Chap. 13 Pile Load Transfer and Limit States
13.1 The toe bearing capacity in piles is similar to the ultimate bearing capacity of spread footings.
However, the side friction capacity has no equivalent in spread footing design. Why do we
ignore the friction acting on the side of spread footings?
Solution
Because the height of spread footings is very small compared to their width, the contribution of
Chap. 13 Pile Load Transfer and Limit States
13.2 When designing shallow foundations, we add the weight of the foundation to the applied column
load (for example, see Equation 6.1). However, with downward loads on piles, this weight is not
explicitly computed (see Equation 13.4). Explain how we account for the weight of piles
subjected to downward loads.
Solution
Shallow foundations are generally designed using gross bearing stresses which would include the
Chap. 13 Pile Load Transfer and Limit States
13.3 A prestressed concrete pile is being driven into a saturated cohesive soil with a hammer blow
count of 17 blows per foot of pile penetration. Unfortunately, the pile driving rig breaks down
before the pile reaches the required depth of penetration, resulting in a 15–hour delay. Once the
rig is repaired, pile driving resumes but the blow count is now 25 blows per foot. Explain the
primary reason for this change in blow count.
Solution
Driving of the pile caused large shear stresses in the soil. Since the soil was a saturated cohesive
soil, positive pore pressures were generated during driving and therefore decreased the effective
Chap. 13 Pile Load Transfer and Limit States
13.4 Sometimes pile driving contractors use predrilling when installing piles. This method consists of
drilling a vertical hole that has a smaller diameter than the pile, then driving the pile into this
hole. Could predrilling affect the side–friction resistance? Why? Is the diameter of the predrill
hole important? Why?
Solution
Predrilling can definitely affect the side–friction resistance by loosening the soil and by reducing
Chap. 13 Pile Load Transfer and Limit States
13.5 Why is it important for drilled shaft contractors to place the concrete soon after drilling the shaft?
What detrimental effects can occur if the contractor waits too long before placing the concrete?
Solution
There are two potential detrimental effects which can be caused by delaying concrete placement.
The first is related to reduction in lateral earth pressure. The drilled shaft construction process
Chap. 13 Pile Load Transfer and Limit States
13.6 Why are there such a wide variety of geotechnical resistance factors in Tables 13.3, 13.4 and
13.5? Why not use a single resistance factor for each failure mode?
Solution
Because the resistance factors account for both the uncertainty of the analysis model as well as
Chap. 13 Pile Load Transfer and Limit States
13.7 Why is it not advisable to use the geotechnical resistance factors in Tables 13.3, 13.4 and 13.5
with the ASCE–7 load factor from Equations 5-12 through 5-18 for geotechnical design of piles?
Solution
Because the load factors and resistance factors are calibrated together to provide an acceptable