Form and Function
Retaining walls for extreme sites.
Small retaining walls are frequently used in today’s residential subdivisions to separate building lots, or for landscaping purposes. In many cases, these walls are constructed by the homeowner and only offer aesthetic value. But for large commercial and road building projects, retaining walls are more than just a pretty face. They are often used to support the soil structure or embankments that are below buildings and roadways, and in these instances, it is time to call in the professionals who use sophisticated software and engineering techniques to ensure these walls make the grade for many years to come.
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Photo: International Erosion Control Systems |
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Photo: Schnabel Foundation Co. |
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Photo:Stone Strong Systems |
Saving a North Carolina Interstate
When Hurricane Frances and Hurricane Ivan both hit land within a two-week period in September 2004, the Pigeon River in western North Carolina couldn’t handle the resulting increase in flow, causing the toe of the adjacent embankment to erode. That embankment was supporting several miles of Interstate 40, a major highway connecting North Carolina with neighboring Tennessee. Between mile markers 3 and 4, the slope and one of the eastbound lanes of the highway washed away, leaving only a guardrail hanging in midair.
“The North Carolina Department of Transportation [NCDOT] was faced with a tough problem,” notes Rich Hall, director of marketing at the Schnabel Foundation Co.’s corporate headquarters in Sterling, VA. “They needed to build a wall with a 100-year design life that would prevent future floods from eroding the slope again; keep the remaining lanes of Interstate 40 open to traffic; protect a sensitive environmental site; and build it fast. Several options were considered, including a tied-back soldier beam wall with a concrete face, a temporary soil nail wall to allow the construction of an MSE [mechanically stabilized earth] wall, and a tied-back micropile wall with a shotcrete face.”
In this case, the NCDOT elected to go with the tied-back micropile wall and contracted the Schnabel Foundation Co. to design and build the wall. The Schnabel Foundation Co. is a national design-build contractor specializing in the construction of excavation support systems, soil stabilizations, and retaining walls supported by soil nails, tie-backs, mini-piles, tied-back elements, and soil mixing. “The tied-back micropile wall had several important advantages over the other wall systems considered for the extremely difficult site conditions at this location,” adds Hall. “The difficulties included limited access, the need to maintain at least two open lanes for traffic on the highway, and space limitations that made the use of large caisson drills prohibitive. The embankment consisted of cobbles, boulders, extremely hard bedrock, and a soft soil matrix, making it difficult for drilling.”
By implementing the tied-back micropile wall, crews could use small track drills to stabilize the mass and prevent a landslide on the disturbed slope. The drills are tooled with rotary percussive duplex drilling heads, which allowed the drilling to progress through extremely difficult and variable conditions. “The wall design transfers the vertical loads from the tie-backs and wall components through the micropiles to the stable rock below, and uses the loads from the tiebacks anchored in the stable material to resist the driving forces of a slide,” explains Hall.
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Photo: Schnabel Foundation Co. |
| Crews minded the sensitive environmental site during construction along Interstate 40 in North Carolina (near the Tennessee border). |
The project was completed in the late spring of 2005 within the 100-day time frame that was specified by the NCDOT. Hall points to the joint effort of the companies and organizations involved for the project’s success, including Schnabel, general contractor Phillips and Jordan Inc., engineering firm Wilbur-Smith Associates, and the NCDOT.
Emergency Repairs in Long Island
Farther north, David N. Rackmales, P.E., a structural engineer with the New York District US Army Corps of Engineers (USACE), was also faced with the challenge of building a wall that would maintain the integrity of an embankment that supported a roadway. In this case, it was in Northport, NY, a suburban community approximately 40 miles from New York City on Long Island.
“The roadway is located on the top of a very high bluff, which had experienced some slope failures and erosion in the past, primarily due to coastal storms and, to a lesser extent, wave action at the toe of the bluff,” explains Rackmales, who has been with the USACE for 13 years and has a master’s degree in ocean engineering from the Florida Institute of Technology. “The road was in danger of failing due to coastal erosion, and there had been some collapse of the pavement, so we wanted to arrest the erosion before it got any worse.”
