| III. GEOTECHNICAL
INVESTIGATION
A. NUMBER
OF BORINGS - The number of exploration
holes is a function of the proposed crossing length and the complexity
of the strata. If the crossing is about 1,000 ft. a bore hole made on
each side of the crossing may suffice. If an examination of these borings
indicates that conditions are likely to be homogeneous on both sides,
it may not be necessary to conduct further sampling. If the report indicates
anomalies discontinuity in the strata, the presence of rock or large
concentrations of gravel it is advisable to make additional borings
to better define the strata. Longer crossings (especially large diameter
pipelines) that indicate gravel, cobble, boulders or rock should have
samples taken about 600-800 ft. apart unless significant anomalies are
identified that might necessitate more borings. All borings should be
located on the crossing profile along with their surface elevations
being properly identified. If possible the borings should be conducted
at least 25 ft. off of the proposed centerline. The bore holes should
be grouted upon completion. This will help prevent the loss of drilling
slurry during the crossing installation.
B. DEPTH OF BORINGS - All
borings should be made to a minimum depth of 40 ft. below the lowest
point in the crossing or 20 ft. below the proposed depth of the crossing,
whichever is greater. In some instances, it may be beneficial to the
owner and the contractor to install the crossing at a greater depth
than the owner requires for his permit. It is suggested that all borings
be through the same elevation to better determine the consistency of
the underlying material and note any patterns which may be present.
C. STANDARD CLASSIFICATION OF SOILS - A qualified technician
or geologist should classify the material in accordance with the Unified
Soil Classification System and ASTM Designations D-2487 and D-2488.
It is beneficial to have a copy of the field drilling log completed
by the field technician or driller. These logs include visual classifications
of materials as well as the driller's interpretation of the subsurface
conditions between samples.
D. STANDARD PENETRATION TEST (SPT) - In order to better
define the density of granular materials the geotechnical engineer generally
uses the Standard Penetration Test (SPT), in general accordance with
ASTM Specifications D-1586. This is a field test that involves driving
a 2-in. split spoon sampler into the soil by dropping a hammer of a
specific weight (usually 140 LB) a specified distance (usually 30 in.)
to determine the number of blows necessary to drive the sampler 12 in.
In very dense soils, the field technician may note the number of blows
required to drive the sampler less than the required 12 in. (i.e., 50
blows for 3 in.). The number obtained is the standard penetration resistance
value (N) and is used to estimate the in situ relative density of cohesion
less soils. Some geotechnical firms will conduct these penetration tests
in cohesive materials and rock, and to a lesser extent, the consistency
of cohesive soils and the hardness of rock can be determined.
E. THIN
WALLED "SHELBY" TUBE SAMPLING - Most geotechnical
firms prefer to use a Thin walled Tube Sampling method for obtaining
samples of cohesive materials. These tests are conducted in general
accordance with ASTM Specification D-1587. This test is similar to the
Standard Penetration test except the sample is collected by hydraulically
pushing a thin-walled seamless steel tube with a sharp cutting edge
into the ground. The hydraulic pressure required to collect the sample
is noted on the field log. This produces a relatively undisturbed sample
that can be further analyzed in the laboratory. These samples can be
field tested with handheld penetrometers, but more accurate readings
of density and consistency can be obtained by performing unconfined
compressive strength tests where the results are noted in tons per square
foot. Generally, for directional drilling contractors a standard penetration
test using the split spoon sampler described above will suffice in both
materials.
F. SIEVE
ANALYSIS OF GRANULAR MATERIALS - A sieve analysis
is a mechanical test of granular materials performed on samples collected
in the field during the standard penetration test with the split spoon
sampler. The split spoon samples are taken to the laboratory and processed
through a series of screens. The sample provides a percentage analysis
of the granular material by size and weight. It is one of the most important
tests undertaken.
G. ROCK
INFORMATION - If rock is encountered
during the soils investigation borings, it is important to determine
the type, the relative hardness and the unconfined compressive strength.
This information is typically collected by the geotechnical drilling
firm by core drilling with a diamond bit core barrel. The typical core
sample recovered with this process has a 2-in. diameter. The type of
rock is classified by a geologist. The geologist should provide the
Rock Quality Designation (RQD) which rates the quality of the rock based
on the length of core retrieved in relation to the total length of the
core. The hardness of the rock Mohs' Scale of Hardness) is determined
by comparing the rock to ten materials of known hardness. The compressive
strength is determined by accurately measuring the core and then compressing
the core to failure. This information pertaining to the underlying rock
formation is imperative to determine the type of down hole equipment
required and the penetration rates that can be expected
|
