Shallow foundations

Question 1

Name and describe three types of footings

Answer 1

Strip footing: long compared to its width. usually used for masonry walls.

Pad footings: support isolated loads such as columns. good for sand sites.

Raft or mat foundations: used where pad footings would need to to be very large or in places with swelling. can be several meters thick.

Question 2

Describe Ultimate limit state and serviceability limit state.

Answer 2

ULS - so the footings don’t fail

SLS - so the footings don’t deform excessively

Question 3

What is Poisson’s ratio?

Answer 3

the ratio of the lateral strain to the vertical strain.

V = - /\Ex / /\Ey

Question 4

What is Hooke’s law in 3D?

Answer 4

/\Yxy = 1/E (2*(1 + Y)*/\Txy)
/\Yxz = 1/E (2*(1 + Y)*/\Txz)
/\Yzy = 1/E (2*(1 + Y)*/\Tzy)
T = shear stress
Y = Shear strain
/\Evol = (1-2V)/E * ( /\sigmax + /\sigmay +/\sigmaz)

Question 5

Young’s modulus (E) = ?

Shear modulus (G) = ?

Bulk Modulus (K) = ?

Answer 5

E = sigma / e

G = T / Y  (G' = Gu (shear unrdained) as water can't shear)
Y = shear strain 
T = shear stress

K = isotropic stress (P) / Volimetric strain (evol)

Question 6

Describe :
Secant modulus (Esecant).

Tangent Modulus (Etangent).

Answer 6

Esecant is found by drawing a straight line from a datum point on the non-linear stress-strain curve. Esecant = stress/strain at this point.

Etangent is found by drawing a tangent to the stress-strain curve at a particular point.
Etangent = /\stress / /\strain

Question 7

What is:
Gibson soil?

Layered soil?

Answer 7

For normally or consolidated soils, soil stiffness tends to increase with increasing mean effective stress (stiffer with depth). This is Gibson soil.

Often, with different geological formations, there may be a layer of different stiffness overlaying each other. Treat each layer differently.

Question 8

What assumptions are there in Boussinesq equations?

Answer 8

soil is:
Isotropic: same properties in every direction
Homogeneous: Same properties through the soil
Half-space: assuming a body of infinite depth and lateral extent such that it occupies half of all space.

Question 9

What can affect Boussineq’s equation?

Answer 9

Where discontinuous lenses/ inclusions of high or lower stiffness exist
Where extensive plastic yielding of the soil occurs sue to relatively high surcharges or soft soil.

Question 10

How do you use Newmark’s Method?

Answer 10

1. Draw a plan sketch of the foundation outline such that the length of the scale line equals the depth of interest z and so that the point of interest x is in the centre of the hart.
2. Count the number of blocks Nq covered by the foundation. group together parts of blocks.
3. The vertical stress /\sigmaz at depth z beneath point x is given by:
   /\sigmaz = 0.005 * sum(Nq*q)

Question 11

How to us Fadum’s chart to find change in stress then settlement?

Answer 11

1. Find the dimensionless parameters M and n. Where
   m = L/Z and n = B/z. ( B and L are interchangeable)
2. Use the chart or table to find the influence factor Isigma.
3. change in stress = q * Isigma
4. settlement = (/\sigmav’ * h)/Eo’
   h= soil depth

Question 12

What do you have to do to use Fadum’s chart?

Answer 12

The point of interest must be on the corner of the footing.