To stabilize the site, Rackmales says several measures were taken. “The corps designed an anchored PVC sheet pile bulkhead wall with toe protection, and in front of that we have riprap and a rocky beach. On the slope itself, soil stabilization measures were taken, including clearing the trees and re-grading and installing a geogrid cellular stabilization system, which was attached to the slope, filled with soil, and then seeded and planted with native species.”
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Photo: Schnabel Foundation Co. |
| Lanes of I-40 were kept open during construction of the wall. |
The PVC sheet pile, which was supplied by Crane Materials International in Atlanta, GA, was driven into the surface and was also anchored to the embankment with helical steel anchors drilled up to 100 feet into the bluff. “This was necessary to achieve the strength that would prevent the anchors from pulling out under extreme conditions,” adds Rackmales.
The project took less than a year from putting the plans out for bidding in August 2004 to completion in May 2005. The USACE and the New York State Department of Environmental Conservation (NYSDEC), which Rackmales says is a similar state entity to the department of environmental protection in most states, jointly funded the project, which cost under $1 million. “This project is an emergency project, not of the same extent as a disaster like Hurricane Katrina, of course, but it is still an emergency,” he explains. “There is congressional legislation in place under Section 14 of the Flood Control Act of 1946, as amended over the years, that allows for federal funds to be released to the Army Corps of Engineers to do a quick study of a shoreline protection problem and to design a solution. These projects have to be of a magnitude that they can be constructed quickly, and there has to be cost sharing between the USACE and a local government. In this case, that was the State of New York whose citizens benefit from this project on a daily basis.”
Rackmales is part of an in-house design staff for the New York District of the USACE. He says the staff also retains the services of architect-engineer (A/E) firms. Rackmales’s responsibilities included providing input to the design scope of work; reviewing of the A/E firm’s design calculations, plans, and specifications; providing structural engineering support to the project during construction; and preparing an operations and maintenance manual that was turned over to the local municipality when the project was completed.
“A cost study was initially done to make sure the project was economically viable,” adds Rackmales. “Typically these types of projects have to fall under $1 million, and in this case, if nothing was done, we could have lost the road. Sooner or later, that would have added up to much more than $1 million in construction and economic costs, and more importantly, emergency vehicles may not have been able to get through on the road if there was a washout, and you can’t put a monetary cost on that.”
When asked about the end results, Rackmales says the citizens of Northport seem to be pleased with the project. “From the feedback we have received, I would say the local residents are happy with the results. In my personal opinion, I think it is beautiful. It is quite an aesthetically pleasing project and, as an engineer, I have to say it has nice lines, it is pleasing to the eye, and it does its job.”
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Photo: International Erosion Control systems |
| For large projects, retaining walls offer more than just aesthetic value; often they are used to support embankments below buildings and roadways. |
 |
Photo: International Erosion Control systems |
Creating Space for a Retail Store
In Mount Pleasant, NY, which is in Westchester County and about 30 minutes outside of New York City, Northeast Mesa had the challenge of designing, supplying, and installing a tiered structural retaining wall for a Home Depot retail store. “This project includes 46,000 square feet of wall that is up to 30 feet high at some points,” explains Giulio Burra, general manager of Northeast Mesa, one of the initial licensees of Mesa Retaining Wall Systems. “This store had been eight or nine years in the planning and zoning stages due to the significant grade changes that were required, and the city had selected the color for the required retaining walls, which is a tan that Mesa could supply.”
As the licensee for Mesa in five states, including New York, New Jersey, Connecticut, Massachusetts, and Rhode Island, the 10-employee Northeast Mesa had the experience to complete this complex project, which required two tiered walls to support the entrance roadway and another two walls to support the building pad and parking lot.
“There was very steep grade from route 9A down to the site, so we had to fill in much of the hole and then use our Mesa block retaining wall to hold the earth in place,” notes Burra. “Although the building is still below highway 9A, this allowed us to get a reasonable grade from the roadway into the site. The second retaining wall supports the soil that is below the building pad.”
