and sloped retaining walls, freestanding noiseb a r ri e r s, and re vetments for rocky slopes. They can beinstalled with backhoes or mobile cra n e s. Ac c o rding tothe designer of the wall, a 4- or 5-person crew with a hy-d raulic exc a vator can erect 40 square feet of wall eve ry 10to 15 minutes.

Precast counterfort wall

This patented system consists of triangular counter-f o rts and curved or flat facing panels. Wo rkers first setthe precast counterf o rts perpendicular to the wall face atequal interva l s. Then they place the precast facing pan-els between the counterf o rts and backfill the panels andc o u n t e rf o rt s. Local soil usually can be used as backfill,and it usually can be placed with normal heavy equip-ment, such as front-end loaders and bottom-dumph a u l e r s. This minimizes or eliminates hand placing andcompacting. Also, no mechanical fasteners or cast-in-place concrete are used.

This system was first used in 1975 along Interstate 70in Co l o ra d o’s Vail Pa s s. A tiered wall with curved fro n tpanels was built 56 feet high. Since then, this system al-so has been used for seawalls along Ut a h’s Great Sa l tLake and Te x a s’ Gulf Coast. Ve rtical or tiered walls up to70 feet high are possible. One crew can place up to 3,500s q u a re feet of wall per day.

Precast cantilever wall

One precaster has developed large L-shaped pre c a s twall units. Wo rkers set the units on a granular base andbackfill them to create a cantilever wall. The wall unitscome in 81⁄2- and 10-foot heights. Both front and backfaces are ribbed. Bo l t spassed through theribs of adjacent panelsand tongue-and-g ro ove joints betwe e npanels help keep themaligned during con-s t ruction. Each unit,h owe ve r, is self-sup-p o rt i n g .

Since the pre c a s t e rd e veloped these unitsabout 10 years ago, hesays miles of wallsh a ve been installed. Ac rew can erect 200 to300 feet of wall a day.

Precast counterfor ts can create vertical or tiered walls up to70 feet high. Workers install the counterforts at equalintervals, then they set the precast facing panels betweenthem.

“WALL CITY,” GEORGIAThe Georgia Department of Transportation (GA DOT) has

built miles and miles of retaining walls to keep within exist-ing rights-of-way as it expands and modernizes Atlanta’s in-terstate system. In fact, so many miles of walls haverisen—or cut—through Atlanta that State Bridge EngineerCharles Lewis has nicknamed the city “Wall City.” We sus-pect a few motorists have, too.

On each retaining wall project, the GA DOT offers a con-tractor four options: two proprietary mechanically stabilizedearth (MSE) systems, a proprietary bin wall system, or aMSE system called Georgia Stabilized Embankment(GASE), which was developed by the GA DOT. Contractorsthen bid the project based on the construction methodthat’s most economical for them. According to Lewis,some projects have been let with three different retainingwall systems. On one side of an expressway you can find aMSE wall and on the other side a precast bin wall.

Costs vary with each project, but all four systems rangefrom $30 to $40 per square foot of wall face. Nine yearsago, before Georgia used MSE and bin walls, a 30- to 40-foot-high cast-in-place wall cost $60 to $70 per squarefoot. Georgia still uses cast-in-place gravity walls for wallsup to 7 feet high. And for the few 7- to 17-foot-high wallsthat are needed, cast-in-place cantilever walls are still eco-nomical at $27 to $30 per square foot. For walls higherthan 17 feet, a MSE or bin wall has always been more eco-nomical in Georgia.

Precast panels with integral columns

Retaining walls up to 12 feet high can be built using asystem more frequently used for screening and noiseb a r rier walls (see Co n c rete Co n s t ruction, May 1987,page 485). Fo u r-inch-thick re i n f o rced concrete panelsa re each precast with an integral 12x12-inch column atone end. A gro ove cast into the column re c e i ves thetongue formed in the opposite end of the adjoining pan-el. To resist the lateral earth pre s s u re s, each column iswelded to a cast-in-place pier. The pier and column actas one rigid unit.

Us u a l l y, workers construct all the piers first. Theyauger holes, install re b a r s, and place concre t e. Then theyunload each panel from the truck and slip it directly in-to the tongue-and-gro ove connection of the pre v i o u s l yset panel.

For noise and sight barrier walls, the columns usuallya re spaced eve ry 12 feet. For retaining walls, this spac-ing is reduced as wall height is increased. For the maxi-mum 12-foot-high wall, columns are spaced eve ry 5 feet(and panels are 5 feet wide).

Precast columns and stretchers

In this system, workers place precast columns with in-t e g ral spread footings 9 feet on center. Then they slide1-foot-deep precast stretchers down the slots pre f o rm e din the columns. Next they backfill and compact an 18-inch-thick blanket of gra vel behind the wall in 1-footl i f t s. They use lasers to align the columns and wedges tokeep the stretchers in position before backfilling.

Because none of the precast pieces weighs more than2 tons, the same machine that exc a vates can install thecolumns and beams. The inventor of the system says

walls can be constructed up to 10 feet high, and 40 feet ofwall can be installed in 1 day. Double and triple tiere dwalls also are possible. Each tier is stepped back a dis-tance equal to the height of the tier below. In the 3 ye a r si t’s been ava i l a b l e, the system has been used in morethan 10 installations.

Block walls

Se ve ral precast companies use block or paver pre c a s t-ing machines to make special precast units for re ve t-ments or small retaining walls. Most of these walls areused for landscaping. They usually cannot be built high-er than 6 feet, but the large interlocking concrete unitsd e veloped by one precaster can be stacked up to 25 feethigh. When these large 1x2x6-foot blocks are placedhigher than 8 feet, special precast headers are installedb e t ween the face units in eve ry other course.

A pro p ri e t a ry system designed for walls up to 10 feethigh uses 8x8x24-inch precast units for footing blocks,wall blocks, and deadmen. Wo rkers set the units on their

sides to construct the footing. They stack them in a ru n-ning bond to create the wall. For walls higher than 2 feet,they also bury the units in the backfill as deadmen. Theends of the steel rods that connect the wall and dead-men slide into T-shaped slots pre f o rmed in the back faceof the units.

All these block walls are laid up without mort a r. Mo s tcan be placed by hand, but even the largest re q u i res nom o re than a light cra n e. Many are self-aligning; oncethe first course is properly placed and leveled, the batterand alignment of following courses is automatically en-s u red by the interlocking of the individual units. So m eunits create a different look simply by installing themupside dow n .

—by F. A. Randall, Jr., and Mark Wallace

Workers slide precast stretchers into slots in precastcolumns to create retaining walls up to 10 feet high.Columns are precast with integral footings.

Used for revetments or small landscaping walls, concreteblocks vary in shape, size, and color. Most can be placed byhand. Many interlock for automatic alignment. None requiresmortar.

Reference1. Leary, Robert M., and Gary L.Klinedinst, “Retaining Wall Alternates,”Federal Highway Administration, No-vember 9, 1983.

Editor’s noteTo learn about another idea for precastretaining walls, read “Precast TwinTees Make Attractive Retaining Wallsfor Highway,” Concrete Construction,March 1987, pages 304-306.

In researching thisarticle, we learned aboutmore than 20proprietary concreteretaining wall systems.Several precast systemsare described here. Part I,which appeared in July,described cast-in-place,mechanically stabilizedearth (MSE), ground-anchored, soil-nailed,and deadmen-anchoredwalls. Even combined,though, both parts don’tcover every existingsystem. If you know ofanother concreteretaining wall system,please write and tell usabout it.