A flownet is drawn to obtain (i). exit gradient (ii). uplift pressure (iii). coefficient of permeability
- Aonly (i)
- B(i) and (ii)Correct
- C(ii) and (iii)
- D(i), (ii) and (iii)
Solution
Concept
A flow net gives seepage quantity, pore or uplift pressure, hydraulic gradient and exit gradient. Permeability k is an input, not an output.
Formula
q = k H (Nf / Nd); exit gradient ie = Δh / l
Step-by-step solution
1. From a flow net we get the seepage discharge q = k H Nf/Nd. 2. We get the pressure head at any point, so the uplift pressure under the structure. 3. We get the hydraulic gradient, including the exit gradient (for piping check). 4. Coefficient of permeability k must be known before; it is found by lab or field permeability tests. 5. So (i) and (ii) are correct.
Explanation
A flow net shows flow lines and equipotential lines for seepage. It helps find seepage, uplift pressure and exit gradient. Permeability is needed to use the flow net, so we do not get it from the flow net.
Why the other options are wrong
A: Uplift pressure is also found from the flow net. C and D: Coefficient of permeability is not found from a flow net.
Shortcut
Flow net gives q, u and i. The value of k comes from a permeability test.
Common mistake
Thinking permeability is found from the flow net because k appears in the seepage formula.
Correct answer: B) (i) and (ii)