The project design was completed by Tensar Earth Technologies, which is a full-service provider of specialty products and engineering services, including patented Tensar geogrids that are designed to interact with Mesa segmental concrete blocks, creating a positive, mechanical, end-bearing connection and having compressive strength that exceeds American Association of State Highway and Transportation Officials (AASHTO) standards (>4,000 psi). “In the design, Tensar addressed seismic and global stability through the use of compaction methods and reinforcing geogrids,” adds Burra.
 |
Photo: Stone Strong Systems |
| Precast modular blocks, massive in size, allow for gravity retaining walls as high as 12 feet depending on soil and surcharge conditions. |
The project was started in February 2006, and the retaining walls, which cost approximately $1.4 million, were completed in mid-May. The rest of the store construction is scheduled to be completed this fall. “The job was done in conjunction with Shawn’s Lawns, which is a large dirt-moving company based in Stamford, Connecticut,” says Burra. “They did all of the site preparation work and drainage, and Geostructures in Carmel, New York, did the actual construction of the walls.”
Working Without Geogrid Reinforcement
At a single-family residential home development near Papillion, NE, Omaha-headquartered Thiele Geotech Inc. is working on a retaining wall project that entails a great deal more than your average subdivision wall. The project, called Shadow Lake, is being developed by Shadow Lake LLC, also in Omaha, but the retaining walls are being installed as a public improvement by Sanitary and Improvement District (SID) 264 in Sarpy County, NE. The SID is a type of municipal corporation that sells bonds to install public improvements and will eventually be annexed when the debt is paid.
“The walls are being installed as wing walls and head walls for two box culverts that were required for street crossing over a drainageway upstream of a planned lake,” explains Daniel J. Thiele, P.E., a licensed engineer in 13 states and president of Thiele Geotech. The company has been in business for 10 years and employs a staff of 55, including engineers, geologists, and technical people. Most of its work is in Nebraska and Iowa, but it has completed projects in over 20 states from the Northeast to the Southwest. The company provides geotechnical, material, and environmental engineering solutions to a wide range of clients, including developers, architects/engineers, contractors, and owners.
“For the Shadow Lake project, the developer and the city desired an attractive ‘signature’ structure at a reasonable cost,” says Thiele. “In addition, the City of Papillion, through extra-territorial buffer for zoning and infrastructure authority, required that no geogrid be used for the retaining walls within 9 feet of finish grade level.”
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Photo: Ultrablock |
| Using a block system as opposed to a cast-in-place wall can save companies considerably when it comes to the cost of materials. |
This stipulation was due to a concern the city had about future maintenance of sewers and other utilities in the right of way. “They did not want the walls to be compromised by damage that might occur to the geogrid reinforcement,” says Thiele, whose client, Linhart Construction of Omaha, is installing the walls as a subcontractor to M.E. Collins Contracting of Wahoo, NE. “To achieve the signature appearance desired by the developer, the walls were constructed to form a ‘land bridge’ across the drainageways. The walls include above-grade, dual-faced walls with a cap that gives the appearance of a stone rail.”
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The retaining walls range in height from 21 to 27 feet, plus the 3-foot dual-face wall above grade, and were constructed using Stone Strong precast modular blocks, which are large wet-cast blocks that are 3 feet high and 8 feet wide for a surface area of 24 feet. Synteen geogrids were utilized in the lower section of the walls, while the upper sections were constructed as a gravity wall with no geogrid reinforcement. “This configuration presented two challenges,” says Thiele. “This created very high grid stresses in the upper reinforced zone, and to provide sufficient geogrid reinforcement, a course of 6-square-foot [1.5-foot-tall by 4-foot-wide] blocks was used at the top of the reinforced zone to accommodate an extra reinforcing layer. In addition, very heavy SF110 geogrids were used throughout the reinforced zone. The second challenge was to provide sufficient overturning and sliding resistance in the upper gravity zone, given the native clay soils used for backfill and the traffic surcharge loads. To resolve this problem, precast ‘mass extender’ blocks were used for the first course of the gravity section. This is an enlarged block with an extra 12 inches added to the buried tail of the block, increasing the block depth [measured face to tail] from 44 inches to 56 inches. The result is an attractive, sound solution.”
 |
Photo: Walltek |
| Properly designing and constructing your wall is critical, especially when cutting into a mountainside is necessary to complete a project. |
Construction of the retaining walls at the Shadow Lake development got under way in March 2006 and is ongoing. The two walls for the first structure were substantially completed by late spring and the two walls for the second structure in the summer. Street paving was occurring elsewhere on the project while the walls were built and proceeded across the culverts when the walls were completed. The finished walls, which total 10,600 square feet, will be stained to provide the appearance of natural limestone or sandstone.
