The terraced wall rises approximately 30 feet befor reaching an elevation just below the hospital roadway.
A tunnel burrows under the roadway directly into the emergency room.

By Mary E. Kremposky, Associate Editor

Survival Flight, the University of Michigan Health System’s airborne medical transport program, operates on the frontlines of trauma care, evacuating burn victims, transporting critically ill newborns and even carrying transplant organs to appropriate medical facilities. In 1999, this “air ambulance” was named the best air medical program in the country by the Association of Air Medical Services. Thanks to a team of dedicated design and construction professionals, this nationally recognized program might save even more lives. A former helipad contained one landing site and rested on the rooftop of University Hospital; today an intricate segmental retaining wall, built on a slope below the hospital entrance, now supports a new helipad with two landing sites and swift access to the emergency room.

Oxford-based Walltek installed 11,000 face feet of segmental retaining wall for the health system’s new helipad. The terraced wall rises approximately 30 feet in four steps to an elevation just below the hospital roadway. Merriman Construction then bored a tunnel under the roadway and directly into the emergency room. The new helipad is an engineering solution that admirably serves one of the region’s most important trauma centers with an efficiency designed to save lives.

In addition to Walltek, the project team included Albert Kahn Associates, Inc., architect; Hanson Engineering, the Plymouth-based engineer for the segmental retaining wall; The Christman Company, construction managers; W.P.M., Inc., prime subcontractor for all site work; and Unilock, the supplier of the Pisa II and Stratagrid 300 segmental retaining wall system.

A CHALLENGING SITE
The helipad is constructed on a difficult slice of land sloping almost 50 feet down to the edge of a retention pond. Working on a sloped site, all the equipment – including the excavator – had to be steel tracked. “There was nothing on site with wheels in it,” said Todd Garris, owner of Walltek.

The core challenge to the retaining wall’s construction was limited site access and insufficient material storage areas. A gated side road was the site’s sole access point for all contractors. More importantly, the constructed site, an incline sloping down to a retention pond, did not offer room for Walltek to store its usual stockpiles of sand, gravel and block on site. In fact, only one day’s supply of block was allowed on the cramped site. Walltek had to order new block daily, giving Unilock only 24 hours notice for the next day’s block supply.

Both Garris and Burke Hyde, Unilock’s territory manager, give high praise to Salud Villagomez, Walltek’s astute foreman, who ordered and maintained the necessary supply of block on a daily basis throughout the course of the three-month project.

“Walltek did a good job of coordinating with us,” said Hyde. “They gave us at least 24 hours notice, and we realized as a supplier that when Walltek called, they needed it the next morning. We operated in this way because there wasn’t a lot of room for error as far as scheduling. …Everything had to fall right into place or it would have a domino effect on the project schedule.”

Walltek with its crew of six men and a foreman played a pivotal role in the project’s scheduling. The firm had to build the retaining wall up to the level of the planned tunnel entrance before other contractors could begin boring the tunnel. “Everything was dependent on us building up to this pad elevation, so the other contractors could bore their holes and pour some concrete walls near the tunnel entrance,” said Garris.

Walltek worked as the retaining wall sub-subcontractor to W.P.M., Inc., the Grand Blanc-based prime subcontractor responsible for controlling all the site work. W.P.M. was instrumental in keeping the schedule on track and in synchronizing the activities of all site work trades. “Because there are so many incremental steps in building a wall, we are usually waiting on another subcontractor,” said Garris. “For instance, we could be waiting for the mass grader to cut out the site or for the sand supplier. W.P.M. did an excellent job of coordinating the work. They kept our guys moving.” Walltek built the terraced wall in three months, beginning in March 2000 and completing their work near the end of June.

“The project almost seemed choreographed,” commented Hyde. Both Walltek and Unilock attended weekly or biweekly meetings throughout the course of the project. “We are the fourth firm down on the totem pole, but even as the sub of a sub we had a key role and attended quite a few meetings,” said Garris. Even before construction began, the endeavor required a year of design and engineering with Albert Kahn and Unilock, the firm that in turn contacted Hanson Engineering for the wall’s required geo-technical engineering design and support. Walltek provided a cost estimate for the wall’s construction a year before the first block was installed on this intricate segmental retaining wall system.

