Vibrating Wire Piezometers Installed with HDD
Ellingson recently completed the installation of four vibrating wire piezometers (VWPs) beneath a shallow channel inlet of San Francisco Bay. Project objectives included monitoring soil pore water pressure for a wetland remediation project. These specialty piezometers detect changes in soil pore water pressure as an indicator of soil stability. The remediation project involves moving and staging contaminated bay sediments and covering them with an engineered cap.
View from HDD rig installing vibrating wire piezometers under a slough
HDD Vibrating Wire Piezometers Measure Pore Pressures in Otherwise Inaccessible Location
As pore water pressure increases, there is a loss of soil strength. Additionally, this leads to an associated increase in the risk of catastrophic slope failure (landslides).
This effect is particularly important for layered soils with contrasting conductivities, like sands interbedded with silts and clays. Similar issues present themselves in CCR basins with layered coarse coal ash (bottom ash) with fine fly ash. Really any matrix layering that might trap pore water poses a risk.
VWPs are a common geotechnical tool, traditionally installed in vertical bores. However, this is, to our knowledge, the first VWP installtion using horizontal directional drilling (HDD).
Tidal Marsh No Match for Special Shoes
Project challenges included working in the tidal zone overlying the borings and the presence of approximately 15 feet of historical fill. Originally emplaced beginning in the 1850s, the fill helped develop the former marsh into a residential and light industrial area. In order to keep from getting stuck in the mud, the locator wore special shoes. Additionally, tidal fluctuations dictated close coordination of site drilling activities.
Walkover locating to install vibrating wire piezometers underneath a tidal marsh
The drilling crew encountered construction debris in most of the boring locations. Even though this required repositioning the rig multiple times and caused the loss of mud circulation in some borings, they perservered.
The VWP installation borings were each approximately 200 ft long and utilized single-ended installation methods. The HDD rig was set up on stable dry ground and oriented such that the drilling trended out into bay muds under the channel, accessing an area inaccessible to vertical rigs. Although two shallow borings targeted the Young Bay Mud (a soft, water-saturated clay) two deeper borings targeted the underlying Old Bay Mud (a wet, consolidated, much stiffer clay).
Tags: coal ash, geotechnical, HDD, pore pressure, slope stability, vibrating wire piezometers