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September-October 2006
Form and Function
Retaining walls for extreme sites.
Small retaining walls are frequently used in today’s residential subdivisions to separate building lots, or for landscaping purposes. In many cases, these walls are constructed by the homeowner and only offer aesthetic value. But for large commercial and road building projects, retaining walls are more than just a pretty face. They are often used to support the soil structure or embankments that are below buildings and roadways, and in these instances, it is time to call in the professionals who use sophisticated software and engineering techniques to ensure these walls make the grade for many years to come. |
Photo: International Erosion Control Systems |
|
Photo: Schnabel Foundation Co. |
|
Photo:Stone Strong Systems |
Saving a North Carolina Interstate
When Hurricane Frances and Hurricane Ivan both hit land within a two-week period in September 2004, the Pigeon River in western North Carolina couldn’t handle the resulting increase in flow, causing the toe of the adjacent embankment to erode. That embankment was supporting several miles of Interstate 40, a major highway connecting North Carolina with neighboring Tennessee. Between mile markers 3 and 4, the slope and one of the eastbound lanes of the highway washed away, leaving only a guardrail hanging in midair.
“The North Carolina Department of Transportation [NCDOT] was faced with a tough problem,” notes Rich Hall, director of marketing at the Schnabel Foundation Co.’s corporate headquarters in Sterling, VA. “They needed to build a wall with a 100-year design life that would prevent future floods from eroding the slope again; keep the remaining lanes of Interstate 40 open to traffic; protect a sensitive environmental site; and build it fast. Several options were considered, including a tied-back soldier beam wall with a concrete face, a temporary soil nail wall to allow the construction of an MSE [mechanically stabilized earth] wall, and a tied-back micropile wall with a shotcrete face.”
In this case, the NCDOT elected to go with the tied-back micropile wall and contracted the Schnabel Foundation Co. to design and build the wall. The Schnabel Foundation Co. is a national design-build contractor specializing in the construction of excavation support systems, soil stabilizations, and retaining walls supported by soil nails, tie-backs, mini-piles, tied-back elements, and soil mixing. “The tied-back micropile wall had several important advantages over the other wall systems considered for the extremely difficult site conditions at this location,” adds Hall. “The difficulties included limited access, the need to maintain at least two open lanes for traffic on the highway, and space limitations that made the use of large caisson drills prohibitive. The embankment consisted of cobbles, boulders, extremely hard bedrock, and a soft soil matrix, making it difficult for drilling.”
By implementing the tied-back micropile wall, crews could use small track drills to stabilize the mass and prevent a landslide on the disturbed slope. The drills are tooled with rotary percussive duplex drilling heads, which allowed the drilling to progress through extremely difficult and variable conditions. “The wall design transfers the vertical loads from the tie-backs and wall components through the micropiles to the stable rock below, and uses the loads from the tiebacks anchored in the stable material to resist the driving forces of a slide,” explains Hall.
|
Photo: Schnabel Foundation Co. |
| Crews minded the sensitive environmental site during construction along Interstate 40 in North Carolina (near the Tennessee border). |
The project was completed in the late spring of 2005 within the 100-day time frame that was specified by the NCDOT. Hall points to the joint effort of the companies and organizations involved for the project’s success, including Schnabel, general contractor Phillips and Jordan Inc., engineering firm Wilbur-Smith Associates, and the NCDOT.
Emergency Repairs in Long Island
Farther north, David N. Rackmales, P.E., a structural engineer with the New York District US Army Corps of Engineers (USACE), was also faced with the challenge of building a wall that would maintain the integrity of an embankment that supported a roadway. In this case, it was in Northport, NY, a suburban community approximately 40 miles from New York City on Long Island.
“The roadway is located on the top of a very high bluff, which had experienced some slope failures and erosion in the past, primarily due to coastal storms and, to a lesser extent, wave action at the toe of the bluff,” explains Rackmales, who has been with the USACE for 13 years and has a master’s degree in ocean engineering from the Florida Institute of Technology. “The road was in danger of failing due to coastal erosion, and there had been some collapse of the pavement, so we wanted to arrest the erosion before it got any worse.”
