290
BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
11.1
CHARACTERISTICS OF SOIL AND RESPONSE
TO EXTERNAL LOADING
An investigation of soil characteristics is a necessary initial step preceding design. Prior to taking boring equipment to the site, the engineer will investigate
the engineering geology, will gain as much information as possible from previous
borings, and will develop a preliminary plan about the number of borings, their
depth, and investigative techniques. The provision of a marine vessel necessary
to sustain the soil-boring equipment is costly so engineers give attention to ail
details that can facilitate and limit the time of the soil investigation. Seismology
is sometimes used to augment the soil sampling and testing.
For the Gulf of Mexico, Fisk (1956) presented an important overview that
has been valuable in giving engineers a general understanding of the near surface
geology. An example of a feature requiring careful attention is the existence of
stream beds that were later filled with soft sédiments, leading to the possibility
that supporting piles could vary significantly in length across the platform.
McClelland (1956) published the results of a sériés of soil investigations in
the Gulf of Mexico that also gave valuable general information. While such
results are insufficient for design, useful information is given on the detailed
planning for an efficient deployment of the marine equipment.
While indirect methods of characterizing soils are being used, such as the
instrumented cône, many engineers prefer to hâve tube samples for examination
and testing. The common procedure is to lower to the sea floor a motioncompensated drill string with an interior diameter to allow sampling tubes to
be lowered on a wire line. A weight is dropped with a wire line to acquire
samples of cohesive soil and to get the number of blows to drive the sampler a
given distance into granular soil. Hvorslev (1949) made a comprehensive study
of soil sampling and noted that the most favorable way to sample clays is with
a steady, continuous push of the tube. Emrich (1971) made a study of the
réduction in strength of clays caused by driving the sampler and found the loss
to be significant.
Spécial tools, operated hydraulically, can be latched to the bottom of the
drill string for performing spécial tests, such as vane shear, cône pénétration,
or pressuremeter. Nevertheless, the sampling by dropping a weight remains
popular because of the speed of the work. When a sample is retrieved, the drill
string is advanced by rotary drilling to the next depth.
The engineer may make a visual classification and perform some tests in the
field, such as using a miniature vane on samples of clay, in order to détermine
the required depth of the boring so piles can be designed to sustain the expected
loads. Tests in the laboratory, including grain-size distribution and Atterberg
limits, allow the soil to be classified accurately. Water content and unit weight
is obtained for ail samples. For cohesive soils, a détermination of the shear
strength and sensitivity is necessary, along with some information on stiffness,
if possible. Consolidation characteristics are normally a secondary considération. For cohesionless soils, undisturbed samples are usually unavailable and an
BEHAVIOR OF PILES SUPPORTING OFFSHORE STRUCTURES
11.1
CHARACTERISTICS OF SOIL AND RESPONSE
TO EXTERNAL LOADING
An investigation of soil characteristics is a necessary initial step preceding design. Prior to taking boring equipment to the site, the engineer will investigate
the engineering geology, will gain as much information as possible from previous
borings, and will develop a preliminary plan about the number of borings, their
depth, and investigative techniques. The provision of a marine vessel necessary
to sustain the soil-boring equipment is costly so engineers give attention to ail
details that can facilitate and limit the time of the soil investigation. Seismology
is sometimes used to augment the soil sampling and testing.
For the Gulf of Mexico, Fisk (1956) presented an important overview that
has been valuable in giving engineers a general understanding of the near surface
geology. An example of a feature requiring careful attention is the existence of
stream beds that were later filled with soft sédiments, leading to the possibility
that supporting piles could vary significantly in length across the platform.
McClelland (1956) published the results of a sériés of soil investigations in
the Gulf of Mexico that also gave valuable general information. While such
results are insufficient for design, useful information is given on the detailed
planning for an efficient deployment of the marine equipment.
While indirect methods of characterizing soils are being used, such as the
instrumented cône, many engineers prefer to hâve tube samples for examination
and testing. The common procedure is to lower to the sea floor a motioncompensated drill string with an interior diameter to allow sampling tubes to
be lowered on a wire line. A weight is dropped with a wire line to acquire
samples of cohesive soil and to get the number of blows to drive the sampler a
given distance into granular soil. Hvorslev (1949) made a comprehensive study
of soil sampling and noted that the most favorable way to sample clays is with
a steady, continuous push of the tube. Emrich (1971) made a study of the
réduction in strength of clays caused by driving the sampler and found the loss
to be significant.
Spécial tools, operated hydraulically, can be latched to the bottom of the
drill string for performing spécial tests, such as vane shear, cône pénétration,
or pressuremeter. Nevertheless, the sampling by dropping a weight remains
popular because of the speed of the work. When a sample is retrieved, the drill
string is advanced by rotary drilling to the next depth.
The engineer may make a visual classification and perform some tests in the
field, such as using a miniature vane on samples of clay, in order to détermine
the required depth of the boring so piles can be designed to sustain the expected
loads. Tests in the laboratory, including grain-size distribution and Atterberg
limits, allow the soil to be classified accurately. Water content and unit weight
is obtained for ail samples. For cohesive soils, a détermination of the shear
strength and sensitivity is necessary, along with some information on stiffness,
if possible. Consolidation characteristics are normally a secondary considération. For cohesionless soils, undisturbed samples are usually unavailable and an