Level Drill Pad - Prepare drilling sites before
arrival of the drill crew. The kelly and mast of the drill
rig are fixed to the truck bed and cannot swing, as some
auger rigs can. Level the bed of the drill truck in order to
drill a vertical hole. The truck is equipped with hydraulic
jacks that can lift the front of the truck one foot off the
ground and one foot on either side to accommodate uneven
terrain. If the slope of the site is steeper than one foot,
prepare a work pad 16 ft. wide and 70 ft. long for leveling
the rig and providing a safe place for the crew to handle
the drill stem. For safety reasons, the crew is not allowed
to block up the jacks to accommodate greater slope angles.
The mud pan must be level or slightly down slope. Before
extensive site work, consult the driller who performs the
work for specific instructions. See the following figure for
drill site requirements.
Overhead Clearance - Overhead must be clear of obstructions. Trees
cannot block the raising of the mast. It is not safe to work within 25 ft. of an
overhead power line. If necessary to work closer, contact the power company to
cut the power or install insulating safety boots.
Underground Utility Locations - You must know the exact location of
underground utilities including the following:
High pressure gas lines
Sewer and storm lines
Electrical and telephone conduits and cables
The driller will be available to inspect locations and make recommendations
on site preparation. Often it is possible to begin drilling easy sites while
preparing more difficult sites.
Access - Ensure that permission to enter private property has been
secured before drilling.
Barge Work - When a bridge must cross large bodies of water, barges
are used to obtain foundation information. Barge work is complex and expensive,
so coordination with the driller should begin well before start of drilling.
Dry Barrel or Single-Wall Sampler - Use the dry
barrel sampler to obtain core samples for visual soil and bedrock classification
and logging. The core sample obtained is generally in a disturbed condition due
to the pressure applied when cutting the core and packing it into the barrel for
recovery. The core is extracted from the barrel by water pressure. When used for
sampling in practically all foundation materials except very soft clay (muck)
and cohesionless sand, the dry barrel sampler obtains a sample containing all
components in the original formation. The amount and degree of disturbance
depends upon the consistency and density of the material. Although this method
is called the dry barrel method, circulating water is used. In hard formations,
a smaller volume of water is circulated while cutting the core.
Core Barrel - Use diamond core barrels to obtain intact rock samples for
field or laboratory tests and classification. The diamond barrel sampler has an
inner and outer barrel. The inner barrel is slightly oversized with a
spring-loaded core retainer at the bottom.
Push Barrel or Shelby Tube Sampler - Use the push barrel sampler to
obtain relatively undisturbed soil samples for field and laboratory tests and
soil classification. The device consists of a thin-walled tube 24 to 36 in. long
with one end sharpened to a cutting edge and the other end reinforced and
designed for easy attachment to the drill stem coupling. The thin-walled tube is
steadily pushed into the formation with the hydraulic pull-down of the drill
rig. This sampler recovers good undisturbed samples where it is adaptable, but
its usefulness is limited to materials that it can be forced into and that have
sufficient cohesion to remain in the barrel while the sampler is being withdrawn
from the hole. Use the device as follows:
Force sampler into
formation with slow, steady push to within 3 to 4 in. of length.
Rotate sampler several
turns to shear off core at bottom before withdrawing it.
Bring push barrel to
Detach barrel from
Mount barrel on the
hydraulic sampler extruder.
Cut core into 6-in.
lengths, and wrap in thin plastic (plastic wrap for food) to retain
Place samples in
cartons for transport to the laboratory for testing.
For samples of soft soil, sample disturbance can be a problem during
transport to the testing location. To ensure minimum disturbance, support soft
samples in their cartons. Fine dry sand poured around the sample in the carton
provides excellent support during transport. Store samples that are not
immediately tested in a moist room.
Wash Sampling or Fish-Tailing - Of the many methods for penetrating
overburden soil, consider only those that offer an opportunity for sampling and
testing the foundation materials without excessive disturbance. Do not use wash
sampling or fishtail drilling unless absolutely necessary. Attempts to classify
the soil materials by watching the wash water may lead to erroneous conclusions
about the subsurface soil being penetrated.
