01 Foundations

Building Construction1. Foundations

CE 2330Civil Engineering Materials and Construction

Function of foundation

Transmit loads to sub-soil in such a way that• Settlements are within permissible limits,

without causing cracks in the superstructure, and

• Soil does not fail in shear

Foundations…

• Reduce load intensity (by distributing over large area)

• Distribute loads evenly (minimize unequal settlements)

• Provide level surface• Provide lateral stability• Protect against undermining and soil

movements

Can you draw some foundations on your own???

Some examples from the past..

What was the function of this building???

Next time you visit an old temple, try to figure out how the foundation system works!

Broad Classification• Shallow foundations – when depth <= width (in open

excavation, sometimes w>d is also allowed)- Spread footings- Combined footings- Strap footings- Mat foundations

• Deep foundations - Piles- Piers- Wells

Shallow Foundations

Depth of footing• Footings should be carried below top (organic)

soil, miscellaneous fill, abandoned foundation, debris, or muck

• According to NBC of India, D depends on:- Securing adequate bearing capacity- How deep shrinking and swelling soils exist (clays)- How deep is the frost penetration (in sand and silt)

Strip footing design

Width of footing B = W/qs, where W = total load, and qs = safe bearing capacity

d depends on type of concrete, projection of block and soil bearing pressure

Design of isolated column footings similar to strip wall footingsUsed for continuously running walls

Construction of strip footing

Excavation complete; reinforcement laid in position

Concreting in progress for the strip footing

Isolated footings

• Used for columns

Grillage foundations

• Used when load is heavy and bearing capacity is poor

• Primarily for steel columns (primarily industrial structures)

• Consists of two layers of steel I-sections in transverse directions, encased in concrete (to prevent corrosion)

Combined footings

• Why? – Columns too close, bearing capacity low, end column near property line

• Typically, rectangular or trapezoidal plan• Rectangular CG of footing = CG of

combined loads of the two columns• Trapezoidal – Unequal loads on two

columns

Continuous footings for more than 2 columns similar to strip footings

Raft foundations

Used when:• Soil bearing capacity is low• Building loads are heavy• Spread footings cover large area• Difficult to control differential settlement• Reduce settlement on highly compressible

soils (Weight of structure + raft ~ Weight of soil excavated)

For heavier loads, thickened bands may be provided along column lines in both directions

2 way grids with cellular construction may also be used for heavier loads

Also, basement walls could be used as deep beams

Rafts

Design of raft footings

• Rafts are designed as RC flat slabs, and the weight of the raft is not considered in the design (carried by the soil)

• However, for differential settlements (related shear and moment) rafts are reinforced more heavily than required by analysis

• Rafts may undergo large settlements without showing differential settlement (double settlement allowed for rafts)

Rafts – some examples

Reinforcement for 1.5 m thick raft

Slab on grade

Machine foundations• For heavy machines• Dynamic loads• Rough guide: ratio of weight of foundation to

machine weight may be kept between 2.5 – 3.5• Permissible bearing pressure under dynamic

loads may be taken as ½ to ¼ of pressure for static loads

• Covered in IS 2974

Get some pictures of machine foundations from our labs!

Deep Foundations

Why Deep Foundations?

• Heavy loads• Top soil has poor bearing capacity• High water level, with large fluctuations• Tough to retain soil in open excavations• Structure close to sea (scouring may occur)• Foundation close to canal or deep drainage lines• Top soil of expansive nature

Piles

Classification of piles• Concrete piles

- Precast- Cast in situ

Driven piles (cased/uncased)Bored piles (pressure, under-reamed, and compaction piles)

• Timber piles• Steel piles

- H-pile- Pipe pile- Sheet pile

• Composite piles- Concrete and timber- Concrete and steel

Cased cast in situ pile(a) Raymond standard pile

(tapered)

Taper of 1 in 30

L: 6 – 12 m

Dia: 40 – 60 cm (top); 20 – 30 cm (bottom)

Mandrel is withdrawn, leaving shell, which is filled with concrete

(b) McArthur cased pile (no taper)

An additional steel casing of heavy gauge is driven with the core. The core is withdrawn, replaced by shell. Concrete is placed in the shell can casing is removed

Uncased cast in situ pileSimplex Pile (for both soft and hard soils)

After being driven in, steel tube is removed and space is filled with concrete

New shoe needed for every pile

Alternatively, alligator jaw point could be used; opens up as concrete is poured, and withdrawn with steel tube

Soil needs to be firm around setting concrete

Vibro-piles: Similar to Simplex, except that while extracting tube, upward extracting and downward tamping blows given; used in soft soils

Bored PilesProcedure:

Ground is bored with auger

Concreting is done with/without casing

Pressure piles:

Compressed air forces concrete down, while steel tube is withdrawn; Pile dia > dia of bore; hence, soil is compressed; better friction

3 sizes: Dia 340 mm, 440 mm, and 500 mm

Depth < 25 m

Piles – some examples

Excavation for pile in progress

Piles being driven

Alaskan residence on piles

Sheet piles

Used for supporting earth load in an excavation

Can also be used to dam water for construction

New Orleans after Hurricane Katrina

Under-reamed pileBored cast in situ pile having one or more bulbs formed by enlarging the borehole for the pile stem using an under-reaming tool

Bulbs increase bearing capacity

Diabulb ~ 2.5 Diastem

Pile cap and pile group

• Pile cap: Rigid, deep RC slab acting monolithically with the pile group

• Piles in a group are arranged symmetrically about the axis of the column

• Pile cap has uniform thickness, and extends at least 10 – 15 cm beyond the edge of exterior piles

• Piles in the group are embedded at least 15 cm into the pile cap

Piles and pile cap

Transmission tower -superstructure

Pile cap

Piles

Well Foundations or Caissons

• French ‘Caisse’ meaning a chest or box• 3 types:

- Box caisson- Open caisson (well)- Pneumatic caisson

Box caissonBuilt on land and floated to site where it is sunk in position

Used when load bearing stratum is available at shallow depth, and where loads are not very heavy

Mainly used for breakwater and seawall

Open Caissons

Very popular for bridges in India

Many shapes possible; easy and uniform sinking for circular sections

Important features:

1. Well cap

2. Top plug

3. Sand filling

4. Steining

5. Curb

6. Cutting edge

7. Bottom plug (no reinforcement)

CaissonsExtensively reinforced caissons under construction

Completed bridge – showing caissons supporting piers

Next time you travel by train / road, try to see if you can spot some caissons

Well sinking operations

• Laying the well curb• Masonry in well steining• Sinking:

- Excavate material from inside the curb- As well sinks deeper, upward force due to skin friction kentledge required- Uniform dredging (avoid any tilts)- Tremie concreting for bottom plug

References

• Various web sources• Wikipedia• Building Construction by B C Punmia• Building Design and Drawing by Balagopal

T S Prabhu et al.