## 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