2. Introduction:
Well foundations are one of the types of deep foundations that
provide a solid and massive foundation typically for bridges
and heavy structures
Caissons or well have been in use for foundations of bridges
and other structures since Roman and Mughal periods.
“Caisson” means a box like structure, round or rectangular,
which is sunk from the surface of either land or water to some
desired depth
3. Types of Caisson
Open caisson
Box caisson
Pneumatic caisson
Open Caisson:
Open cassion is a box
opened both at top and
bottom.
It is made up to either
timber, concrete or steel.
The open cassion is called
well.
4. Types of Caisson
Box caisson:
It is open at the top and
closed at the bottom and
is made of timber,
reinforced concrete or
steel.
This type of caisson is
used where bearing
stratum is available at
shallow depth.
5. Types of caisson
Pneumatic caisson
Pneumatic caisson has its
lower end designed as a
working chamber in which
compressed air is forced to
prevent the entry of water
and thus excavation can be
done in dry conditions.
6. Shape of Well:
Double-D
Twin hexagonal
Twin octagonal
Single circular
Twin circular
Dumb well
7. PROBLEM
1) A bridge pier in a sand deposit with external diameter d=8.5 and the
depth of well below scour level D=15m is subjected to the following
loads
Vertical Load, W= 14000 kN
Horizontal Load, H= 2000 kN
Moment about base level, M= 42000 kN
The value Ø of the sand = 30˚
Wall friction, δ= 20˚
Allowable bearing = 60t/m²
kh/kv = m = 1
Assume the weight of soil is 20 kN/m³
Check the lateral stability of the well under the above forces according to
IRC 45 (1972) recommendations.
8. Solution:
We check the four conditions for stability in the elastic state and also check the
ultimate state
ELASTIC ANALYSIS
(1) H > (M/r)(1+µµ´)-µw
(2) H < (M/r)(1-µµ´)+µw
(3) Mm/I ≯γ(Kp – KA)
Where , γ - Submerged unit weight of soil
Kp – Passive earth pressure
Ka– Active earth pressure
(4) σ1, σ2 =
Where, σ- soil pressure
IB
MBW
2A
µ´P-
±
9. STEP-1:DETERMINE THE PARAMETER
Projected dimension, L=0.9d = 0.9 X 8.5=7.65 m
IV = LD³/12 = 7.65 X 15³ /12 = 2151m 4
where ,Iv – moment of inertia of vertical area of L * D
IB = πd 4 /64 = π(8.5) 4 /64 = 256 m 4
where, IB - moment of inertia of rectangular or circle base
α = d/ πD = 8.5/ π X 15 = 0.18
µʹ= tan δ = tan 20 = 0.18
where, µʹ - Side friction
µ = tan Ø = tan 30 = 0.58
where,µ - friction at base
I = IB + MIv(1+2µ´α)
= 256+2151(1+2*0.36*0.18) = 2685.7
r = ID/2mIV
= 2685.7*15/2*1*2151
r = 9.36m
10. Kp = tan² (45+ø/2) = 3.69 (Passive earth pressure)
Ka = tan² (45+ø/2) = 0.27 (Active earth pressure)
STEP-2:
Condition -1: Check whether the Base friction is safe or not
H > (M/r)(1+µµ´)-µw
>
> -4182 kN
2000kN > -4182kN (satisfied).
1400070.0)36.070.01(
36.9
42000
12. Condition – 4 :(M =external moment at base of well)
σ =
A=πd²/4 = 56.72m²
σ =218 ± 66 =284 and 152 kN/m²
Being less than the value of 600 kN/m²,these value are acceptable .
BI
MBW
2A
µ´P-
±
r
M
P =
kNP 4487
36.9
42000
==
7.26852
5.842000
72.56
448736.014000
×
×
±
×
=
13. Ultimate Analysis:
The following two condition should be satisfied.
(1)
(2) Mr =0.7(Mb + Ms + Mf) and M < Mr
M = actual moment acting on the well
Step 1: Calculate stress at base in kN/m²
qu = 600 (ultimate bearing capacity)
246 ≯ 600 (kN/m²) is satisfied.
Step 2: Calculate Mb (base moment)
Mb = (QB) (W tan ø )
uq
A
W
/>=
246
72.56
14000
==
A
W
14. D/d = 15/18.5 =1.76
Q = 0.53 *0.6 = 0.32 (refer the value of constant Q and also multiply the
shape factor 0.6 for circular base)
Mb = 0.32*8.5*14000*0.70 = 26,656 kNm
Step 3: Calculate Ms (side earth pressure moment)
Ms = 0.1γD³ (KP –KA)L
= 0.1*10(15)³ *(3.69-0.27)*0.9*8.5
Ms = 88,300 kNm
Step 4: Calculate Mf (side friction moment) δ = 20˚
Mf = 0.11γ(KP-KA) B²D² sin δ
= 0.11*10*3.42*(8.5)²*(15)²*0.34
Mf = 20,793 kNm
15. Step 5: Find moment to be resisted at base
M = 42,000 kNm (given value)
Step 6: Estimate total resisiting moment
Mr = 0.7(Mb + Ms + Mf)
= 0.7(26,656+88,300+20,246)
Mr = 94,641 kNm
94,641 kNm > 42,000 kNm
Mr > M (satisfied)
Step 7 : Final results
All condition are satisfied. Hence safe.