To stabilize the site, Rackmales says several measures were taken. “The corps designed an anchored PVC sheet pile bulkhead wall with toe protection, and in front of that we have riprap and a rocky beach. On the slope itself, soil stabilization measures were taken, including clearing the trees and re-grading and installing a geogrid cellular stabilization system, which was attached to the slope, filled with soil, and then seeded and planted with native species.”
|
Photo: Schnabel Foundation Co. |
| Lanes of I-40 were kept open during construction of the wall. |
The PVC sheet pile, which was supplied by Crane Materials International in Atlanta, GA, was driven into the surface and was also anchored to the embankment with helical steel anchors drilled up to 100 feet into the bluff. “This was necessary to achieve the strength that would prevent the anchors from pulling out under extreme conditions,” adds Rackmales.
The project took less than a year from putting the plans out for bidding in August 2004 to completion in May 2005. The USACE and the New York State Department of Environmental Conservation (NYSDEC), which Rackmales says is a similar state entity to the department of environmental protection in most states, jointly funded the project, which cost under $1 million. “This project is an emergency project, not of the same extent as a disaster like Hurricane Katrina, of course, but it is still an emergency,” he explains. “There is congressional legislation in place under Section 14 of the Flood Control Act of 1946, as amended over the years, that allows for federal funds to be released to the Army Corps of Engineers to do a quick study of a shoreline protection problem and to design a solution. These projects have to be of a magnitude that they can be constructed quickly, and there has to be cost sharing between the USACE and a local government. In this case, that was the State of New York whose citizens benefit from this project on a daily basis.”
Rackmales is part of an in-house design staff for the New York District of the USACE. He says the staff also retains the services of architect-engineer (A/E) firms. Rackmales’s responsibilities included providing input to the design scope of work; reviewing of the A/E firm’s design calculations, plans, and specifications; providing structural engineering support to the project during construction; and preparing an operations and maintenance manual that was turned over to the local municipality when the project was completed.
“A cost study was initially done to make sure the project was economically viable,” adds Rackmales. “Typically these types of projects have to fall under $1 million, and in this case, if nothing was done, we could have lost the road. Sooner or later, that would have added up to much more than $1 million in construction and economic costs, and more importantly, emergency vehicles may not have been able to get through on the road if there was a washout, and you can’t put a monetary cost on that.”
When asked about the end results, Rackmales says the citizens of Northport seem to be pleased with the project. “From the feedback we have received, I would say the local residents are happy with the results. In my personal opinion, I think it is beautiful. It is quite an aesthetically pleasing project and, as an engineer, I have to say it has nice lines, it is pleasing to the eye, and it does its job.”
|
Photo: International Erosion Control systems |
| For large projects, retaining walls offer more than just aesthetic value; often they are used to support embankments below buildings and roadways. |
|
Photo: International Erosion Control systems |
Creating Space for a Retail Store
In Mount Pleasant, NY, which is in Westchester County and about 30 minutes outside of New York City, Northeast Mesa had the challenge of designing, supplying, and installing a tiered structural retaining wall for a Home Depot retail store. “This project includes 46,000 square feet of wall that is up to 30 feet high at some points,” explains Giulio Burra, general manager of Northeast Mesa, one of the initial licensees of Mesa Retaining Wall Systems. “This store had been eight or nine years in the planning and zoning stages due to the significant grade changes that were required, and the city had selected the color for the required retaining walls, which is a tan that Mesa could supply.”
As the licensee for Mesa in five states, including New York, New Jersey, Connecticut, Massachusetts, and Rhode Island, the 10-employee Northeast Mesa had the experience to complete this complex project, which required two tiered walls to support the entrance roadway and another two walls to support the building pad and parking lot.
“There was very steep grade from route 9A down to the site, so we had to fill in much of the hole and then use our Mesa block retaining wall to hold the earth in place,” notes Burra. “Although the building is still below highway 9A, this allowed us to get a reasonable grade from the roadway into the site. The second retaining wall supports the soil that is below the building pad.”
