DHARMSETTLEMENT ANALYSIS 417Estimate:
(i) the ultimate settlement,
(ii) the settlement at the end of the loading period, and
(iii) the settlement 2 years after completion of the building.
From preliminary analysis, the ultimate settlement is 25 cm. This will change because
of the altered conditions given in (b) and (c).
Settlement S varies in direct proportion to the thickness of stratum.∴ Modified value of ultimate settlement will be obtained by using the factor120
100or 6/5.Due to lowering water table, effective stress increases. In this case the increase in effec-
tive stress is γw. H or 9.81 × 1 kN/m^2.
Approximately, settlement varies linearly with an increase in effective stress. There-
fore, the modified value on this count will be got by using the factor 33.81/24.
∴ Final value of ultimate settlement = 250 ×6
533 81
24×. = 422.6 mmSimilarly, modified value of settlement in 6 years = 60 ×6
533 81
24×.
= 101.4 mmBut since the loading is also to be considered this settlement is supposed to occur in
(6 + 3/2) or 7.5 years.SinceS
St
t1
21
2= ,Settlement at the end of loading period, =101 4
7515.
.×. = 45.35 mmSimilarly, settlement 2 years after completion of the building is:
101 4
75. 35
.
.. = 69.27 mm.Example 11.9: The plan of a proposed raft foundation 18 m × 54 m is shown in Fig. 11.7(a).
The uniform pressure from the foundation is 324 kN/m^2. Site investigation shows that the top
6 m of subsoil is saturated coarse sand with a unit weight of 18.0 kN/m^3. The ground water
level occurs at 3.00 m from the top of sand. The standard penetration value of the sand taken
at a depth of 4.50 m is 18. Below the sand there exists a clay layer of 30 m thickness (E = 16.2
MN/m^2 , Eswelling = 63 MN/m^2 ). The clay rests on hard rock. Determine the total settlement
under the foundation.
Vertical stress increments:
Net pressure = 324 kN/m^2
Relief due to excavation of 1.5 m sand = 18 × 1.5 = 27 kN/m^2
Gross pressure = (324 + 27) = 351 kN/m^2.