2017 Archive : Forest Lake Estates

Project Details

Fabric 1

Mirafi
Producer: TenCate Geosynthetics

Engineer Company 1
ETTL Engineers

Design Company
Rusty Gibson

Project Manager Name
David Hagood

Installation Company
Stan Excavating

Please describe the project specifications

The roadways within the Forest Lake Estates subdivision were initially constructed back in the early 80’s with a five inch (5”) section of concrete placed directly over a high PI clay subgrade. The roadway sections over time have disintegrated due to the expansion and contraction of the high plasticity clay soils underneath. The use of TenCate Mirafi® RS580i material was proposed as the best solution for use with the fine grained select fill materials. Although the RS material can be used with a wide range of structural fill materials, the use of locally available fine grained fill and the effective cost savings of this fill over the initially proposed aggregate created the best engineered section. The proposed design of the section included the subgrade CBR support of 1.5 and the fill support of 10.

What was the purpose of this project? What did the client request?

The roadways needed to be reconstructed and initial design for the reconstruction of the roadways specified the use of a geogrid with a thick layer of imported crushed aggregate fill. This layer would serve as the buffer to minimize the loading caused by the expansive clay subgrade. The issue with the design was the cost of the imported aggregate. Due to limited local supply, the aggregate had to be imported and the extra cost created an expense overrun of more than $1 million USD.

It was proposed by a local engineer that the use of Mirafi® RSi reinforcement along with a local, fine grained select fill, could create a similar section. This proposed fill would be much less expensive and easily available due to the proximity of the borrow pit. On the morning of May 25th, a test section was constructed. This section would test the applicability of using the locally available fill described as a clayey sand (SC) over the saturated expansive clay subgrade.

What is unique or complex about the project?

The site conditions showed evidence of recent rain, with standing water in drainage structures and wet subgrade soils. The subgrade conditions within the subdivision are comprised of high PI clays with poor drainage potential. An excavation depth of 18-20” was utilized for the roadway structure, with moderate molding of the subgrade surface. The surface, immediately after excavation, was viewed to be extremely soft and with high moisture content. A Site Dynamic Cone Penetrometer (DCP) Investigation was made to ensure the parameters used in the proposed design aligned with actual field conditions. Based on the initial geotechnical investigation, a subgrade CBR of 1.5 could be used. Two different investigations were conducted at different locations on the test section. Both DCP investigations indicated an initial soft area going to a depth of 15-20” and falling within the design parameter of 1.5 for subgrade CBR support. Beyond this depth, the subgrade did show a slight improvement, but still not enough to mitigate the need for geosynthetic improvement.

What were the results of the project?

Based on the variables and the design based around them, the test section was successfully constructed and tested for competence. Even with only an initial fill placement of 8”, vs. the designed 18”, the foundation over the RS580i with select fill performed extremely well. First, the initial layer was tested for density, passing the required 95% density. It was then loaded with fully loaded construction equipment and visually inspected for deformation. It was determined by those onsite that the test section, even with it only being an initial 8” of the final 18” section, exceeded expectations and that the remainder of the project will be constructed using the same cross-sectional foundation design.

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