02/09/23 - Laserfiche WebLink

Home

EXPLORATION/CLASSIFICATION METHODS SAMPLING METHODS Split -Spoon Samples (SS) - Calibrated to N6o Values Standard penetration (split -spoon) samples were collected in general accordance with ASTM: D1586 with one primary modification. The ASTM test method consists of driving a 2" O.D. split -barrel sampler into the in -situ soil with a 140-pound hammer dropped from a height of 30". The sampler is driven a total of 18" into the soil. After an initial set of 6", the number of hammer blows to drive the sampler the final 12" is known as the standard penetration resistance or N-value. Our method uses a modified hammer weight, which is determined by measuring the system energy using a Pile Driving Analyzer (PDA) and an instrumented rod. In the past, standard penetration N-value tests were performed using a rope and cathead for the lift and drop system. The energy transferred to the split -spoon sampler was typically limited to about 60% of its potential energy due to the friction inherent in this system. This converted energy then provides what is known as an N60 blow count. Most of today's drill rigs incorporate an automatic hammer lift and drop system, which has higher energy efficiency and subsequently results in lower N-values than the traditional N60 values. By using the PDA energy measurement equipment, we are able to determine actual energy generated by the drop hammer. With the various hammer systems available, we have found highly variable energies ranging from 55% to over 100%. Therefore, the intent of AET's hammer calibrations is to vary the hammer weight such that hammer energies lie within about 60% to 65% of the theoretical energy of a 140-pound weight falling 30". The current ASTM procedure acknowledges the wide variation in N-values, stating that N-values of 100% or more have been observed. Although we have not yet determined the statistical measurement uncertainty of our calibrated method to date, we can state that the accuracy deviations of the N- values using this method are significantly better than the standard ASTM Method. Disturbed Samples (DS)/Spin-up Samples (SU) Sample types described as "DS" or "SU" on the boring logs are disturbed samples, which are taken from the flights of the auger. Because the auger disturbs the samples, possible soil layering and contact depths should be considered approximate. Sampling Limitations Unless actually observed in a sample, contacts between soil layers are estimated based on the spacing of samples and the action of drilling tools. Cobbles, boulders, and other large objects generally cannot be recovered from test borings, and they may be present in the ground even if they are not noted on the boring logs. CLASSIFICATION METHODS Soil classifications shown on the boring logs are based on the Unified Soil Classification (USC) system. The USC system is described in ASTM: D2487 and D2488. Where laboratory classification tests (sieve analysis or Atterberg Limits) have been performed, accurate classifications per ASTM: D2487 are possible. Otherwise, soil classifications shown on the boring logs are visual -manual judgments. Charts are attached which provide information on the USC system, the descriptive terminology, and the symbols used on the boring logs. The boring logs include descriptions of apparent geology. The geologic depositional origin of each soil layer is interpreted primarily by observation of the soil samples, which can be limited. Observations of the surrounding topography, vegetation, and development can sometimes aid this judgment. WATER LEVEL MEASUREMENTS The ground water level measurements are shown at the bottom of the boring logs. The following information appears under "Water Level Measurements" on the logs: • Date and Time of measurement • Sampled Depth: lowest depth of soil sampling at the time of measurement • Casing Depth: depth to bottom of casing or hollow -stem auger at time of measurement • Cave-in Depth: depth at which measuring tape stops in the borehole • Water Level: depth in the borehole where free water is encountered • Drilling Fluid Level: same as Water Level, except that the liquid in the borehole is drilling fluid The true location of the water table at the boring locations may be different than the water levels measured in the boreholes. This is possible because there are several factors that can affect the water level measurements in the borehole. Some of these factors include: permeability of each soil layer in profile, presence of perched water, amount of time between water level readings, presence of drilling fluid, weather conditions, and use of borehole casing. SAMPLE STORAGE Unless notified to do otherwise, we routinely retain representative samples of the soils recovered from the borings for a period of 30 days. 01 REP051 C (12/08) AMERICAN ENGINEERING TESTING, INC.