What a difference a growing economy makes. Omaha, Neb., once considered a quiet hamlet in flyover country, is now anything but a sleepy-eyed community.
Look at any list of top city rankings and you will find Omaha among America’s hottest spots for business expansion, careers and even having babies. During the past six years, Omaha’s central business district has benefited from more than $2 billion in development. All of this has Omaha roads bursting at the seams. Once bucolic county byways lined by cornfields have become bustling urban thoroughfares with thousands of commuters making their way to and from the city center. Keeping up with this growth means improving the roadways.
It is Tim O’Brien’s job to make sure the city’s road budget stretches as far possible, so the Omaha construction engineer in charge of the construction division is ever on the lookout for new methods and materials that will make roadwork faster, cheaper and better. One of the biggest challenges in upgrading Omaha’s country roads to city standards comes with leveling the gentle bluffs characteristic of rural highways. City standards require longer sight lines. Leveling existing roads often means building miles of retaining walls so that the new road can run an even course without disrupting existing subdivisions and adjacent streets.
When O’Brien sat with city engineers to design improvements to Harrison Street, an east-west county road along the Douglas and Sarpy county lines, the task of widening the rural route from two lanes to four was complicated by buried power, cable and phone lines flanking the city right-of-way. With nearly 2,000 ft of 16-ft-high retaining walls needed, the 13-ft-wide cantilever footings and sloped bank cuts required for such tall walls meant relocating about 13 city blocks of buried cable, disrupting adjacent properties and slowing construction to a crawl. Homeowners did not want their yards torn up and a historic cemetery boarded along one 700-ft stretch of the road. Fortunately, O’Brien came across a new retaining wall product that proved not only efficient, but saved the city coffers nearly $250,000.
A load of gravity
Every contract using federal dollars contains a “value-engineering” clause. Value engineering means finding new, more cost-effective ways of building a desired product. Contractors and engineers are encouraged to find better and cheaper ways of achieving performance goals through financial incentives. In simple terms, if a contractor can devise a means of reducing cost without compromising quality, the contractor gets to keep half of the savings. This is why Tom Crockett, vice president of Hawkins Construction, Omaha, and project manager for the Harrison Street project, was delighted when he had the opportunity to help the city find a better way to build the retaining walls along Harrison Street. Not that the task did not have challenges.
The Harrison Street retaining walls were originally designed as cast-in-place monoliths with huge cantilever footings. To install these footings, Hawkins would have to coordinate with three utilities to remove thousands of lineal feet of cable, excavate, build forms, tie steel, pour concrete, backfill, re-compact and then coordinate with the utilities to relay the cable—all of this during a Midwest winter. O’Brien had seen a presentation about a new, large-scale gravity-wall retaining block by Stone Strong Systems that promised certain advantages. It did not require over-excavation or cantilevered footings.
The system consisted of interlocking hollow precast blocks with unit sizes available from 3 sq ft (24 in. x 18 in.) to 24 sq ft (96 in. x 36 in.). Unlike other gravity walls, this one did not require a geogrid tieback or gravel backfill and would allow Hawkins Construction to place retaining walls without disrupting existing utility lines or digging up the residential yards. The absence of geogrid also had long-term benefits near utility easements, explained Tom Glow, project manager for the Omaha Roads Department. “After a few years, when a utility contractor runs a trencher or borer along the easement and gets hold of the geogrid and cuts it, the wall’s no good anymore,” he said.
Although the large gravity-wall blocks looked like a sure-fire solution, problems remained. The walls were designed to sit on a gravel footing and stack about 8 ft high without tiebacks. The Harrison Street walls had to be 12 to 16 ft tall. Moreover, some of the walls doubled as sound barriers, meaning stretches would remain exposed on both sides. The chosen gravity wall blocks had only one finished face, the backside was plain, coarse concrete. Crockett called Daniel J. Thiele, P.E., of Thiele Geotech Inc. for suggestions. Thiele helped design the block and thought the unique challenges of the Harrison Street project not only had solutions but also offered an opportunity to develop a new product line.
The height problem was not new; Thiele had coordinated construction of the wall system up to 30 ft tall. This was a question of filling the bottom two courses of open cells with concrete and adding steel and solid filled cores to resist the soil pressures. Nevertheless, the sound walls exposed on two sides presented a challenge. After consulting with the manufacturer, Thiele met with John Gran, vice president of Stone Strong Systems, the precast producer that would supply block for the project, and began the process of designing a new double-sided block.
On both faces, front and back, a split granite appearance would enable contractors to combine retaining wall and sound barrier into one. This also would add flexibility to the construction process, given that along some stretches of Harrison Street the grade switched from high to low relative to the adjacent streets, meaning the exposed area of wall altered from front to back. Once the double-faced block became a reality, O’Brien gave Hawkins the green light to proceed with the gravity-wall system.
Since Crockett no longer had to coordinate with the utility companies for line relocation, work started about a month ahead of schedule. A typical gravity-wall installation involves excavating a 60-in.-wide trench, 9 in. deep, filled with 11?2-in. compacted limestone or lean concrete. In this case, the added height made it necessary to pour a concrete footing, but even at 5 ft it was small in comparison to the 13-ft-wide behemoth needed for a cast-in-place wall. Excavation was not a problem, given the 16-ft-tall wall only required about 40 in. of working space for stacking. After workers set the first course of block over the footing and leveling to assure proper line and grade, they poured additional concrete into the open cells and started assembling the rest of the wall.
Although Hawkins Construction had never used the gravity-wall block product, installation was simple. With a three-man crew and a Caterpillar 320B excavator, the work proceeded smoothly with no time lost to a learning curve. “You’re just stacking blocks, there’s not that much to it,” said Crockett.
Made with ordinary 4,000 psi air-entrained concrete, the blocks come in 3-, 6- and 24-sq-ft sizes to accommodate large and small structures in straight or serpentine patterns. The blocks interlock precisely and feature built-in reinforcement bar handles that make it easy to lift and place each 5,800-lb unit with a standard excavator or rubber-tire loader. Two men and an excavator can lay about 2,000 sq ft of block in a day—nearly six times the production of a cast-in-place or small-block installation. After reaching the desired height, simply backfill with dirt or granular fill.
Along Harrison Street, the tall wall section required grout in specified cells, but even this provided certain advantages. The large hollow sections allowed engineers to cast in traffic lights and fence posts. The final product looks like a large hand-hewn stone wall, said Crockett. Eventually, Omaha plans to stain the walls for an even more natural appearance. “People hate looking at gray concrete,” said Glow, “and we wanted this project to appeal to the neighborhoods.”
In the end, the large gravity-wall blocks shaved a month off the project schedule. The precast blocks also allowed wall construction through winter. “The schedule impact would have been even greater during the summer,” said Crockett. Although large segments of wall stood complete, paving still had to wait for good weather. During paving season, the ready speed of a large block system would have allowed the Harrison Street project to move faster.