THE SITUATION 

No one ever wants a wall failure to occur, but when it happens it is very important to have a thorough

analysis and the right people on hand to ensure it doesn’t happen again. After an especially heavy and wet snowfall in April 2018, approximately 45 lineal feet of the 72,000 +/- square foot Woodbury Lakes retaining wall project, originally started constructed in 2003, failed causing a significant amount of the wall to need to be reconstructed.

This reconstruction project provided a variety of challenges for the design and construction teams to ensure the future stability and integrity of the wall. Project Owners, Ramco-Gershenson Properties Trust called in Daniel Spederspiel of RPT Realty to assist in finding the right design and construction team to do the job.  After some initial meetings Spederspiel secured Matt Barron with Hardscape Construction and Kyle Huerd, PE with GeoWall Designs to make up the project construction team and get the wall reconstructed.

THE PROBLEM

Before the wall section could be removed and rebuilt, the team had to first determine why the wall failed and what must be done to repair and reinforce the structure.  The segmental retaining wall industry has transitioned since the project started in 2003.  With standards of construction, design, and oversite changing from the time the wall was originally built, Barron and Huerd had to create a stable structure that would meet requirements today and into the future.  Best Practices for both construction and design were followed in order to achieve the best solution for the owner.  Although only a 45 ft section of the wall had catastrophically failed, multiple other portions of the wall were showing failed conditions by industry standards.  Although reconstruction hadn’t begun yet, it appeared that settlement was one of the multiple reasons the wall was compromised.

Internal, external and failure mechanics were run to determine slip arcs and find the exact point at which the wall failed in order to determine exactly how much of the existing structure would need to be removed and replaced. The original estimate of settlement was approximately 14-20 inches which took place at the top of the structure.  Since the soils below the wall did not appear to fail in bearing, the settlement would decrease while nearing the bottom of the wall. Originally, the retaining wall bid was to replace the top 25 courses (~17 ft) for a 600 LF span of wall, but after the full-scale design analysis, it was determined that roughly 40 courses (~27 ft) would need to be replaced. This depth of reconstruction followed the clear demarcation from the 4-course (32 inch) grid spacing with lighter grid at the top of the wall met with the closer grid spacing and heavier grid used at the bottom of the wall.

THE SOLUTION

Once the initial assessment of the site was complete, Hardscape Construction secured Frattalone Companies to excavate the site and get it set up for rebuild.  Before large scale excavation began, Barron and Huerd conducted test digs following the path where the grid had ruptured to where the geogrid was still intact. These field verifications were completed not with a surveyor team but by using field construction equipment and the precision of the operators. It is with these verification tests and the communication between excavators, field personnel, and designers that critical information was

discovered. A plan had been theorized to this point, but the field investigation allowed the construction team to qualify those Ideas and create a proper plan to eliminate unforeseen issues before reconstruction began.  The reconstruction process would now include the top 39-40 courses of the wall in the highest portion of the 600 LF section.

With careful site considerations and Frattalone creating a 3D image of the excavation prior to digging, the team was able to handle the challenges of the catch basins and water detention system located within the cut zone. It is with these drawings and the communication with the rest of the team that they were able to remove an unnecessary catch basin and realign storm sewer pipes to straighten the remaining and new storm sewer system.  With major excavation needed, the construction team was pressed for time to have the site closed prior to the Minnesota fall/winter months.

When it came time to begin the reconstruction, Hardscape Construction followed the industry Best Practices guidelines for segmental wall construction by using select granular backfill, 2-course geogrid spacing, and a high level of compaction. This process was selected by the contractor and approved by the geotechnical engineer before reconstruction began.  Construction began at the lowest point of the southern wall and was planned to move north, towards the fallen portion of the wall.

Reconstruction moved smoothly along the 600 LF of wall to be replaced, but when excavating into the fallen portion, they found some of the softest and wettest soils.  Field measurements had shown 6-10 inches of settlement in a large portion of the wall to this point, but the fallen area had 12-16 inches of settlement where the reconstruction project was supposed to end. Due to this development, the team determined the wall reconstruction would need to extend past the original parameters until better soil and no further geogrid layers were ruptured. In addition to these challenges, the team had to face perhaps the biggest challenge of all – Mother Nature. While rainfall during construction is always a challenge and requires planning for surface water diversion, consistent storms during the 2019 summer of reconstruction was shown to be the wettest year on record for Minneapolis.  Without the proper planning, continuous site analysis throughout the process and the excellent communication between the teams on the project, the weather alone could have delayed or derailed the project.

