Page 68 - Handbook GEOTEKNIK_Final_Neat

Pile Bearing Capacity (Pile in Rock) Negative Skin Friction, Q
nsf

• Allowable pile bearing capacity, Q a • Negative skin friction or downdrag happens, if settlement of surrounding soils is more than
Q = A*f pile settlement
a
Q = f *A
n
nsf
n
2
Where A = Surface area of bond (mm )
2
f = Allowable bond strength (working bonding stress) (N/mm ) Where f = Unit negative friction (kN/m )
2
n
2
= 0.05q A = Area of negative skin (m )
n
uc

< 0.05f (f = grout/concrete strength characteristic) Table 12: Relative difference between soil settlement and piles
uc
uc

Table 11: RQD and Allowable Bond Strength for Different Type of Rock > 5 – 10 mm <<<< 5 mm

Type of rock RQD (%) f (N/mm ) f = k P (B B Broms) f = k P (Bjerrum)
2
o
n
n
o
Where; Where;
Limestone <25 0.3 P = Effective overburden P = Effective overburden pressure
o
o
1 25 - 70 0.6 pressure k = 0.20 for low plasticity clays
2 k = 0.20 for n-c clay with k = 0.15 for clays of medium
PI>50%
plasticity
3 >70 1.0 PI>50% k = 0.10 for highly plastic clays
BS 8081 recommend 0.26 – 1.18 k = 0.35 for sand and gravel
k = 0.40 for rock fill
Granite > 80 2.0
Hard Shale > 80 0.65 • Hence,
2
• Working bond stress should be limited to f = 0.6N/mm , unless detailed information of rock Q = (Q /2 + Q /3) - Q
all
s
nsf
b
socketed is known.

52 PUBLIC WORKS DEPARTMENT MALAYSIA GEOTECHNICAL ENGINEERING HANDBOOK 53