SINGLE PILES UNDER AXIAL LOADING
293
For a particular applied load, values of settlement wb at the base of the pile
can be found by procedures given above. The corresponding values of unit end
bearing load q can be found by dividing the load at the base of the pile by the
area of the base. The full q/wb curve can be obtained by analyzing ail of the
loadings, provided the pile settles sufficiently. If a number of such experiments
in a variety of soils for a variety of piles are performed, corrélations can be
developed between soil properties and the load-transfer curves.
Referring to Figure 11.2a, the load test should be performed to cause plunging, where additional deflection results in no increase in load. The resuit will be
the development of the load-transfer curves to ultimate values, as shown by the
curves for the depths Zi, z2, and z3. The ultimate values of /ui( can be correlated
with the shear strength of the soil in order to develop équations for computing
the ultimate load in skin friction Qs. If the end bearing has reached an ultimate
value, the ultimate value of q can be correlated with the shear strength at the
base of the pile and yields the ultimate load in end bearing Qb. The capacity
of the pile under axial load Q is Qs + QbThe procedure for computing the capacity and settlement of a pile in clay,
found offshore at many locations, is illustrated in the paragraphs below. Let
Q,= [ fuitCdz
(11.1)
Jo
and
Qb = ^quNcAb
(11.2)
where L is the pénétration of the pile; fuit — qu^z/l ; Qu is the unconfined
compressive strength of the clay and assumed to be equal to twice the undrained
shear strength cu: Nc is the bearing-capacity factor which is taken as 9.0 for
ail except very short piles; and Ab is the area of the base of the pile. The
value of az can be interpolated from the following list: az = 1.0 at qu = 0
tons/ft2; 0.9 at 0.75; 0.8 at 1.12; 0.7 at 1.45; 0.6 at 1.82; 0.5 at 2.36, and 0.43 at
3.0. To simplify calculations, these numbers represent average values, selected
from a curve with a wide range (see page 288 of Peck et al., 1974; Tomlinson,
1980). For the behavior of piles in clay under axial loading, the sélection of
the value of az varies among authors, and even the method of computing the
axial capacity varies among investigators (American Petroleum Institute, 1993).
The lack of agreement among investigators for piles driven into clay, and even
more disparity for piles driven into sand, is due to the scarcity of high qualitv
experimental data. The models for ultimate capacity and for settlement will
remain useful even as more data become available.
The pile selected for analysis is an open Steel pipe with a diameter b o. 36
>d. and assumed to hâve been driven to a pénétration of 140 ft into clay at an
offshore location with the following properties: 0 to 50 ft, qu — T0 tons/ t ,
t0 100 ft, qu = 1.8 tons/ft2; 100 to 175 ft, qu = 2.5 tons/ft2. Interpolatton of
values of az from the list in the previous paragraph yielded the following values
293
For a particular applied load, values of settlement wb at the base of the pile
can be found by procedures given above. The corresponding values of unit end
bearing load q can be found by dividing the load at the base of the pile by the
area of the base. The full q/wb curve can be obtained by analyzing ail of the
loadings, provided the pile settles sufficiently. If a number of such experiments
in a variety of soils for a variety of piles are performed, corrélations can be
developed between soil properties and the load-transfer curves.
Referring to Figure 11.2a, the load test should be performed to cause plunging, where additional deflection results in no increase in load. The resuit will be
the development of the load-transfer curves to ultimate values, as shown by the
curves for the depths Zi, z2, and z3. The ultimate values of /ui( can be correlated
with the shear strength of the soil in order to develop équations for computing
the ultimate load in skin friction Qs. If the end bearing has reached an ultimate
value, the ultimate value of q can be correlated with the shear strength at the
base of the pile and yields the ultimate load in end bearing Qb. The capacity
of the pile under axial load Q is Qs + QbThe procedure for computing the capacity and settlement of a pile in clay,
found offshore at many locations, is illustrated in the paragraphs below. Let
Q,= [ fuitCdz
(11.1)
Jo
and
Qb = ^quNcAb
(11.2)
where L is the pénétration of the pile; fuit — qu^z/l ; Qu is the unconfined
compressive strength of the clay and assumed to be equal to twice the undrained
shear strength cu: Nc is the bearing-capacity factor which is taken as 9.0 for
ail except very short piles; and Ab is the area of the base of the pile. The
value of az can be interpolated from the following list: az = 1.0 at qu = 0
tons/ft2; 0.9 at 0.75; 0.8 at 1.12; 0.7 at 1.45; 0.6 at 1.82; 0.5 at 2.36, and 0.43 at
3.0. To simplify calculations, these numbers represent average values, selected
from a curve with a wide range (see page 288 of Peck et al., 1974; Tomlinson,
1980). For the behavior of piles in clay under axial loading, the sélection of
the value of az varies among authors, and even the method of computing the
axial capacity varies among investigators (American Petroleum Institute, 1993).
The lack of agreement among investigators for piles driven into clay, and even
more disparity for piles driven into sand, is due to the scarcity of high qualitv
experimental data. The models for ultimate capacity and for settlement will
remain useful even as more data become available.
The pile selected for analysis is an open Steel pipe with a diameter b o. 36
>d. and assumed to hâve been driven to a pénétration of 140 ft into clay at an
offshore location with the following properties: 0 to 50 ft, qu — T0 tons/ t ,
t0 100 ft, qu = 1.8 tons/ft2; 100 to 175 ft, qu = 2.5 tons/ft2. Interpolatton of
values of az from the list in the previous paragraph yielded the following values