Soil confinement system supports access over soft subgrade, protects tree roots
Copt Hewick Hall is a privately owned “special interest” building in North Yorkshire, England, on 86 acres of land. As part of an extensive renovation, a new service road was required for vehicles to access areas of the estate. Protecting large, mature trees from root damage during construction of the road was a major concern. Relatively poor soils meant additional reinforcement would be required to support vehicles over the soft ground. A road solution was sought to minimize the depth of excavation, support heavy vehicles over soft subgrades, and prevent expected loading stresses that would cause compaction and potential damage to the trees’ critical root zone.
The critical root zone/tree protection zone is the minimum area beneath a tree that must remain undisturbed to preserve a sufficient amount of root mass in order to assure the tree’s survival. When construction equipment and vehicles intrude a tree’s critical root zone, they can cause negative impacts to the soil environment, including compaction of the soil, damage to near-surface roots and, ultimately, endangerment of the structural integrity of the tree. The majority of a tree’s root system is contained within the top 3 ft of the surface, and construction excavation and compaction can damage or even destroy roots to the point where trees may not survive.
Less Excavation, Less Loading Stress
The Presto Geoweb 3D soil confinement system was chosen for the access road. Its ability to reduce base requirements at least 50% results in less excavation and less disturbance to the subgrade soils. Loads are distributed laterally—not vertically—over the roadway reducing point loads and compaction of the subsoil. These roads perform as a load-supporting structural layer, reducing loading stresses off of the underlying soil and critical root zone. With the permeable aggregate infill planned for the site, the roadway also allows natural water infiltration.
Installation Sequence & Components
The service road installation consisted of a geotextile underlayment, the Geoweb material and aggregate infill. The completed road was finished with concrete decorative edging.
Excavation: Only 8 in. of subgrade needed excavation, compared to 20 in. required for conventional road construction and subgrades of similar California Bearing Ratio value. That 60% reduction saved 12 in. of excavation.
Geotextile soil separation layer: A 10-oz. geotextile was installed as a separation layer between the subgrade and the aggregate fill; it offers a good balance between infiltration of water from the permeable surface to the subgrade and strength.
Geoweb surface layer: Geoweb panels were expanded and connected with weather-resistant ATRA keys. Temporary anchors held the panels open for infilling. Bends in the roadway were accommodated by overexpanding the outer cells and under-expanding the inner cells until the desired radius was achieved. Once secured in position, the panels were infilled with aggregate, and the cells overfilled slightly to ensure a slight pavement layer wear course (0.5 in) once compaction was completed.