FOOTINGS AND PILE FOUNDATIONS - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 196

TABLE 1 2.2 VALUES FOR PILE FACTORS
pile type c e c s

0.6 to 0.9 *
1
0.8
1
0.1
0.5

0.012
0.01
0.0075
0.0075
0.006
0.005
* depending on the way of installation: 0.6 for percussion driving

concrete (prefab)
timber
screw
steel
auger
bored

Practical design formulas for the shaft resistance are based on a large number of
pile loading tests, performed on different piles in different soils and with different
installation methods. Special reference is made to the API-code (American
Petroleum Institute), which is widely used. It states that in undrained soil the shaft
resistance q s can be expressed by

q s =Q s /A s = s c u (12.19)

Here, A s is the pile shaft surface area . The API-code gives the relation between

s and the undrained shear strength c u (see Fig 12.5b), which may vary with the
vertical soil stress. In drained soils the shaft resistance can be found from

q s = K v ' tan ? (12.20)

Here, K is the earth pressure coefficient and ? the apparent skin friction angle

(½ ). There is much to say about the value of K , as it may represent any

situation between an active and a passive stress state. It may dramatically change
during pile installation and partly recover afterwards. There are many practical
empirical methods in use to establish the end bearing and skin resistance
(Meyerhof, Vesi, Vijayvergiya, Burland, Jardine, Frank, Lehane, Xu, Van Tol).

< ? <

Dynamic testing
Monitoring of the dynamic behaviour of a pile during driving can be used, in the
first place, to optimise production, but also to estimate the static bearing capacity,
in the case of end-bearing piles.
Pile driving data (blowcount) and static and dynamic soil investigation methods
(CPT and SPT) reveal that the results show some similarity, but the differences
represent a complication for precise design (Fig 12.6a).
A dynamic soil response is fundamentally different from a static response. Soil
absorbs shear waves, which may decay in depth and which may run along the
surface (Raleigh waves) or along interfaces of soil layers with different impedance
(Stonely waves). These surface waves show relatively little damping. In principle,
the soil response can be approximated with shear and/or rotational springs and
dashpots (Wolf and Deeks). A comprehensive field test and sophisticated
numerical simulation has shown the pattern of dynamic energy dissipation during
pile driving (Fig 12.6b). While analysing the dynamic response during pile driving,
it may give information about the static bearing capacity, but soil variation
(layering) and soil constitution (friction, plasticity) make a reliable analogy a rarity.