Civil Engineering Reference
In-Depth Information
The degree of sophistication of the data-logging equipment provided depends to
some extent on the volume of data to be recorded and on the number of piles
to be tested. Some piling contractors and specialist pile testing companies such as
PMC Limited have developed equipment, and the quality of the output can be sig-
nificantly better than that achieved with conventional test methods. Specialized pile
testing companies will use pre-programmed pile tests with automatic load mainte-
nance and stage increases together with settlement recording and detection of cessation
of pile movement. The basic elements of such a system are shown schematically in
Figure 9.14.
To provide more information on the behaviour of the test pile, additional instru-
mentation is occasionally deemed necessary. This type of instrumentation is described
in section 9.2.5.
9.2.3 Pile load test procedures
The pile loading test that is most commonly carried out is the maintained load test
but alternatively, the constant rate of penetration test is sometimes employed. The
maintained load test is convenient for testing end-bearing piles and for determining
the load-settlement characteristics in clay soils. Usually, however, it is not as suitable
for determining the true ultimate capacity of a pile.
The constant rate of penetration test procedure is best suited to determining the
ultimate bearing capacity of a pile, as the method of testing is closely related to the
test procedures used to obtain the shear strength of the soil. In clay soils, rapid pile
testing may approximate to undrained loading conditions, whilst a constant load for
several days may be necessary to allow full dissipation of pore pressures.
9.2.3.1 Maintained load tests
The maintained load test requires careful specification of loading increments and
periods for which these increments are held constant. The ICE
Specification for Piling
sets out a suitable minimum scheme (Table 9.2), and the ASTM Test Designation
D-1143 specifies a similar procedure. In addition, limits are placed on the rate of
movement before the next load stage is added. The ICE
Specification for Piling
lim-
its the rate of movement to 0.24mm/h, provided the rate is decreasing, for pile
head displacements greater than 24mm, but less than 0.1mm/h for head displace-
ments of less than 10mm. Between these pile head displacement values, a rate of
less than 0.1%
pile head displacement/h is recommended. Some engineers pre-
fer a limit of 0.1mm/h, and a frequent requirement is to hold the load constant
for 24 h at design load. The ISO/DIS 22477-1 Draft document (2006) recommends
a cessation of movement criterion of 0.1mm per 20 min and a time interval of
at least 60 min per loading step. It also permits a pre-load of 0
×
.
05
Q
max
to check
the loading train and instrumentation, unloaded to a small residual value of the
order of 10 kN prior to starting the test proper. The particular details of the load-
ing path followed are not of great significance, but it is important that the holding
time at each load increment is the same so as to lead to the same degree of soil
consolidation.