THE RESULTS

The Woodbury Lakes reconstruction project presented a wide variety of challenges for the engineering and design teams as well as the excavation and construction teams. Without the teamwork and communication throughout the process and the meticulous testing and evaluation of the area, this project might have had a different story. However, the combine efforts from Hardscape Construction and GeoWall Designs and their years of experience and industry knowledge, allowed this project to be completed efficiently and completely.  In the end, the reconstruction of Woodbury Lakes project was completed on time, using updated construction best practices techniques, thorough testing throughout the design and construction process and the superior communication of the entire team. The Woodbury Lakes retaining wall will now stand tall and proud for a very long time.

DOWNLOAD CASE STUDY

The parking lot built in the late 1980’s was originally designed to be a raised single-story lot for a commercial building space. With native swamp land so close to the property, the wetlands were filled in near the building allowing for the retaining walls and parking lot to be built. As time went on, an additional parking level was added to the structure to make room for the growing need for space. Now, over 30 years later, the upper parking lot is stable in its deep pilings while the original parking lot has settled more than 2 feet causing the need for the retaining walls to be replaced and the parking lot to be restored to its original elevation.

Project owners called in Dave Klein with Hardscape Construction and Kyle Huerd with GeoWall Designs to assess the situation and come up with a plan of action. This reconstruction project provided a variety challenge for the design and construction teams to ensure the future stability and integrity of the parking lot.  With boggy ground, bulging wooden retaining walls and a sagging parking lot to contend with, there was more to the reconstruction than simply rebuilding it if the settlement issues were to be contained.

THE SITUATION

When the original parking lot was constructed, over 8 feet of swamp land was filled in to create the space. So, before a new design for the lot could be developed, the soil needed to be tested to determine exactly where the peat was located and how much more settlement would be expected to the area over time.   WSB Engineering was called in to do some testing of the soil under the lot.  They used a Shelby tube to collect undisturbed soil samples to determine soil density and water content in order to evaluate the current level of strength and compressibility.  They also did some consolidation testing to get an idea of how much more settlement could be expected with the existing soil. This data helped the design team to better understand what was going on under the lot and allowed them to come up with solutions for the settlement issue.

The initial testing determined that the lot was basically floating on top of the bog, and they were going to need to be careful about adding any additional surcharge to the structure if they were going to keep the lot from settling further.  The original retaining walls holding the parking lot in place were a set of terraced timber walls. These were to be replaced with a single concrete block retaining wall using the Veteran Pro block with the hewn face.  This meant that the aggregates used to fill in behind the new wall were going to need to be lighter so as not to add additional pressure.  This is where Hardscape Construction and GeoWall Designs had to think outside the box to find the right solution.

THE SOLUTION

Using lighter fill options for retaining walls is not a new concept and there are a variety of options available on the market. GeoWall Designs suggested foam to Hardscape Construction as an initial solution to the project. They determined that the cost and the learning curve for the crew would be over the budget and went back to the drawing board for other ideas.  They next came up with a lightweight aggregate that was approximately 40 lbs. per cubic foot but ruled this out as well do to cost. Which led them to their third option, recycled tire chips. Typically used in road applications in swamp areas with a great level of success but had never been used in a wall application such as this.

In order to consider these recycled tire chips as a viable option, Kyle Huerd with GeoWall Designs needed more information and contacted Liberty Tire Recycling, based out of Pittsburg, PA.  Tim Landers, the Midwest sales director based out of their Savage, MN manufacturing location, provided Huerd with MN DOT testing, and a variety of other information of what this type of aggregate was capable of.  Through their communication they determined that the recycled tire aggregate in combination with granular sand could meet the need for a lighter aggregate and still provide the right amount of friction needed while staying within the cost constraints of the project.

Since tire chips had never been used before on a retaining wall and there is no compaction testing, the design and construction teams, knew they would need to do extensive field testing to make sure they had the right mix. The plan was to use a mix of 75% locally manufactured recycled tire chips and 25% granular sand behind the retaining walls. This mixture would be hand mixed on site as they backfilled the wall.

