One of the most common applications is base course stabilization in roads. As traffic loads pass over the roadway, the tensile forces can cause cracking and rutting as the pumping action induced beneath the surface causes fines and coarser aggregate to mix.
A separation geotextile prevents those aggregate layers from mixing. A reinforcement geogrid redistributes those tensile forces and interlocks with the aggregate to prevent lateral migration. In both cases, roadway service lives are extended, performance improved and standard maintenance greatly reduced.
Active roadways, however, are not the only installations that might require these functions from geosynthetics. Many construction projects themselves require separation and reinforcement geosynthetics to support the construction activities, such as when work is performed over weak soils and heavy equipment is used.
This need was exemplified by a project in Brunei. During the construction of an interchange, multiple cranes were placed on site to handle various aspects of the construction, such as bore piles and materials movement for piling and bridge construction. The area experienced heavy rains during construction operations, rains which not only delayed general site work but caused significant erosion. One of the cranes sank and toppled when it ventured too close to the unreinforced edge (see picture).
When the project team investigated the sunken crane, they discovered that the reinforcement and separation geosynthetic installed beneath the staging area had not been extended far enough. Two of the primary cranes were unaffected because of the geosynthetic beneath them. The third crane was situated upon only the weak soil, and when enough soil washed out from beneath it and what remained was saturated the soil modulus became too weak to support the crane.
Shortcuts Lead to a Longer Road Brunei’s soft soils are primarily composed of sandy, silty clays with high montmorillonite content or silty, peat soils. Annual precipitation is up to 1500mm (59in.). This often requires multiple geosynthetics to enable safe, heavy construction.
For the project site that experienced the collapse of one of its cranes due to weakened soils, a unique geosynthetic had been specified. Manufactured by NAUE GmbH & Co. KG, the Combigrid® product provides both a separation geotextile and a reinforcement geogrid in a single-layer product that can simply be unrolled into position.
The needlepunched nonwoven layer is secured in the material during the manufacturing process and is sandwiched between the transverse and longitudinal high-strength geogrid monolithic bars. This allows the product to provide reinforcement, as well as filtration, separation and drainage functions.
On site, the geosynthetic-supported zone experienced some erosion, but the base modulus was held in tact by the geogrid. Water penetrating the soil was able to drain through the geotextile, which also provided separation to prevent mixing. The cranes were supported.
Where the Combigrid® had not been installed, the base course on which the crane was standing mixed with the subbase and the platform basically sunk. Also the water washed out the soil and both this caused the crane to topple.
With one storm and one photograph, we find an essential lesson about geosynthetic utilization depicted clearly. Not only can separation and reinforcement geosynthetics provide more durable and more economical roadways, but the construction process itself can be made more efficient and less subject to disruption.