In a previous blog, we talked about how pore pressure data is used to correct and adjust soil behavior type characterizations – but this is only one application of this important and revealing information. Pore pressure data can also be used to estimate the depth of the water table and the direction and rate of groundwater flow. This information is useful both for site characterization and for geo-environmental and remediation applications.
What is a Pore Pressure Dissipation Test?
As a CPT cone is pushed into saturated subsurface soil, it creates a localized increase in pore pressure (denoted excess pore pressure, ui) as groundwater is pushed out of the way of the cone. In a pore pressure dissipation test, the downward movement of the cone is paused and the time it takes for the pore pressure to stabilize is measured. This stable pore pressure is called equilibrium pore pressure, uo. This information allows the user to identify important hydrogeologic features:
The water table (or phreatic surface) depth is defined as the distance below the soil surface at which pore pressure is equal to atmospheric pressure. This can be roughly visualized as the level below which subsurface materials are fully saturated with groundwater.
- Especially in fine-grained soils, estimating the water table can be more complex than simply detecting moisture, since surface tension draws groundwater upwards, creating negative pore pressures. This is effect is called capillary rise.
Very low or negative pressures can be difficult to measure precisely with the piezocone, which is primarily designed to measure high pressures below the water table. In this case, the water table depth can be calculated by the following formula:
dwater = dcone – hw
dwater = water table depth
dcone = depth of piezocone
hw = water head
The water head, hw, is the height of the water above the cone, which can be calculated based on the pore pressure and the unit weight of water:
hw = u/ɤw + z
hw = water head
uo = equilibrium pore pressure
ɤw = unit weight of water
z = distance, if any, between pressure sensor location and depth reference point on the piezocone
- The rate of dissipation indicates the permeability or hydraulic conductivity of the soil – that is, the tendency of the soil to allow or resist the flow of groundwater.
A rapidly dissipating pore pressure indicates the presence of an aquifer (a porous region where groundwater tends to flow), while a slowly dissipating pore pressure indicates an aquitard (a compacted region that resists the flow of groundwater).
Multiple CPTu soundings can be used to generate a three-dimensional model of subsurface hydrogeologic conditions, including a method to accurately predict the localized groundwater table, estimate the direction and velocity of groundwater flow, and estimate the distribution of hydraulic conductivity. These parameters can then be used to predict the characteristics and spread of shallow aquifers: this information is critical to deep foundation design, site investigations and remediation design.
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