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.


C. Selection of structural system


The superstructure of a building should be selected such that it had flexibility i.e. If subjected to deformation under soil movement it behaves compatible with the foundation. The objective of flexible superstructure is to perform structure under expansive soil movement such that

  • Overall functionality of structure remain uninterrupted
  • Minimum maintenance is required
  • Contributes aesthetically to remain compatible to environment. 

Structures susceptible to settlement


Buildings consist of load bearing walls are found more susceptible to shear failure and subsequent damage as compared to that have more flexible framing system. The suggestion is to provide

  • Frame construction
  • Truss roofs
  • Open plans.



The effects of differential movement are reduced with such structures. As an example, lateral thrust can be minimized on walls by designing roof truss system with timber overhead beams as tension members. Relative flexibility offered by different types of superstructure systems, more precisely components, are presented below


  • Rigid
  • Semirigid
  • Flexible
  • Split construction

Rigid-Slab on grade; unreinforced masonry with or without plaster; precast concrete block etc.



Semirigid-Reinforced masonry; brickwork reinforced with vertical and horizontal tie bars, bands provided in brickwork by steel bars or applying RC beams with provision of vertical reinforcement on either sides of opening like windows and doors, slab on grade with provision for isolation from walls. 


Flexible-Brick veneer having articulated joints; wood, metal or vinyl panels; gypsum boards fixed with metal or wood studs; vertical construction joints; wood, steel framing; all types of drains and pipes run through the structure having flexible joints; suspended floors or slabs supported on grade having proper isolation from walls designed considering probable heaving and subsequent cracking of slab; metal panels as walls or strip windows provided to separate rigid wall section (spacing should not be more than 25 feet).

Spit construction-Modular construction where walls, rectangular sections move as a unit, walls or components which are separated by joints at a spacing of 25 feet or less, slab-on-grade or suspended floors isolated from walls omitting probable damage to slab-on-grade; all drains and water pipes having flexible joints; construction joints in stiffened and reinforced slabs with a spacing of 150 ft and provision for cold joints, if required, at not more than 65 ft spacing.


The flexibility of superstructure components has been discussed above; now question is 



Why is flexibility of structure required?




The flexibility of structure is required on expansive soil to eliminate unexpected distress may be offered by flexible components and joints. According to requirement of differential movement, the joints in the wall is provided; walls neither should be tied with the ceiling nor should slab-on-grade be ties to columns or foundation walls. The isolation should be done by expansion joints or impervious and flexible compounds should be used to fill the gaps. 


The brick, masonry block and plaster walls should be replaced by stud frames and paneling, gypsum board and RC walls to perform structure against distortions. The foundation of walls may be made flexible by additional reinforcement and making walls to behave as structural member having resistance to bending like RC shear walls.

Ability of structure to tolerate deformations 

This tolerance of structure depends on 
  • Brittleness of construction materials
  •   The ratio of length to height
  • Relative stiffness of the frame in shear & bending
  • Mode of ground deformation whether settlement i.e. dish-shaped deformation or heave i.e. dome-shaped deformation.
The ratio Δ/l  is an important parameter for selecting right type of structure.

Where Δ =differential displacement

              l =distamce between columns as well as footings

Thus Δ/l   is a measure of differential movement, Δ over a length l which is known as vertical angular deflection to span length ratio. The ration Δ/l  that can be tolerated by a structure is dependent on above variables. 


For a slab supported on ground the ratio of A/ is controlling parameter

Where A= deflection length
and = length of slab.

Δ/l     2  A/l

Propagation of cracks


It depends on degree of tension restraint developed in the structure and supporting foundation. So building frames with wall panels are more restraint to relative deflections i.e. tension restraint than URM buildings.

Generally structural damage in buildings subjected dish-shaped deflection (settlement at center) is found less frequent than that subjected to edge down warping ( heave in the center) as the foundation has better ability to resist tension forces or respond better than the walls.

Tolerable angular deflection limits

Limits for Δ/l , as discussed above, for different superstructure systems are presented below:


Superstructure system
Acceptable vertical angular deflection/span length ratio, Δ/l
Rigid
1/600 to 1/1000
Semi-rigid
1/360 to 1/600
Flexible
1/150 to 1/360
Split construction
1/150 to 1/360

A safe value of Δ/l is 1/500 is very familiar to avoid generation of cracks in single and multistoried structures. Plaster in walls, masonry or precast blocks (concrete) and brick walls are often subjected to damage for Δ/ratio of 1/600 to 1/1000. But sometimes cracks may not developed in such walls when the rate of deflection is slow enough to permit frame and foundation to adjust for the new distortions. A common method to reduce cracking is to apply lean mortar over soft bricks.

Reinforced concrete beams and walls, steel frames and reinforced masonry can tolerate angular deflection

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