Standard Penetration Test (SPT) - The SPT uses a 2-in. diameter pipe (split
spoon) driven with a 140-lb. hammer at a drop of 30 in. The test is described in
ASTM procedure D 1586. This test is recommended mainly for granular soil but has
been used in cohesive soil. It cannot be used in rock. It correlates roughly
with the TCP test as follows:
Clay: Ntcp = 1.5 Nspt
Sand: Ntcp = 2 Nspt
Test correlations presented here are only for approximate evaluation of
design adequacy from outside sources and not for normal foundation design work.
Observation Wells and Piezometers - Observation wells and piezometers
are used to measure ground-water levels. Observation wells are essentially water
wells and are sometimes pumped to determine the permeability of the soil to
predict seepage volumes in excavations. Piezometers are instruments which
measure water pressure at the elevation of the installed sensor.
For short-term observations of water levels, leave exploration core holes
open for several hours to several days to monitor the ground-water level and
note the depth to water in the hole. Cover the hole to protect people or
livestock from injury.
For long-term observations, install either observation wells or piezometers.
Observation wells are most useful where the groundwater conditions are fairly
stable, and in relatively porous soils or rock. They are simple to install and
read, however they must be placed in a location where the top of the well is
accessible. Piezometers are useful where access is difficult, since they may be
read from a remote location. Piezometers are also more sensitive to groundwater
changes in fine-grained soils. Many types of piezometers are available, with
each having advantages and disadvantages. Consult with the designer regarding
selection and installation of piezometers.
Some typical applications for piezometers are to evaluate ground-water levels
in future depressed roadway sections and ground-water effects on slope
Future depressed roadway sections. The construction and long-term
performance of depressed roadway sections can be affected adversely by
ground-water. The final installation may need special drainage features to
control water inflows and provide a stable pavement section.
Slope stability. Ground water affects slope stability by reducing the
effective stresses in the soil through buoyancy. This applies to both side
slope stability and bearing capacity of embankments and retaining walls.
Drill the hole with no
water if possible. If not possible, drill with clear water. If hole
stability continues to be a problem, add small amounts of drilling mud
to the water.
Place the assembled
observation well piping into the hole. Either use a slotted screen, or
drill holes in a section of the pipe and then wrap them with filter
fabric. The upper sections of the pipe are not perforated
Place the granular
media in all but the upper 5-10 ft. of the hole. Use a fairly coarse
sand or pea gravel to allow easy placement through water.
Seal the remaining
upper portion of the hole with grout or bentonite pellets. When using
bentonite pellets in a dry hole, pour several gallons of water over the
pellets for 10-15 min. to start expanding the pellets to seal the hole.
Finish the well in
such a manner as to not be a hazard to the public. Use a locking cover
if vandalism is possible.
Take a reading immediately and weekly thereafter until the water level
stabilizes. Monthly readings thereafter are normally sufficient unless the site
exhibits large fluctuations in readings.
Inclinometers - Inclinometers measure horizontal movements within a
soil mass over time. The inclinometer is a sensitive device that measures
deviations from vertical. Record these deviations at periodic intervals along a
special casing grouted into a bore hole to determine the horizontal deviation of
the casing from the bottom of the casing to the top.
The most common application is for monitoring slope failures to determine the
failure plane depth. Install inclinometer casing at several points in and
adjacent to the slope failure, and use information from inclinometers in
stability analyses. In order to be effective, the bottom of the inclinometer
casing must extend well below the failure plane.
Take an initial set of readings immediately after casing installation to
establish the baseline reading. Compare all subsequent readings to the baseline
to determine direction and amount of movement. Base frequency of readings on the
rate of failure of the slope.
The installation of casing, operation of the inclinometer, and data reduction
is quite complicated. Consult Bridge Division geotechnical engineers if
inclinometer measurements are required.