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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