Test: Soil Mechanics- 4

The design of an embankment dam and other hydraulic structures, the choice of soils are aimed at reducing or eliminating the detrimental effects of seeping water. Where high hydraulic gradients exist there is a possibility that the seeping water may cause internal erosion within the dam, especially if the soil is poorly compacted. Erosion can work its way back into the embankment, creating voids in the form of channels or ‘pipes’, and thus impairing the stability of the dam. This form of erosion is referred to as piping and occurs when water flow upwards resulting in zero effective pressure.

Yield of well means the discharge from the well excavated through given aquifer

Q = nv_a A = KiA

where

n is porosity

v_a is actual flow velocity of ground water

A is area of the aquifer opening onto the wells

K is permeability of soil

Quick sand condition can be prevented by lowering of water table at site before excavation.

• It can also be prevented by increasing the upward flow length by providing sheet pile wall.
• Another method adopted at sites to avoid quick sand Condition is addition of surcharge (i.e additional weight) on the excavation side to increase the net weight of the soil mass.

For a Quicksand Condition

Confined aquifers are permeable rock units that are usually deeper under the ground than unconfined aquifers. They are overlain by relatively impermeable rock or clay that limits groundwater movement into, or out of, the confined aquifer. Groundwater in a confined aquifer is under pressure and will rise up inside a borehole drilled into the aquifer. The level to which the water rises is called the piezometric surface.

(h_L)_x + (h_L)_y = Total head loss

Total head loss from B to A = - 20 - 5

= - 25 cm

or H = 25 cm

(h_L)_x = 5 - [-17.5] = - 22.5 cm

or (h_L)_x = 22.5 cm

(h_L)_y = 25 - 22.5 cm

(h_L)_y = 2.5 cm

V_x = V_y

K_x i_x = K_y i_y

3 × 10^-4 × 1.125 = K_y × 0.25

k_y = 13.5 × 10^-4 cm/sec

An aquifer of unit width and thickness B carries a discharge = KB; where K is the co-efficient of permeability and B is the width. This discharge is termed as transmissibility T and has dimension L²T.

The transmissibility of a stratified formation = K_e∑B
Here K_e =  3.5m/day
Transmissibility = 3.5 x 60 = 210 m²/day

F.O.S against boiling is giving by

F.O.S =  
 

i_er = critical hydraulic gradient

i_per = permissible hydraulic gradient

 

η = percentage air voids

a_c = air content

η = porosity

calculation:

i_cr = i_per x FOS

i_cr = 0.315 × 3

i_cr = 0.945

 

η = 0.427 = 42.7%

a_c = 15%

η_a = 0.427 × 0.15

η_a = 0.06405

η_a = 6.405%

At C - C

∑ F_B = 0

(15 - 12.70) × γ_sat - 5 × 9.81 = 0

γ_sat = 21.33 kN/m³

For artesian (or) confined aquifer

= 3832.37 m³/day

= 159682.26 lph