Week 5 Groundwater Problems and Permeability Flashcards
(19 cards)
What is the primary issue with groundwater during engineering works?
The presence of the water table in soils can lead to problems during excavations.
What are potential solutions for water inflow into excavations?
- Sump
- Cofferdam
- Dewatering
What occurs when uplift pressures from artesian groundwater cannot be balanced by soil weight?
Heaving of the soil may occur, allowing further water flow and increasing erosion.
How does increased water pressure affect structures?
Increased water pressure acts upwards on the base of any structure.
What is running sand?
A condition where water flows into excavations and carries soil particles, causing slumping and collapse.
What are the three approaches to determine soil permeability?
- Indirect determination
- Laboratory tests
- In-situ tests
What does indirect determination of permeability rely on?
Estimates based on soil properties like particle size distribution, density, and void ratio.
What is a common laboratory test for permeability?
The constant head permeability test.
What is measured in the constant head permeability test?
The volume of water discharged (Q), time interval (T), and difference in head (H or Δh).
What does Darcy’s law relate to in the context of permeability?
It relates flow rate (q) to permeability (k), hydraulic gradient (i), and cross-sectional area (A).
What is the formula for permeability (k) in the constant head test?
k = (Q L) / (T A H)
What type of soils is the constant head permeability test suitable for?
Soils with k between 10-2 to 10-5 m/s.
What is a limitation of the constant head permeability test?
Sample preparation may alter soil properties and small samples may be unrepresentative.
What is the falling head permeability test typically used for?
Fine grained soils.
What is the purpose of the hydraulic cell test?
To determine vertical and horizontal permeability using undisturbed soil samples.
What is the most accurate method for in-situ determination of permeability?
Pumping tests.
What occurs when the rate of water flow into a borehole matches the pumping rate?
A steady state condition occurs, creating a symmetrical cone of depression.
What is the formula for permeability in an unconfined aquifer?
k = [q / π (h2 – h1)] x ln r2/r1
What is the formula for permeability in a confined aquifer?
k = [q / 2πL (h2 – h1)] x ln r2/r1
