Foundation
❖ Functions of foundation: -
1. To transmit the structure loads safety to soil, this transmit of load may be through
..
• Shallow foundation
• Deep foundation
2. To transmit lateral earth pressure through retaining wall
❖ Foundation types: -
a- Shallow foundation
• strip footing
• isolated footing
• combined footing
• strap footing
• Raft
b- deep foundation
▪ Piles
-they are used when the top soil is so weak to use shallow foundation .
o Types of piles :
1- End bearing piles
2- Friction piles
• Factors govering the choice of the type of foundations :-
1- Structure configuration and function
( residential building , tank , pump station,…. )
2- Loads type and magnitude
For example ,
- Foundation for 5 story building will not the same for 20 story
building
3- Soil condition
( bearing capacity , settlement )
4- Ground water table
5- Estimate cost of foundation and available budget
•Failure causes of foundation :-
1- Shear failure
Applied pressure ˃ qall
2- Excessive settlement
Large settlement or differential settlement
3- Error in design or during construction
4- Instability of structure due sliding or over turning
• Choice of foundation depth :-
1. Structure configuration
2. Depth of top soil (( fill ))
3. Location of different utilities , sewer pipe, water pipe , cable pf electricity.
4. Location of grand water table
5. Location of the foundation of existing building
❖ Choose the type of the foundation: -
Ratio
𝐴.𝐹
𝑅𝑎𝑡𝑖𝑜 =
𝐴.𝐿
𝑓𝑜𝑟𝑐𝑒
𝑠𝑡𝑟𝑒𝑠𝑠 =
𝑎𝑟𝑒𝑎
𝐵𝑒𝑎𝑟𝑖𝑛𝑔 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑜𝑖𝑙
𝑞𝑎𝑙𝑙. 𝑞𝑎𝑙𝑙. 𝑛𝑒𝑡
𝑃𝑐𝑜𝑙.
𝑞𝑎𝑙𝑙 / 𝑞𝑎𝑙𝑙. 𝑛𝑒𝑡 =
𝐴. 𝐹
Ratio foundation
˂ 75% isolated
75-100 % Raft
˃100 % Deep Foundation
Settlement
Settlemen foundatio
t n
˂ 3 cm isolated
3-10 cm Raft
˃10 cm Deep Foundation
- S= 𝑚𝑣 ∗ 𝐻𝑐 ∗ ∆𝜎
𝐶𝑐
- S= ∗ 𝐻𝑐 ∗ 𝑙𝑜𝑔(𝜎°+∆𝜎 )
1+𝑒 𝜎°
• The difference between (𝑞𝑎𝑙𝑙 ) and (𝑞𝑎𝑙𝑙. 𝑛𝑒𝑡)
𝑃 𝑐𝑜𝑙.
• 𝑞𝑎𝑙𝑙. 𝑛𝑒𝑡=
𝐴.𝑓
𝑃 𝑐𝑜𝑙.∗ 1.1
• 𝑞𝑎𝑙𝑙 =
𝐴.𝐹
Mv >> coefficient of volume change
𝑐𝑚2 /𝑘𝑔
➢ Very high compressible 0.1
➢ High compressible 0.1-0.02
➢ Low compressibility 0.005-.002
➢ Very low compressibility ˂.002
Hc >> clay layer thickness
Hc = effective depth = 2B " width of footing "
𝐻𝑐
Z= 2
𝑓𝑜𝑟𝑐𝑒 𝑃 𝑐𝑜𝑙. 𝛴𝑃 𝑐𝑜𝑙.
∆𝜎 = = ( 𝐿+𝑍)(𝐵+𝑍) = (𝐿+𝑍)(𝐵+2)
𝑎𝑟𝑒𝑎
given
𝐶𝑐
Cc=.009(L.L- 10 %)
e = void ratio
σ° =Σɤ.h from (G.S) to ( c.L of clay layer )
(0.0)
example: - Fill
γ=12.5 kN/m3
it's required to choose the type of foundation
(-2.0)
suitable for the site, allowable settlement in case
Med. Clay
isolated = 3 cm, Raft = 10 cm
2
qu=100 kN/m
• site area =16*15
mv = 3*10-4 m2 /kN
• loaded area = 4*5
• P col. =1000 KN. (-12.0)
Site Area = 16*15, loaded Area = 4*5 Med. Sand
𝑠𝑖𝑡𝑒 𝐴𝑟𝑒𝑎 16∗15 qall=150 kN/m
2
No. of col . = = = 12 𝑐𝑜𝑙.
