Test: Bligh's Creep Theory - Civil Engineering (CE) MCQ

The value of Bligh’s creep coefficient (C) is different for different soil.

Bligh’s Creep Theory:

• Bligh assumed that the percolating water follows the outline of the base of the structure which is in contact with the subsoil.
• The length of the path traversed by the percolating water is called the length of creep or creep length.
• He further assumed that the head loss per unit length of creep (i.e. H/L) which is called hydraulic gradient is constant throughout the percolating passage i.e. Loss of head is proportional to length of the creep.
  
• The reciprocal of hydraulic gradient (i.e. L/H) is known as Bligh’s Coefficient of creep.

Where, C lies between 5 and 15 and depends on the type of soil.
To ensure safety against piping failure, L_req ≥ CH
Note:-
Percolation Coefficient:
It is defined as the ratio of coefficient of permeability to porosity of soil.

Bligh's Creep theory:
(i) This theory is base on the assumption that seeping water through the soil below the weir.
(ii) Follows the path along with the contact of the base, with the underlying sub-soil.
(iii) The length of the path of seeping water from the point of entry into the sub-soil from the U/S of the impervious.
(iv) Apron to the point at the D/S end of the impervious apron is know creep length.
(v) Bligh also assumed that loss of head of the seeping water is proportional to the length of its travel irrespective of whether.
(vi) If H is the total head causing seepage or total loss of head, and L the total creep length, the loss of head per unit length of creep (C) is given by

C = H/L
Where C is the Blighs Creep Coefficient
Safe values of coefficient of creep for different soils are given below:

For Micaceous sand, the safe hydraulic gradient is less than 1/15 and for Coarse-grained soil is 1/12. For light sand and mud, the safe hydraulic gradient should be less than 1/8.

The thickness is generally increased by 33% so as to allow a suitable factor of safety. The water on the upstream side is more than the downstream side and it counterbalances the uplift caused on that side. Hence, only a nominal thickness is required.

The hydraulic gradient i.e. HL/L is equal to the coefficient of creep 1/C.
Bligh’s coefficient = 12
So, safe hydraulic gradient = 1/12.

According to the theory, safety against piping can be ensured by providing sufficient creep length. It is given by –
L = CH where, C = Bligh’s coefficient and H = seepage head
OR
Hydraulic gradient (i) ≤ 1/C.

The thickness to be provided by taking a factor of safety of 4/3 i.e. Thickness (t) = 4/3 x h / (G-1) where h = ordinate of HGL from top of floor and G = specific gravity of floor material.

Total creep length, L = b + 2(d₁ + d₂ + d₃)
d₁ = 6 m, d₂ = 4 m, d₃ = 8 m and b = 22 m
L = 22 + 2 x (6+4+8) = 58 m.

The worst condition is that when the water level on U/s side is maximum and no water on D/s side. It would be more economical to provide more creep length on U/s side instead of D/s side. A vertical cut-off at U/s end is more useful than at the D/s end of the floor.