Foundation design can be done using the following two methods.
1. Using Elastic Theory, loads are assigned and applied onto the soil surrounding the foundation. The soil, with its particular Young’s Modulus and Poisson’s Ratio, is calculated for its deformation, which is dependent on the soil’s stiffness, due to a given pressure over a given area.
2. Using Subgrade Reaction Method, a series of springs are assigned onto the soil. These independent springs, that simulates as soil mass with different stiffness properties, function as resistance to loads applied to the superstructure or foundation elements. This method is usually used for raft foundations.
The foundation design is based on unfactored loads. It will be designed to have load bearing capacity capable of supporting the superstructure loads and, at the same time, have limited amounts of settlement. Bearing capacity is the ability or the maximum pressure of the foundation to support loads, without failure in shear.
Stress can be induced by the differential settlement between new and existing foundations, between different foundation types, and unexpected ground conditions. Maximum allowable settlement for low rise buildings should be 25mm. And the allowable settlement to distance ratio should be less than 1:300. For tall buildings, differential settlement should govern where settlement should be limited to a maximum of L/150 for differential settlement (where L is the distance between columns or walls)
Groundwater must be controlled, during and after construction. This is because the change in groundwater can change the water table of the soil and affect the moisture content.
Typical soil properties
Undrained conditions are when pore water pressures within soil are supporting applied forces in the short term. Undrained conditions are often considered for fine grained soils. However, for granular soils above the water table, pore water pressures are not accounted for due to pore water draining quickly. Drained conditions will then be used for granular soils, as well as, for conditions where pore water have drained in the long term.
Typical allowable bearing capacity
|
Description |
Safe bearing capacity (kN/m^3) |
|
Limestones and hardstones |
Up to 3000 |
|
Stiff clays |
150-300 |
|
Firm clays |
75-150 |
|
Soft clays and silts |
<75 |
|
Compact or gravel |
600 |
|
Medium dense gravel and sandy gravel |
200-600 |
|
Loose gravel and sandy gravel |
<200 |
|
Compact sand |
300+ |
|
Medium dense sand |
100-300 |
|
Loose sand |
<100 |
|
Firm organic |
20-40 |
Source: Structural Engineer's Pocket Book, 2nd Edition: British Standards Edition
Equations for allowable bearing for shallow foundations
Area of foundation = Applied loads/Allowable bearing pressure
Shallow foundations on undrained cohesive clay
Allowable = 2Cu (on undrained cohesive soil yf = 2.5)
Shallow foundations on drained gravel
Allowable = 10N Pad footing on dry soil (yf =3)
Allowable = 7N Strip footing on dry soil (yf =3)
Allowable = Allowable/2 when the foundation is on or below the water table
Source: Structural Engineer's Pocket Book, 2nd Edition: British Standards Edition
Equations for allowable bearing for piles
Concrete piles are made of cast insitu, prestressed, precast or reinforced concrete. Steel piles are needed when the foundation is required to have slender or lightweight piles. Even though steel piles require corrosion protection, they are good in supporting lateral forces.
Pile diameters generally range from 100mm to 2000mm and are 5m to 100m in length.
Typical pile capacity for bored piles = 300 to 1800 kN
Typical pile capacity for driven piles = 500 to 2000 kN
Typical pile capacity for minipiles = 50 to 500 kN
Piles should be spaced about 3 diameters apart (between pile faces). Pile capacities increases at deeper depths. However, for piles in granular soils, negative skin friction and pile capacities peak at around 110,000 kPa when pile capacities are taken at around 20 pile diameters deep.
For bored piles in granular soils, the bearing capacity is less than that of driven piles since there is loosening effect as a result of boring.
Other considerations
Negative skin friction is the additional drag force acted on the piles by surrounding cohesive soil as its soft layer consolidates and compresses when the piles are being driven and compressed to reach a firm strata layer below. Negative skin friction is also induced by the drag force of the soil weight on the piles.
NSF used for group of piles
Qskin friction = AH/Np
A = area of pile group
H = thickness of consolidation layer of soil
Np = number of piles in group
Pile caps assist in transmitting applied loads from the superstructure into the piles below. One constant pile cap depth should be used for one project, even with different piles with different diameters, in order to reduce cost and labor.
|
Pile diameter (mm) |
300 |
350 |
400 |
450 |
500 |
550 |
600 |
750 |
|
Pile cap depth (mm) |
700 |
800 |
900 |
1000 |
1100 |
1200 |
1400 |
1600 |