Pärnu Graniit Creates Custom Solution for Europe’s First Redi-Rock Bridge Abutments

The Problem

Redi-Rock retaining wall manufacturer Pärnu Graniit -- who had been subcontracted to design, supply, and install abutment walls for the Mustla Bridge as part of the realignment of Highway 2 in Estonia -- rose to the challenge when it was discovered that insufficient space had been included on the site plan for the traffic barrier.

“We could have let them redesign the road and the road barrier, but we wanted to show how flexible the Redi-Rock system can be in our hands,” said Teet Rehtla, sales specialist for Pärnu Graniit.

Shifting the wall to create space for the traffic barrier meant that only 350 millimeters (14 inches) of space would remain between the front face of the top course of precast modular blocks (PMB) and the two load-bearing support columns for the bridge along the north wall. Pärnu Graniit was going to use Redi-Rock Positive Connection (PC) blocks, which measure 710 mm (28 inches) in depth, for the geogrid reinforced walls, so a creative solution was necessary.

The Solution

“Redi-Rock is a simple idea, but when it gets complicated, we can adjust it to work around a problem,” said Redi-Rock European technical manager Liam Donohoe.

Donohoe helped with the initial concept for the project, the first of its kind in Europe although many similar projects had been completed in North America since PC blocks launched in 2011. With the initial idea in hand, Donohoe then connected Rehtla to Geoman Ltd., a geotechnical engineering firm with locations in Northern Ireland and Spain to dive into the specifics.

One benefit of the PC blocks, which use 300-millimeter (12-inch) wide strips of geogrid that wrap through a center core slot cast into the block instead of sheets of geogrid, is that it makes it easier to accommodate obstructions in the reinforced fill. The strips of geogrid can be splayed up to 15 degrees without compromising the performance of the reinforcement.

“You can usually for small obstructions just splay the grid around it. There's also another standard detail for large obstructions where you use a waler beam along the back of the blocks, but neither of those worked well for this particular case because of the limited space,” said Ken Knox, EIT, of Geoman Ltd.

With both of the standard options off of the table, Knox, Rehtla, and Donohoe created a plan to reduce the width of the blocks where they conflicted with the bridge columns. Pärnu Graniit would create custom block faces about 300-millimeters (12-inches) thick with rebar that protruded from the back of the blocks. The rebar would be utilized to bolt the blocks together with steel channels, as well as thread behind the columns into a system of galvanized steel pipes for the geogrid to wrap around.

The Outcome

The wall construction was completed in early June 2019. The two walls in the Ledgestone texture stretch 90-meters (295-feet) long and stand 4.16-meters (13.7-feet) tall. The near vertical walls contain 10 courses of Redi-Rock PC blocks.

Pärnu Graniit exceeded expectations for the project, cutting the overall cost for the clients compared to MSE panel walls and finishing the project ahead of schedule.

“In the end the wall needs to retain earth, but it also needs to look good,” said Rehtla.
To read a more in-depth case study on this project, visit www.redi-rock.com.

Find out how Redi Rock manufacturers teamed up for emergency landslide repair in our previous case study here.