The project design was completed by Tensar Earth Technologies, which is a full-service provider of specialty products and engineering services, including patented Tensar geogrids that are designed to interact with Mesa segmental concrete blocks, creating a positive, mechanical, end-bearing connection and having compressive strength that exceeds American Association of State Highway and Transportation Officials (AASHTO) standards (>4,000 psi). “In the design, Tensar addressed seismic and global stability through the use of compaction methods and reinforcing geogrids,” adds Burra.
|
Photo: Stone Strong Systems |
| Precast modular blocks, massive in size, allow for gravity retaining walls as high as 12 feet depending on soil and surcharge conditions. |
The project was started in February 2006, and the retaining walls, which cost approximately $1.4 million, were completed in mid-May. The rest of the store construction is scheduled to be completed this fall. “The job was done in conjunction with Shawn’s Lawns, which is a large dirt-moving company based in Stamford, Connecticut,” says Burra. “They did all of the site preparation work and drainage, and Geostructures in Carmel, New York, did the actual construction of the walls.”
Working Without Geogrid Reinforcement
At a single-family residential home development near Papillion, NE, Omaha-headquartered Thiele Geotech Inc. is working on a retaining wall project that entails a great deal more than your average subdivision wall. The project, called Shadow Lake, is being developed by Shadow Lake LLC, also in Omaha, but the retaining walls are being installed as a public improvement by Sanitary and Improvement District (SID) 264 in Sarpy County, NE. The SID is a type of municipal corporation that sells bonds to install public improvements and will eventually be annexed when the debt is paid.
“The walls are being installed as wing walls and head walls for two box culverts that were required for street crossing over a drainageway upstream of a planned lake,” explains Daniel J. Thiele, P.E., a licensed engineer in 13 states and president of Thiele Geotech. The company has been in business for 10 years and employs a staff of 55, including engineers, geologists, and technical people. Most of its work is in Nebraska and Iowa, but it has completed projects in over 20 states from the Northeast to the Southwest. The company provides geotechnical, material, and environmental engineering solutions to a wide range of clients, including developers, architects/engineers, contractors, and owners.
“For the Shadow Lake project, the developer and the city desired an attractive ‘signature’ structure at a reasonable cost,” says Thiele. “In addition, the City of Papillion, through extra-territorial buffer for zoning and infrastructure authority, required that no geogrid be used for the retaining walls within 9 feet of finish grade level.”
|
Photo: Ultrablock |
| Using a block system as opposed to a cast-in-place wall can save companies considerably when it comes to the cost of materials. |
This stipulation was due to a concern the city had about future maintenance of sewers and other utilities in the right of way. “They did not want the walls to be compromised by damage that might occur to the geogrid reinforcement,” says Thiele, whose client, Linhart Construction of Omaha, is installing the walls as a subcontractor to M.E. Collins Contracting of Wahoo, NE. “To achieve the signature appearance desired by the developer, the walls were constructed to form a ‘land bridge’ across the drainageways. The walls include above-grade, dual-faced walls with a cap that gives the appearance of a stone rail.”
The retaining walls range in height from 21 to 27 feet, plus the 3-foot dual-face wall above grade, and were constructed using Stone Strong precast modular blocks, which are large wet-cast blocks that are 3 feet high and 8 feet wide for a surface area of 24 feet. Synteen geogrids were utilized in the lower section of the walls, while the upper sections were constructed as a gravity wall with no geogrid reinforcement. “This configuration presented two challenges,” says Thiele. “This created very high grid stresses in the upper reinforced zone, and to provide sufficient geogrid reinforcement, a course of 6-square-foot [1.5-foot-tall by 4-foot-wide] blocks was used at the top of the reinforced zone to accommodate an extra reinforcing layer. In addition, very heavy SF110 geogrids were used throughout the reinforced zone. The second challenge was to provide sufficient overturning and sliding resistance in the upper gravity zone, given the native clay soils used for backfill and the traffic surcharge loads. To resolve this problem, precast ‘mass extender’ blocks were used for the first course of the gravity section. This is an enlarged block with an extra 12 inches added to the buried tail of the block, increasing the block depth [measured face to tail] from 44 inches to 56 inches. The result is an attractive, sound solution.”
|
Photo: Walltek |
| Properly designing and constructing your wall is critical, especially when cutting into a mountainside is necessary to complete a project. |
Construction of the retaining walls at the Shadow Lake development got under way in March 2006 and is ongoing. The two walls for the first structure were substantially completed by late spring and the two walls for the second structure in the summer. Street paving was occurring elsewhere on the project while the walls were built and proceeded across the culverts when the walls were completed. The finished walls, which total 10,600 square feet, will be stained to provide the appearance of natural limestone or sandstone.