ENGINEERING A WALL
The original concept called for a single wall approximately 28 feet in height. After consulting with Hanson Engineering, the wall was broken into four terraces, each about 9 to 10 feet in height with roughly 6 feet of level ground forming each step, said Garris. Unlike a large, monolithic wall, terracing produced a structurally stronger wall, more effectively spreading and distributing the load, added Hyde. The terraced wall also offers some built-in safety features in the event of a fall. “A fall 9 or 10 feet to a terrace step is inherently safer than a 30-foot fall from the top of the wall,” said Hyde.

Each terrace level is built of three elements: the block itself, compacted Class 2 sand fill and a series of geogrids linking the block courses of the lower wall to matching courses on the wall above. Block, soil and grid interact as a cohesive system to create an evenly distributed load.

Garris explains: The geogrid locks into the ridge-shaped tongue and groove connection in the middle of the block, providing a continuous connection of the grid along the entire length of the wall. The grid is then stretched out perpendicular to the block course. Finally, the grid is buried in layers of the compacted sand behind the block and actually takes the load of the soil.

“We stretched the grid, so that it is now taking the load of that soil that would otherwise be bearing on the face of the wall,” said Garris. “The geogrid actually distributes the load behind the wall which is why this segmental retaining wall is termed a soil-reinforced mass.” This project required three to eight layers of geogrid ranging in length from 5.5 feet to 6.68 feet, depending upon which wall of the four they were installed, according to a Unilock case study.

The project is a perfect marriage of a well-manufactured system with a skilled installer. “Tying the grid from the lower wall into the bottom courses of the next wall rising up is not easy to accomplish,” said Hyde. “You are basically tying the walls together, and your elevations have to be right on the money. You have to line up the top courses of block on the lower wall perfectly with the bottom courses of block on the next wall.”

Walltek employed lasers to achieve these critical elevations. “By far, this is the most complex retaining wall we have ever done,” said Garris. “…Unilock provided great technical support.” As a further complication, radiused walls form each end of the helipad, the only straight stretch being the middle of the retaining wall.

In addition to the geogrid connections, adequate drainage and proper compaction of the fill were key elements in ensuring the structural integrity of this project. A foot of gravel protected by a filter fabric tile was installed behind the block for drainage. “Drainage is key,” said Hyde. Walltek even had to build forms to control the migration of the two soils – the gravel and the Class2 sand, added Garris.

The whole slope is mostly built-up with engineered fill, mainly Class 2 sand that was compacted, measured in place and monitored via compaction tests performed at 6-inch intervals. “As we built the first wall we had to make sure that the compaction of all the soil behind that first wall was adequate to take the load of all the other terraces,” said Garris.

Altogether the retaining wall system required 18,000 cubic yards of Class 2 sand behind the four terraces. “These walls are challenging to begin with because of all of the fill going in behind them,” said Garris. “It’s all a matter of how fast can you get that much sand behind a wall and compact it in place and test it.” About 500 semi loads of sand were shipped to the site over the course of the job. About thirty to thirty-five trucks arrived daily carrying material to supply four contractors on this confined one-acre site.

THE BUILDING BLOCKS OF SUCCESS
The reinforced segmental retaining wall (SRW) system is both aesthetically pleasing and economical to build. The block offers a pleasing texture and color for the sizeable wall on this visible and exposed site. The entire wall is built of roughly 33,000 blocks with 3 per face foot, said Hyde. SRW’s are a dry-stacked, gravity system without the use of mortar.

Horizontal fall protection installed along the rim of the helipad
is safely removed from rotating helicopter blades.

The final project component was installation of proper fall protection. Vertical railings at the top rim of the wall were unacceptable due to the danger of helicopter rotor blades possibly hitting the rails. The solution was to attach a system of horizontal grates to the wall by coring through the wall and anchoring this system behind the wall. After coring small holes through the block, the crew ran supports that actually tied to the deck way back into the system.

An established and vital company, Walltek recently added a civil engineer, Chad Clark, to its team as a means of providing clients with invaluable engineering support services.

Completed in summer 2000, this complex segmental retaining wall now supports the aluminum deck of Survival Flight’s new helipad where a skilled installer, adept engineering, and well-manufactured system together produced a structure designed to help save lives.

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