Civil Engineering Reference
In-Depth Information
pile load is specified. Rates of penetration between 0.5 and 2mm/mm are commonly
used, the lower rate being used in clay soils.
Great care is required to maintain a steady rate of penetration, but this is difficult
to achieve with manual pumping without some practice. If manual control is adopted
time-penetration plots should be drawn as the test proceeds, so that deviations from the
required path are easily identified. For this type of testing it is much more satisfactory
to use a hydraulic power pact, and control the rate of penetration through software
linked to a displacement transducer. Control of the jack movement is achieved by
manual operation of the rate of oil flow. Fully automatic systems have been devised
but are complex, and present difficulties in operation on site.
Loading of the CRP test should be continued to a deflection that goes to or beyond
failure of the pile. This may be, typically, until a constant load has been recorded or
until the penetration is at least 10% of the pile diameter (see Figure 9.15).
For this test, large travel of the jack and deflection transducers should be allowed for.
It may be necessary to reset the transducers during the test, or alternatively, machined
spacing blocks can be inserted under the gauge stems.
Once the equipment has been set up the test may be completed within an hour or
so, and a rapid assessment of the ultimate capacity of the pile made. However, an
experienced team of three or four people is required to carry out the test efficiently.
Tension piles may be tested using a constant rate of uplift procedure, which is similar
to the CRP test. The full tensile capacity of a pile is normally mobilized at a displace-
ment of a few millimetres and the imposed rate of uplift should be correspondingly
reduced to say 0.1 to 0.3-mm per minute.
9.2.3.3 Recording the results
It is essential that all the relevant data are recorded and to this end it is useful to
use a standard proforma (see Table 9.3). Similarly the load, settlement and time data
should be recorded clearly throughout the test. It is important that any problems or
unexpected occurrences are reported in the test records, as this information is most
useful when interpretation of the results is made.
It is customary to present load-time, settlement-time and load-settlement graphs of
the results. It is important that these last two graphs are plotted during a maintained
loading procedure so that modifications to the test can be made if required.
Pile tests are relatively expensive and the results of such tests can have a considerable
influence on the construction of the permanent works. It is therefore imperative that
an adequate number of qualified staff are available to set up and carry out the test with
engineering supervision throughout the test period. In addition at least one technician
should attend the tests, to control the hydraulic jack, etc. Similarly, interpretation of
the results and preparation of a report on the test programme should not be stinted,
and sufficient staff time should be allocated.
Special pile tests (including CRP tests), involving a higher level of instrumenta-
tion generally involve a larger team to carry them out successfully. Setting up the
instrumentation should be carried out by experienced technicians or engineers. Staff
time involved in carrying such special pile tests can be considerable, and represents a
significant cost in the overall test programme.