𝑙𝑜𝑎𝑑𝑒𝑑 𝐴𝑟𝑒𝑎 4∗5
𝐴.𝐹 𝑝 𝑐𝑜𝑙. 𝑞 𝑢𝑙𝑡
Ratio= ≫ 𝐴. 𝐹 = (𝑞𝑎𝑙𝑙 = )
𝐴.𝐿 𝑞𝑎𝑙𝑙 𝐹.𝑂.𝑆
∴ 𝑞𝑢𝑙𝑡 = 𝐶. 𝑁𝑐 + ɤ1. 𝜈𝑓. 𝑁𝑞 + 0.5. 𝐵. ɤ2. 𝑁ɤ (-22.0)
𝑞𝑢 100
𝐶= = = 50𝐾𝑁/𝑚2
2 2
𝑚𝑒𝑑 𝑐𝑙𝑎𝑦 → ∅ = 0 → 𝑁𝐶 = 5.7 , 𝑁𝑞 = 1 , 𝑁𝛾 = 𝑍𝐸𝑅𝑂
∴ 𝑞𝑢𝑙𝑡 = 50 ∗ 5.7 + 12.5 ∗ 2 ∗ 1 + 0 = 310 𝐾𝑁/𝑚2
310
∴ 𝑞𝑎𝑙𝑙 = = 103.3 𝐾𝑁/𝑚2
3
𝐶∗𝑁𝐶 50∗5.7
→ 𝑞𝑎𝑙𝑙. 𝑛𝑒𝑡 = = = 95 𝐾𝑁/𝑚2
𝐹.𝑂.𝑆 3
𝑃 𝑐𝑜𝑙. 1000
∴ 𝐴𝐹 = = = 10.52 𝑚2
𝑞𝑎𝑙𝑙.𝑛𝑒𝑡 95
𝐴𝐹 10.52
∴ 𝑅𝑎𝑡𝑖𝑜 → = ∗ 100 = 52.6% < 75%
𝐴.𝐿 4∗5
∴ 𝐼𝑠𝑜𝑙𝑎𝑡𝑒𝑑
Settlement ( for med clay )
𝑆 = 𝑚𝑣 ∗ 𝐻𝑐 ∗ ∆𝜎
L=3.5 m
∎ mv=3*10−4 KN/𝑚2
B= 3m
∎𝐻𝑐 = 2 ∗ B ⇒ 𝐴𝐹 = 10.52 = 𝐿 ∗ 𝐵 𝑯𝒄 𝟔
𝒁= = = 𝟑𝒎
𝟐 𝟐
Effective depth assume L = 3.5 B= 3 m
Pcol. = 1000 KN
𝐻𝑐 = 2 ∗ 3 = 6 𝑚
𝑃 𝑐𝑜𝑙.
∎ ∆𝜎 ( 𝑖𝑠𝑜𝑙𝑎𝑡𝑒𝑑 ) =
( 𝐿+𝑍)(𝐵+𝑍)
1000
∆𝜎 = = 25.6 KN/𝑚2
( 3.5+3)(3+3)
∴ 𝑆 = 3 ∗ 10−4 ∗ 6 ∗ 25.6 = 0.046 𝑚 = 4.6 𝑐𝑚
( 3 − 10 ) 𝑢𝑠𝑒 𝑅𝑎𝑓𝑡 𝑓𝑜𝑢𝑛𝑑𝑎𝑡𝑖𝑜𝑛 .
L=16 m , B=15 m , Σ Pcol.= No. of col. * Pcol.
=12*1000
=12000 KN
Effective depth = 2*B = 2*15= 30 m ( Hc = 10m)
𝐻𝑐
(𝑍 = =5𝑚)
2
𝜀 𝑃𝑐𝑜𝑙.
∆𝜎 (𝑅𝑎𝑓𝑡) =
(𝐿+𝑍)(𝐵+𝑍)
12000
∆𝜎 = = 28.57 𝐾𝑁 /𝑚2
(16+5)(15+5)
∴ 𝑆 = 3 ∗ 10−4 ∗ 10 ∗ 28.57 = 0.085 𝑚 = 8.5 𝑐𝑚
∴ 𝑢𝑠𝑒 𝑅𝑎𝑓𝑡 𝑓𝑜𝑢𝑛𝑑𝑖𝑜𝑛