At Oak Point Phase 6, a modular retirement housing project in Middleboro, MA, Thiele Geotech also specified Stone Strong precast modular blocks and Synteen geogrids. “This phase of the project required 16 separate retaining walls for grade separation,” notes Thiele. “The majority of the walls were required along wetland areas to allow the lots to be filled and raised. The rolling terrain across the property required wall heights of up to 23 feet tall. The taller walls were constructed as geogrid-reinforced walls, while shorter walls up to 9 feet tall were constructed as gravity walls without geogrids. To maximize the buildable property, the walls conformed closely to a buffer around the wetlands, creating attractive serpentine shapes. This also required some jogs using 45-degree corner blocks. The last two walls were used to create a land bridge crossing the wetland. These walls used dual-face blocks above grade to form a barrier rail, and incorporated a small culvert at flow line.”
Thiele Geotech’s client was MBO Precast in Plymouth, MA, a licensed dealer for Stone Strong Systems. The Contractor was SLT Construction Corp., also from Plymouth, while the project developer was Saxon Real Estate Partners in Middleboro. The entire project included 30,300 square feet of retaining wall.
 |
Photo: Walltek |
| This wall was built using segmental retaining wall units and rock anchors. |
Thiele says he likes to specify the Stone Strong precast modular blocks for several reasons, with size being a main consideration. “The massive size of the precast modular blocks allows for relatively tall gravity retaining walls, up to 9 to 12 feet depending on soil and surcharge conditions,” he explains. “Using the mass extender blocks, gravity walls of 12 to 15 feet can be constructed, again, depending on soil and surcharge conditions. In addition to the main 24-square-foot block, the accessory blocks [6 square feet, 3 square feet, corner blocks, end blocks, and dual-face blocks] create a very versatile system. The chiseled granite face creates an attractive retaining wall, and the blocks can be stained in place if desired to create a specific natural stone effect. Besides the advantages in gravity wall construction, the inherent stability created by its large mass makes the block attractive for geogrid-reinforced wall applications as well. This system is much more stable under construction and service loads that often occur after the wall is completed but are rarely considered in design.”
Challenging Top-Down Construction
Walltek Design Build Inc., a designer and builder of MSE walls based in Clarkston, MI, was awarded a project in Staten Island, NY. It wasn’t a huge undertaking in terms of square feet, but it was definitely challenging when it came to the planning and designing of the wall system. “There was an enormous drop of about 70 feet from the top to the bottom of the site,” explains Chad Clark, P.E., the company’s president. “The plan was to build a high-rise assisted living facility on the site, and prior to this site being developed, land was very limited. In order for the site to be developed for this building, they had to cut back into a mountainside, which is why having a properly designed and constructed retaining wall was so critical.”
The wall is approximately 20,000 square feet and was built using top-down construction, including two 20-foot tiers for a maximum height of 40 feet. “We excavated into the mountainside, created a ledge for the upper tier to sit on, and then built the top 20 feet,” adds Clark. “Once that was completed, we built the lower tier. The wall was built with segmental retaining wall units and rock anchors.”
Todd Garris, CEO of Walltek, says the biggest challenge faced on the project was accessibility and site conditions. “We had to develop a special lifting platform for the workers to be lifted to the 20-foot wall height. The platform was attached to a rubber tire loader that is typically used for moving earth. The excavation had to be temporarily stabilized to keep the upper tier from collapsing in areas where rock was not present.”
Because of the design complexity of this project, typical retaining wall design software was not used. Clark says for most jobs Walltek uses commercially available trademarked programs, including MSEW software for analysis and design of MSE walls and ReSSA software for the analysis and design of reinforced soil slopes. Both programs are from Adama Engineering Inc. in Newark, DE.
