Slip circle theory

Another theory is the slip circle method as shown in Figure 4 (a). With slip circle methods the foundation fails by rotating about some slip surface. The slip surface is taken as the arc of a circle. Almost all foundation failures showing rotational effects. The actual centre of rotation is slightly above the base of the foundation and to one side of it as shown in Figure 4 (b).


Figure 4

This is shown to provide you with some theoretical background knowledge. The derivation of all equations for the different theories is beyond the scope of this subject.

Properties & strength of soils

Properties & strength of soils is shown in the tables below:

Table 1                      Properties of cohesive clay soils
Material State SPT (No) CPT (Mpa) C (kPa) ABP (kPa)
Alluvial clays soft 2 - 4 0.3 - 0.5 20 - 40 < 75
firm 4 - 8 0.5 - 1 40 -75 75 -150
Till and tertiary clays stiff 8 - 15 1 - 2 75 - 150 150 - 300
very stiff 15 - 30 2 - 4 150 - 300 300 - 600
hard > 30 >4 > 300 > 600


Table 2                                  Properties of sand
Packing RD  SPT (No) CPT (Mpa)
SBP (kPa)
very loose <0.2 < 5 < 5 < 2 < 30
loose 0.2 - 0.4 5 - 10 2 - 4 30 - 32 30 - 80
med. Dense 0.4 - 0.6 11 - 30 4 - 12 32 - 36 80 - 300
dense 0.6 - 0.8 31 - 50 12 - 20 36 - 40 300 - 500
very dense > 0.8 > 50 20 40 500
SPT
CPT
C

= Standard penetration test
= Cone penetration test
= Cohesion with respect to    total stress
ABP


SBP
= Allowable bearing pressure
= Angle of internal friction
   (angle of repose)
= Safe bearing pressure