Foundation, Concrete and Earthquake Engineering

Selection of Foundation and Structural System for Expansive Soil

Any one planning to construct foundation for new buildings on expansive soil, some special considerations should account in design and constriction which are somewhat different from construction on normal soils. In the preliminary design of a building to be constructed on expansive soil, the swelling potential should be considered and the acceptance limit of cracking, may be structural or nonstructural, should be set at this time.


Objective of foundation design


• Foundation should be designed such that no unexpected foundation and structural distress are observed throughout its service life

• Foundation should be selected based on availability of building materials, both construction skills and equipment.

• The moisture content below foundation should be made and maintained constant. In this regard, if possible the foundation construction should be followed by wet season and proper drainage should be provided to avoid ponding water. The moisture content of the excavation should be kept constant as far as possible.

Special consideration for bearing capacity, foundation system and superstructure system must be taken and a tolerable a angular distortion have to set during planning and design. To avoid excessive distress to the buildings, necessary flexibility to the structural system must be provided.

a. Evaluation of bearing capacity


Pressure from foundation loading should be more than swelling pressures of expansive soil, if possible; sometimes this foundation pressure may be less than swelling pressure even a proper deign is done. This is due to long construction period while loads are exerted gradually in case of multistoried buildings. The foundation pressure should be less than the predicted bearing capacity to avoid unexpected foundation displacement.The factor of safety is decided to keep safe value of foundation pressure sufficiently lower than bearing capacity.

Current theoretical assessment and empirical relationship furnishes a reliable estimation of ultimate capacity, but which is not applicable for evaluation of differential deformation of the foundation. So factor of safety is used for ultimate bearing capacity to evaluate allowable or safe working loads should be consistent with allowable settlement.

b. Selection of Foundation Systems



A well designed foundation should fulfill all functional requirements of the building and limit differential movement of different parts of the building that may be subjected to damage at optimum cost. A perfect foundation should effectively transmit maximum allowable distortion. The allowable distortion that can be said tolerable depends on design and use of the 
building.


In discussing selection of different types of foundation a new term  is required which is known as effective plasticity index. As heave potential of clay depends on  rather than PI. 

Now what is effective plasticity index? 


When the foundation soil has uniform plasticity index throughput the depth up to 15 ft (≥ feet), the PI is taken as effective PI for foundation design purposes. Thus effective PI is required for layered soil. When layered soil have different PI’s, the calculation of  is done using equation (1).



An example of calculation of PI are presented below for the top 15 feet soil under foundation slab.
Calculation of PI for the top 15 feet soil under foundation slab
Figure-1: An example of calculation of PI for the top 15 feet soil under foundation slab
Some assumptions are made in calculating   are:
  • PI≥15 that is if PI is less than 15, it is taken as 15
  • In case of slope foundation surface slope factor (FS) should be used to increase PI.
         log Fs=0.01S 
         where  
         Fs= Slope factor
         S= Gradient of slope in %

  • Weight factor is applied to modify PI; top and middle portion of the soil layer (segmented by three layer) weighted by 3 and 2 times of bottom one third segment of the layer considering upper layer have more contribution to foundation movement.
  • If PI immediate below the bottom of the stiffening beam of foundation slabs is more than other layers underneath lowest level of the slab, the PI of the topmost layer is taken as  i.e. applicable for entire soil mass. 
Sometimes is not considered reliable basis to design in case of foundation supported on non-expansive soil like sand or rock underneath 5 feet thick highly expansive soil. Previous strata of such type of soil may become a media to flow moisture toward nearly expansive soil. A guideline for selecting different types of foundation based on are provided below:

Shallow foundation



Shallow foundation may be of isolated or continuous footings and stiffened mats; selection of these footings is based on and differential movement are provided below:



Isolated or continuous footings


Isolated or continuous footings are selected as foundation system when it supported on low expansive soil, where predicted differential deflection is not more than 0.5 inches or ratio of predicted angular movement/span length lie between 1/600 to 1/1000.


Stiffened mats


This is a slab stiffened by beam which is used in expansive soil having predicted differential movement of up to 4 inches. Stiffening beams used in the foundation effectively reduce differential movement of that level. Following table provides required spacing and dimensions of stiffening beams which is considered enough to design lightly loaded structure.


Predicted Differential Deflection
Effective Plasticity Index, 
Foundation System
Design Remarks
0.5
 <15
Shallow isolated thin mat thickness (4-5) inches
For residential and lightly loaded building, must have stiffening beams of thickness 10~12 inches. The free area between beams must bit exceed 400 ft2, beam should have 0.5% reinforcing steel. To avoid distortion of corner exterior stirrups to avoid both torsion and shearing failure induced by higher edge forces. In addition beams are placed beneath corners.


Type of Mat
Beam depth
inches
Beam spacing
(feet)
0.5 to 1
15 to 25
Light
16 to 20
20 to 15
1 to 2
26 to 40
medium
20 to 25
15 to 12
2 to 4
41
heavy
25 to 30
15 to 12
Not limit

Thick reinforced concrete mat
Heavily loaded large structure having mats of thickness ≥ 2 ft.

Deep foundation

A pile or drilled shaft connected by overlying beams is appropriate for such type foundation soils but can be used for large variety of foundation soil. A well designed and properly constructed deep foundation effectively eliminates foundation damage due to heaving tendency of such soils. No limits for differential settlement and types of structural system are recommended for properly designed foundation. 


Grade beam connecting piles or shaft should be constructed 6~12 in above ground surface to permit expansion of soil. On grade slabs or supported floors are kept isolated from walls and grade beams. Drilled shaft may sometimes designed straight or under-reamed. Concrete used in RC cast in situ piles or drilled shaft should be at least 3000 psi having 6 inches slump. These type of foundations are reported to have deflection of shaft to spacing ration of not more than 1/600.

No comments:

Post a Comment

Followers