Mirafi 180N Geotextile Fabric

Case Studies:

Mirafi 180N – Otsego, MN

The Objective:

In 2003 the Mississippi River rose 50 feet causing massive landslides and very steep slopes. This area also suffered from smaller landslides over the past few years. Landslides extended to the northbound lane of S.S.A.H. No. 42 leaving a potential risk of losing the southbound lane as well. This required a major project to re-construct over 750 linear feet of slope coming up from the normal river level of 50 feet including a 1H:1V slope angle.

Because large sections of the road needed to be removed, a detour had to be setup in order to replace this section of road after reconstructing the slope. The plan was to complete the project within 4 months which entailed the designing and actual construction. The 4-month window was to ensure the project would be completed before winter set in.

Various regulatory agencies were involved including the Corp of Engineers, the Department of Natural Resources – Division of Waters, and the Division of Fish and Wildlife. Other agencies included Soil and Water Conservation services along with the Mn/DOT State-Aid Disaster Committee. All these agencies were required in order to receive all the necessary permits and approvals.

The Designing Process:

In order to prevent further land loss over the next two months, Gale-Tec Engineering, Inc. needed to place a temporary holding system of helical screw anchors, a Mirafi® 180N geotextile, and chain link fence. This would give them the needed time to evaluate and design the needed repairs. Analyzing the repair systems included gabion walls, lightweight fill, soil, nails, and mechanically stabilized earth systems.

The distance between the river and the top of the slope would allow for a retaining wall or the creation a very steep man-made slope of 1H:1V. It was decided the best way to go was combining high-tenacity polyester TenCate products that would also save money. These products ranged from Miragrid® 5XT to 10XT and a riprap facing with a graded filter system of Mirafi® 180N and sheet pile for added protection in the flood zone. Riprap is loose stone for forming a foundation for a breakwater or other structure. Also implemented was a cellular confinement system facing consisting of a special fertilizer or seed mix for the upper portion of the slope. Miragrid® geogrid is a highly persistent, molecular weight polyester fiber that is capable of flexible reinforcement that is between 2200 and 4300 pounds per foot for long-term strength. Embedding lengths of approximately 40 feet were decided based on their limit equilibrium analysis. Limit equilibrium analysis in rock and soil is the study of the point at which a material has reached its stability.

Construction:

At the beginning of construction, the contractor needed to excavate below the normal river's water elevation to find the necessary base for construction. In order to maintain a water-tree excavation, they used sheet-pile cofferdam. The compaction of soil is an easy way to cause damage to trees because tree roots need well-aerated soil to grow and receive oxygen, water, and nutrients. The implemented graded granular filter included a Mirafi® 180N with a Mn/DOT Class IV quarry run riprap with parts between 8 lbs to 200 lbs within the 500-year flood elevation for the river. From that point, they proceeded up in one-foot lifts.

The back slope excavation was regulated by OSHA standards for Excavation 29 CFR Part 1926 which required obtaining an additional 30 feet construction easement from the property on the side of the road opposite the river. It was required that the contractor complete the project in 60 days in order to allow the replacement of the bituminous roadway. This needed to be done before shutting down the asphalt plants in Minnesota in mid-November due to cold weather.

A special seed mixture of deep-rooted prairie perennial grass was placed within a 6-inch high cellular confinement system web. Topsoil, seed mixture, and a special natural organic plant food were mixed by hand into the web. The fertilizer had a slow releasing nitrogen that is perfect as a grass starter with low burn capabilities. The natural fertilizer had a compost base that was heated to stimulate the microorganism growth.

The Outcome:

The reconstruction restored the majesty and magic of the Mississippi River which was the center point at the Grand Excursion 2004 Celebration, commemorating the transformation of the upper Mississippi River and surrounding communities. According to the Chair of Grand Excursion 2004 and president of the St. Paul Riverfront Corp, the collaboration included more than 50 cities along the river and 23 regional organizations showcasing more than 5 billion of capital, economic, and environmental improvements to the area around the river.

For additional information, and CAD details please see the TenCate website.

Bennington, VT

The Objective:

The Bennington Bypass would act as an alternate route for US Route 7 and Vermont Route 9 in the Bennington area. It would allow through traffic with a high capacity highway going around downtown Bennington and Old Bennington. This bypass would have a significant reduction in delays, increase safety, and decrease congestion for both through traffic and local traffic. It will also be better for the local street system for both residential and commercial traffic and for pedestrians, bicyclists, and other people.

The Designing Process:

The challenge for the designer was to create a support for the 700-foot bridge for the west abutment over the Furnace Brook area of the project. They could use steel piles and compacted earth but the soil boring indicated only medium dense soil which would need steel piles driven down over 25 meters. Therefore, a reinforced soil slope was considered the best way to get the desired support.

Construction:

After excavating and compacting the base elevation for the support pad, a perimeter drain was placed using Mirafi® 180N. It required 14 panels of Miragrid® 24XT which is the strongest manufactured geogrid available. It was installed as the primary reinforcement with embedded lengths of 34 meters. They used a D350 hauler to truck in granular borrowed from the site and then spread it to a thickness of 200mm using a John Deere 550 bulldozer. Then used a CAT 563 vibrator to compact to 95% maximum density. A secondary reinforcement layer of Miragrid® 24XT (400mm), embedding lengths of 1.8 meters were installed. This process took 4 stages and each stage consisted of embedded lengths of 34m, 23m, 14.5m, and 10m using approximately 18 panels in each stage. The height of the final reinforced slope was `17 meters. The face of the slope was covered with Mirafi® 180N and covered with riprap. The side slopes were vegetated.

The Outcome:

The contractor was able to create a support pad for the bridge abutment using a reinforced soil slope over the steel piles. They were able to create this support pad utilizing minimal equipment and labor. They also completed this in a timely manner which saved the state considerable money.