CVLE451

Online Lecture 5
Books referred:Donald Coduto, Foundation Design- Principles and Practice, Prentice Hall; 2nd
Edition, 2001; John Cernica, Foundation Design, Wiley, 1994
In situ test data analysis, BC and Settlement (Past MT question)

A one-story building is to be supported by spread

footings on a soil profile as shown below. The profile is
obtained by soil exploration done to a depth of 17 m,
using a rotary drill.

a. Is this exploration depth sufficient for analyzing the

foundation-soil interaction? (Use de Beer’s
approach).

b. A Shelby tube of outer diameter 7.56 cm and inner

diameter of 7.30 cm is used to recover undisturbed
samples from the clay layer. Could you make a
comment on the quality of these samples?

c. Standard penetration test (SPT) has been conducted

at 1.5 m depth intervals in the sand layer.

i. Determine the average (N1)60 and correlate to 

(angle of internal friction angle) of sand using e. Determine the allowable bearing capacity using Terzaghi’s equation, and compare with (iii).
Hatanaka and Uchida (1996) approach.
ii. Determine Young’s Modulus, Es using Kulhawy f. Vane Shear tests were conducted in the clay layer. The vane dimensions were 63.5 mm x 127
and Mayne (1990) approach. mm. At mid clay, the torque required to cause failure was 0.06 N.m. Estimate the over-
iii. Determine the allowable bearing capacity using consolidation ratio (OCR) of clay.
the average N60 of the sand layer. Is the chosen g. Determine the consolidation settlement of the clay layer using the following assumptions:
foundation size safe to carry the given column
load. Cc=0.009 (LL-10) and if required take Cr= 1/10 Cc
d. Determine the elastic settlement expected to occur h.What will be the total settlement and is it within a reasonable range?
within a time span of 10 years.
a. Is this exploration depth sufficient for analyzing the
foundation-soil interaction? (Use de Beer’s
approach).

b. A Shelby tube of outer diameter 7.56 cm and inner

diameter of 7.30 cm is used to recover undisturbed
samples from the clay layer. Could you make a
comment on the quality of these samples?
c. Standard penetration test (SPT) has been conducted at 1.5 m depth intervals in the
sand layer.
i. Determine the average (N1)60 and correlate to  (angle of internal friction
angle) of sand using Hatanaka and Uchida (1996) approach. (6%)
ii. Determine Young’s Modulus, Es using Kulhawy and Mayne (1990) approach.
iii. Determine the allowable bearing capacity using the average N60 of the sand
layer. Is the chosen foundation size safe to carry the given column load.
(Stolerable= 25 mm)
d. Determine the elastic settlement expected to
occur within a time span of 10 years.
e. Determine the allowable bearing capacity
using Terzaghi’s equation, and compare with
(iii).
f. Vane Shear tests were conducted in the clay layer. The vane dimensions were
63.5 mm x 127 mm. At mid clay, the torque required to cause failure was 0.06
N.m. Estimate the over-consolidation ratio (OCR) of clay.
g. Determine the consolidation settlement of the clay layer using the following
assumptions:
Cc=0.009 (LL-10) and if required take Cr= 1/10 Cc
h.What will be the total settlement and is it within a reasonable range?
Results of a plate load test in sandy deposits are given in the
figure. If the size of the plate used is 0.3 x 0.3 m,
(a) Determine the ultimate bearing capacity of the soil-plate.
(b) What will be the settlement of the plate at this value?
(c) Determine the allowable bearing capacity of the soil-footing if
the footing size is 2 m x 2 m. (FS=2.5)
(d) Would a footing of size 2 m x 2 m carry 1000 kN in this soil
stratum?
(e) What will be the settlement of the footing under this load?
(f) Find the footing size for a tolerable settlement of 25 mm.
A certain column is to be supported on a 1.0 m deep rectangular footing,
subjected to a vertical downward load of 1000 kN, a horizontal load of 500 kN
and a moment of 250 kN-m in the direction of B. Soil properties: = 18 kN/m3, c=
15 kPa, = 25. The groundwater table is at a depth of 1.5 m below the ground
surface.
(a) Compute the allowable bearing capacity of the footing-soil system given,
using the General Bearing Capacity Equation. Use a factor of safety of 3.
(b) Is this size safe in bearing capacity?
(a) What will be the bearing capacity if the water table rises to the ground
level?
(d) Assume a ground inclination of 10◦ and determine the
allowable bearing capacity using Vesic’s equation.
(e) Could General BC Equation be used when ground inclination
exists?
 Problem on BC and Settlement (Past MT question)

(a) Correct the field N values to N60 using the

following field data: standard sampler, safety
hammer (USA), borehole diameter= 60 mm, rod
length= 1.0 m.
(b) Determine the allowable bearing capacity of the foundation- 0.5
uniform sand, using Terzaghi’s approach. Is the chosen foundation
size safe to carry the given column load? (FS=3)

(c) Determine a suitable size of foundation for the given data.

