EI 02G001
CEMP-E
01 July 1997
(4) Prediction of Driven Pile Capacity. Although a great deal of research has been done into the
nature and engineering properties of calcareous sands, predicting the capacity of piles driven into these
soils is highly speculative. Great care must be taken to verify that the soil can in fact support the installed
foundation, both in the design process and especially in the field verification process. This should include
extensive instrumentation and load testing of both indicator and production piling. With driven piles, a
frequent occurrence with these soils is very easy driving; this can be due to temporary conditions (as was
the case with this project) or a more permanent condition, in which case remedial action must be taken.
Blow count cannot be relied upon as a key jobsite control method.
(5) Remediation. Once a situation has been encountered where the piles installed do not have the
required capacity, remediation is necessary. One method, of course, is to drive the piles further. This can
be uneconomical depending upon the situation. Another potential solution is to use drilled and grouted
piles. These can significantly improve pile capacity by pressurizing the surrounding soil. However, as with
any pile of this type comprehensive quality control during installation is essential.
b. Description of Kwajalein Project. Project Overview and Design Requirement. The aim of the project
was to construct a drydock at the Kwajalein Atoll. The foundation designed to support the drydock was
made up of 12 pile groups, designated N-1 through N-6 on the northwest side and S-1 through S-6 on the
southeast side. The layout for these piles is shown in figure 5-1. The piles were to be driven into
unconsolidated bioclastic limestone (coral) debris with sizes that ranged from silt to cobble, although the
predominant size was sand. Some of the piles in each group were battered. These are shown with their
batter orientation noted in figure 5-2. Before pile driving could begin, the drydock area was first dredged to
-26 feet mean sea level (MSL). The area was also drilled and blasted during dredging operations.
c. Soil Investigations. Eleven soil borings were originally drilled. Four of these borings, B1 to B4, were
offshore (that is, in the harbor). After the drydock was moved to the northwest about 100 feet, two
additional borings, B12 and B13, were drilled for a total of 13 borings. Standard penetration test (SPT)
results from these borings indicated N-values ranging from 3 to 35 blows/foot as shown in figure 5-3 for
both onshore and offshore borings. Depths are shown relative to MSL. These constitute very loose to very
dense sands and gravels; most sands were of medium density. The blow counts of borings B5, B6, and B7
in figure 5-3a and are representative of the coral sands in the onshore drydock area. Of the borings whose
blow counts are shown in figure 5-3b, borings B2 and B4 were located about 100 ft southeast of the pile
plan centerline shown in figure 5-1, and borings B12 and B13 are located as shown in figure 5-1. Assuming
that the driving energies delivered to the borings was identical, comparison of figures 5-3a and 5-3b shows
that the blow counts in the offshore borings are as great as or greater than those of the onshore borings.
Additionally, the borings indicated broadly graded coral sand with gravel and lesser quantities of silt and
clay.
d. Pile Capacity and Load Testing. The required design load Qd for the offshore piles was 120 kips
and 160 kips for the onshore piles. These piles were 20 inches square precast prestressed (PCPS)
concrete piles, 85 feet long with an embedment depth of 53 feet to obtain the desired capacity. Proof load
tests were conducted to twice the design depth on piles C2, E2, and E3, using the ASTM D 1143. These
tests indicated adequate reserve capacity with displacements not exceeding 0.1 inch at the design load for
the selected piles. Results of these tests are summarized in table 5-1. As for the driving resistance of
these piles, for example the E2 pile was driven with a Delmag D46-23 hammer with a rated striking energy
of 102,000 foot-pounds and a ram weight of 10,143 pounds. This hammer is a predecessor of the
Delmag D46-32 shown in table 3-2. When first driven on 26 April 1991, the blow count was as low as
2 blows/foot, at which point driving was discontinued 6 feet before final toe elevation. When driving was
resumed the next day, the blow count during driving increased to 17 blows/foot at a toe elevation of -81 feet
MSL.
5-2
Previous Page