THE RESULTS

WSB Engineering was on site to do testing on the soils during every step of construction to ensure they were on track.  The construction teams also made a variety of on-site decisions based on what they found during excavation and construction. In fact, they wound up excavating an additional 2 ft. of depth to hit solid ground when they found a large pocket of debris in the soil at the original depth.

Fabric was used at the base level with double wrapped geogrid and rock to reinforce the base and essentially create a floating platform above the base.  All of this was extremely complicated in application because of the proximity to the ground water and the swamp itself.  When it came time to begin the backfilling behind the wall, they faced their next set of challenges.

On the first day of backfilling, they found that the 75/25 mix just did not have the right friction ratio and based on the years of experience between the design and construction teams they determined a 50/50 ratio would be a more conservative mix.   They used the best practices for retaining wall construction guidelines to install geogrid every two courses throughout the retaining wall and brought in additional granular sand to get out of the ground water at the base. To finish the structure, they added a foot of Class 5 material before installing the asphalt to top off the parking lot.

While this project had an interesting degree of difficulty and a variety of challenges, the project owes its success to the extensive testing by WSB Engineering and the combine efforts of the Design team at GeoWall Designs and the Experience of the crew at Hardscape Construction. Without their “outside the box” solution and the communication between all the teams involved, this project would not have come out quite the same.  The original parking lot and retaining walls settled over 16 inches in the 30 years since construction. The new lot and walls are projected to settle only 1-2 inches in the next 30 years. An amazing improvement indeed.

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Just north of downtown Minneapolis, Minnesota, lies a bustling business center full of industrial properties that have seen a variety of uses over the past 50 years. Revitalizing one such property to improve safety and aesthetics while increasing the usability of the space for the current tenants of the building during one of the wettest seasons in Minneapolis was a distinctive challenge.

THE SITUATION

The property, owned by Capp Properties has a couple buildings they rent out to a dog obedience trainer and a plastics manufacturer and had a large grass berm next to it being held up by a 6-foot treated wood retaining wall that was in dire need of replacement. “That old wooden wall was rotting out and was really close to failing. I was afraid I’d wake up one morning to find the upper parking lot had slid into the lower parking lot,” said Lee Virock, a 21-year veteran of Capp Properties.  “That wall was there before I started working here. The tie backs were rotted out. It was old, and it was a real problem.” That is when they turned to the contractor and Kyle Huerd at GeoWall Designs for help.

THE SOLUTION

The site was a challenging one. “First of all, the plans from the 1950’s weren’t as precise as we would have liked,” Virock said referring to the plans for the water utilities on the site. “We found the fire suppression sprinkler supply line while excavating. It wasn’t where it was supposed to be.” That workaround changed the wall design midstream. “We went from a 90° corner in the wall to a curved wall, but it worked out just fine and we kept the utilities safe.”

The other issue was tight space. Since they were replacing a 6-foot-tall wooden wall and severe sloping berm with a 16-foot-tall near vertical wall next to an active road—and they wanted to gain 8 feet of space, there was no room to excavate for the installation of geogrid reinforcement.  This is where the engineering design from Kyle Huerd at GeoWall Designs came in to play. Continued communication between the engineer and the construction team allowed the project to move smoothly through the process.

The contractor worked on the project from start to finish. He tore the old, rotting wall out and excavated the site. The on-site soils were sandy and wet and over 250 tons of base has to be brought in. The contractor had to excavate as deep as 4 feet in some spots before the engineers felt comfortable, they were on solid ground. One of the main objectives for the project was to gain parking space for the tenants. Something that’s a rare commodity in the close-quartered industrial district.  To do this, the contractor chose a nearly vertical wall system to gain the valuable space for the added parking.

The contractor chose the Sterling Wall from Belgard for its ability to construct nearly vertical walls allowing for this project to only have a setback of 2 feet, which is a tremendous space saver for a 16-foot-tall wall.

To handle the reinforcement needs of a nearly vertical wall and the tight space, GeoWall Designs specified using a stabilized backfill to meet the needs of the project. The construction crew had never used this type of backfill material before but were impressed with the results.

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The residence has an underground garage directly under the backyard. Retaining walls were cracking and failing. Special consideration was needed to ensure garage and neighboring garages tied to the retaining wall did not collapse during demolition and reconstruction of the retaining wall.

Big Block Retaining Wall Application