(d) Determine the elastic settlement of the footing-sand system
under the given load using the strain-influence factor method
within a time period of 10 years (Determine the soil stiffness, Es
using the average CPT results)
(e) Determine the over-consolidation ratio (OCR) of the (f)
clay using the appropriate empirical approach.

(f) Estimate the consolidation settlement of the clay layer

and determine the total settlement to be expected under
the given load and foundation size. Is it an acceptable limit
if the tolerable settlement is 25 mm?
(e)
 Problem on BC solved by Vesic’s Approach
A column is to be supported on a 1.5 m deep square footing,
subjected to a vertical downward load of 1200 kN, a horizontal
load of 600 kN and a moment of 300 kN-m in one direction.
(a) Determine the allowable bearing capacity (qall) using Vesic’s
approach.
(b) What will be the allowable bearing capacity if the water table
rises to the ground level?
(c) What will be the allowable bearing capacity if the ground
inclination = 10?
(c)
• Foundations may be grouped under two broad categories:
Shallow Foundations and Deep Foundations.

• A foundation placed near the surface of the earth or which transfers the
loads at shallow depth is called shallow foundation.

• A Foundation which is placed at a greater depth or which transfers the

loads to deep strata is called deep foundation.
 Types of Shallow Foundations

1. Spread Footings
These types of footings are also known as pad footings, isolated
footings, and square or rectangular footings (for an L/ B ratio
less than 5).

• 2. Combined Footings
• These footings are formed by combining two or more
columns (even with unequal loadings) into one footing.
• This arrangement averages out and provides a more or
less uniform load distribution in the supporting soil or
rock and, thus, prevents differential settlement.
• These footings are usually rectangular in shape but
may be modified to a trapezoidal one to accommodate
unequal column loadings (or columns close to property
lines).
• And provided with a strap to accommodate wide
column spacings or column close to property lines
##(Fig. 15.1b).
• 3. Continuous Footing
• These footings carry closely spaced columns or a continuous
wall so that the load intensity is low and uniform on the
supporting soil or rock.
• In such footings, the load per unit length is considered
accordingly.
• The load intensity is given in terms of force per unit length of
the footing.
• These footings are also referred to as strip footings or wall
footings (for an L/B ratio greater than 5).

4. Mat Foundation or Footing

• These are characterized by the feature that columns frame
into the footing in two directions.
• Any number of columns can be accommodated, and the
number can be as low as four.
• Mat foundations are recommended for poor foundation soils
and when the total area of footings exceeds 50% of the total
plinth area.
5. Floating Foundations

• The total load of a structure may cause a pressure that

is more than the safe bearing capacity of the soil or an
excessive settlement.
• In such cases, instead of changing the foundation size,
the structure may be placed at a greater depth.
• In this way, the weight of the excavated soil reduces
the total load and only a net load (total load — weight
of the excavated soil) is transferred to the soil.
• This technique of reducing the net load by more
excavation is called flotation and the foundation is
called a floating foundation.
• The technique where a load of a structure is partially
adjusted by the relief of load due to excavation is
called partial flotation, and if fully adjusted it is full
flotation.
• These techniques are suitable for light structures on
soft or loose soils and for heavy structures constructed
over a limited area.
Design Criteria for Shallow Foundations

• While considering a shallow foundation for a given loading system, the foundation must
meet certain design requirements.

• The three basic requirements are as follows:

• 1. Foundation placement, which involves the location and depth of foundation, requires a careful
investigation of the past usage of the site and detailed information of the sub-surface stratum. The
foundation placement should be such that any future influence should not affect its performance
adversely.

• 2. Safety against bearing capacity is a requirement that involves suitable proportioning of the
footing to avoid a catastrophic collapse of the soil beneath the foundation. This occurs if the shear
strength of the soil is inadequate to support the applied load. This requirement makes it essential
to have complete knowledge of the geo-technical properties of the soils and rocks involved.

• 3. Tolerable foundation settlement involves keeping a check on the excessive settlement of a

structure. The excessive settlement is caused due to the distortion of the soil mass as a result of
the applied shear stresses and due to the settlement of the supporting soil.
 Spread Footing Design

This type of footing is used when

• Columns are not closely spaced
• Loads on footings are low
• The safe-bearing capacity of the soil is
generally high
• The isolated footings essentially consist of a
bottom slab. There are three basic types of
bottom slabs:
• Pad footing (with uniform thickness)
• Stepped footing (with nonuniform thickness)
• Sloped footing (trapezoidal section)
• Reinforced steel mesh is provided to resist the two major
internal forces namely, shear force and bending moment.

• When there is property line, footings cannot be extended

in neighbour’s property, therefore may be designed as
combined footings or even eccentric footing is used as an
alternative to isolated footings.
Diagonal Tension Shear Check for Square Single Footings
General Procedure for Design of Isolated Footings