“It would be very laborious to do the calculations without these software programs, especially for our cost estimating,” says Clark. “If we were doing all of our costing manually, we would have to do a set of calculations for each type of face or anchor. The software estimates which is the most cost-effective wall system in a matter of seconds.”
Walltek was started in 1994 by Garris. Today, the company has over 75 employees and offices in Clarkston, MI; Monroeville (near Pittsburgh), PA; Weehawken, NJ; Plymouth, MA; Tampa, FL; and Chicago, IL.
Calculating Costs
High tech also plays a key role in the success of Tacoma, WA–based WestBlock Systems. Since the early 1990s WestBlock has provided innovative, cost-effective, design-driven earth retention and barrier wall systems, which are produced under license by a worldwide network of concrete block manufacturers. WestBlock has developed its own software program that allows engineers and designers to plot the wall and calculate the structural requirements and costs.
“This software provides a means for someone to generate reasonable estimates, in a short period of time, for the materials and structural elements they will need when utilizing the GravityStone Modular for retaining walls,” says Scott West of WestBlock. “After minimal input regarding site conditions, it will quickly render results for the quantity of block, gravel, and cell depth. Typically results concur very closely to what a trained engineer will determine after a more detailed evaluation of the site and soil conditions.”
 |
Photo: Ultrablock |
| This site's steep conditions made grade a major factor in construction. |
WestBlock has eight different block products on the market, including blocks for retaining walls, fence walls, and privacy walls. “With some of our retaining wall blocks, especially our GravityStone line, you can utilize the same block components and continue above grade and directly into a fence system,” notes West. “The fence can rest on top of the retaining wall system, and in utilizing the same components there will be no visual break in the wall. This is a good alternative when you want aesthetic continuity and helps minimize the number of subcontractors that are used on the job. Typically this provides a cost reduction because the crews and product are there and it simply involves a few more courses of block to be stacked up.”
The line of products offered by WestBlock ranges from consumer products such as NurseryStone, which can be installed by the do-it-yourselfer, to professional grade products such as GravityStone. To work in conjunction with GravityStone, WestBlock has developed SlideStop. “This is a patented process where we have a concrete foundation with rebar going up through the wall vertically,” explains West. “This provides additional stability if there is an earthquake and it has been certified and approved by engineers.”
Building for Homeland Security
Also on the west side of the country, Oregon City, OR–based Darkhorse Construction LLC installed 102,000 square feet of MSE-type Ultrablock wall for the Department of Homeland Security office in Tukwila, WA, a suburb of Seattle.
“Grade was the big issue on this project due to the steep site conditions,” explains Jeff Campbell, head estimator for Darkhorse, which has a second office in Bonny Lake, WA. “We had to install more than 15 walls, some of which were perimeter walls. Some of the walls were to hold up the banks where we had to cut in to put in a parking lot, and others were holding up the parking lot. We also used the same product to create planter boxes around the building.”
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Photo: Walltek |
| Seismic considerations are always accounted for on sites in the Northwest. |
Originally formed in 1999, Darkhorse Construction specializes in private development and public works projects where MSE walls are required. The company, which employs about 60 people during the peak summer season and about half that in the winter, is a preferred installer for Vancouver, WA–based Ultrablock Inc. and for Keystone Retaining Wall Systems (a Contech Co.) in Minneapolis, MN.
For the Homeland Security project, Campbell says a cast-in-place wall was originally specified, but there were considerable cost savings by switching to a block system. “The budget for the retaining walls was $2.2 million, and by going with the Ultrablock, we were able to save around $500,000, or 25%,” he notes. “Most of the savings were in the cost of the materials.”
Engineering for the project was provided to Darkhorse and included seismic considerations. “Here in the Northwest, seismic is always included in the engineering,” adds Campbell.
The project, which was started in October 2003 and was finished in October 2004 under the direction of Superintendent Wayne Wootan, was done in stages. “We would build one wall and then the other construction crews would come in for a while to build the structures and then we would come back and do another wall,” says Campbell. “Our work was also weather dependent, which can be another big factor in the Northwest, especially during the winter months when we get a lot of rain.”