Soil Mechanics Lecture 7 Flashcards
What is capillary rise (h_c)
When the surface tension of the water, and the attraction of the water to the side of the tube counteract the weight of the water
rise stops when capillary forces = weight of the water
What is the variation of pressure in a capillary tube?
The pressure is zero up until the free water interface, where it increases linearly
In the capillary tube the water is under tension, so the pressure is negative
(remember walls have an attractive force)
What shape does the surface of the fluid form at the top of the capillary tube?
A U shape
What is capillary rise inversely proportional to?
d
so h_c is proportional to 1/d
How does capillary rise apply to soils?
Void space in coarse soils results in less capilalry rise
Void space in clay is much less, so more capillary rise
These void spaces is why capillary rise is recorded in soils
Would capillary rise be higher in clay or sand?
Higher in clay - smaller void spaces
What 3 zones would you fine on a beach?
The area above the capillary zone, poor bearing capacity, less good with loads
- The capillary zone, the ideal place for building
- The submerged zone, pooor bearing capacity, less good for building
2 Weaker zones ‘bracket’ a better zone
What is the equation for pore water pressure in the capillary zone?
u = -h_cgamma_wSr
Why is the capillary zone best to build on?
Because the uplift of water means that the effective stress increases, therefore the strength increases, therefore it is able to provide more support
What is a danger of unfiltered dam?
Collapsing due to the pipe mechanism, where flow causes the erosion of soil particle
How does a dam filter work?
A ‘pipeline’ or drainage mechanism made of coarse material allows the water to flow through to drainage more easily, this stops unecessary structural wear on the dam
What causes slope instability?
Increasing pore pressure, due to the degree of saturation
What can poor drainage cause in roads?
Pavement degredation
Water pumping/ bleeding
What does flow around a sheet pile cause?
An increase in effective stress on the side with the higher water table, and a decrease in effective stress on the lower side.
Downward seepage = increase in effective stress
upward seepage = decrease in effective stress
What happens in excevation with retaining structures on both sides?
The retaining structures need to be supported, to prevent excessive uplift, borehole pumps are used, which lowers the water level of the site, allowing for dry conditions at the bottom of the excevation
