Difficulties with Geotechnical Explorations
June 1, 2009
Limitations and challenges may be encountered during geotechnical explorations, which are common to all exploratory techniques. They usually are a result of site-specific geologic conditions and/or a function of the improper equipment or techniques being utilized.
With regard sample recovery, occasionally, sampling is attempted and little or no material is recovered. In cases where a split-barrel or some other type of sampler is used to recover a disturbed sample, it is appropriate to make a second attempt to recover the material immediately following the first failed attempt. In such instances, the sampling device may be modified to include a retainer basket, a hinged trap valve or other measures to help retain the sample.
In cases where an undisturbed sample is desired, drill to the bottom of the attempted (disturbed) sampling interval and repeat the sampling attempt. The sampling method should be reviewed, and the sampling equipment should be checked to understand why no sample was recovered (such as a plugged ball valve). It may be appropriate to change the sampling method and/or the sampling equipment, such as waiting a longer period of time before extracting the sampler, or extracting the sampler more slowly and with greater care, etc. If recovering a sample at a specific depth is necessary, a second boring may be advanced to obtain a sample at the prescribed depth using the improved technique.
Generally, sample recovery less than 10 percent is considered inadequate for representative sampling. However, this criteria may be waived for the specific situation (i.e., in thick, uniform deposits).
Various sampling devices equipped with check and pressure release valves, sample retaining springs, baskets and lifters should be used. Occasionally, sample recovery may be enhanced by modifying the equipment or the drilling techniques.
Sample DisturbanceThere is no way to obtain a truly undisturbed sample using available soil-sampling techniques. Block sampling continues to be the most reliable method for minimizing sample disturbance. However, because gaining access to the zone to be sampled can be limited by the depth of overlying material, and because the sampling process is fairly rigorous and time-consuming, most samples are obtained via drilling.
The selection of the correct sampling tool, drilling technique and borehole stabilization method should be based on the soil type being sampled and the subsurface conditions. The incorrect preservation and shipment of samples may further disturb the specimens to the point where they no longer are usable.
The termination of an exploration above the required design depth due to boulders, fill material, excessively dense materials or other obstructions may occur during any investigation. When this occurs, it usually implies that the correct exploration method might not have been selected for the anticipated subsurface conditions. Specialized tools and equipment are available to enhance the capacity of conventional drilling equipment. In some cases, when obstacles are anticipated, a solutionis to redrill the boring a few feet away.
Problematic Geologic ConditionsMore thought and care should be given in selecting proper sampling equipment and sampling techniques when conducting subsurface exploration in problematic geologic conditions. A list of some of problematic geologic conditions:
- organic soils
- metastable soils (loess, alluvial deposits and mudflows
- expansive soils or rocks
- sensitive clays
- hydrocollapsible soils
- moving ground (slides)
- meander loops and cutoffs
- abandoned mined areas
- normally consolidated clays
- loose, granular soils
- noxious or explosive gases
- artificial fill
- weathered shale rocks
- wet or saturated soils
Ground Water ConditionsGround water can affect the stability of boreholes, especially in cohesionless soils (sands and silts). Water flowing into the hole could cause caving and quick (liquefying) conditions, which would artificially reduce the standard penetration test blow counts being measured, as well as make drilling and sampling progress difficult. Drill fluids typically are used to stabilize the borehole in such situations.
Where precise water level data is important, the effects of drilling water additives (bentonite) on the permeability of certain soils should be evaluated. In soils with lower permeability and flow rates, such as in silt or silty sand, the use of bentonite mud can dramatically limit the movement of water by coating the walls of the boring. A bentonite coating can reduce the likelihood that piezometer readings will represent true ground water levels or that the water levels in the boring will respond accurately to natural ground water changes. In these situations, alternative drilling techniques – such as using a casing advancer or hollow-stem auger – should be considered to produce a stable borehole without relying on additives that can affect permeability. Following drilling, especially whenever low permeability conditions exist, wait an adequate period of time for the water level to reach equilibrium within the borehole before initiating ground water measurements.
It is preferred that a ground water measurement be taken 24 hours after completing the boring to allow the water level to reach equilibrium. In fine-grained soils, depending on the permeability, this period may not be adequate. The installation of permanent or temporary observation wells, which provide access for measuring the ground water table over a longer period, can be used in this case. Observation wells generally are an inexpensive safeguard against erroneous data regarding the presence and behavior of the ground water conditions.
Contaminated SitesWhen an investigation is to be performed, acquisition records for newly obtained right-of-way indicate the most recent land use for the area. On rehabilitation projects, where the only planned activities are shown on the existing right-of-way, the information available may vary from very complete to none. There are many problems and issues inherent in sampling and handling contaminated soils. However, it is necessary for all involved in geotechnical investigations to be aware of the salient points of these procedures. The U.S. Environmental Protection Agency’s document titled, “Description and Sampling of Contaminated Soils – A Field Pocket Guide,” contains guidelines, background information and a list of useful references on the topic.
During an investigation, if unexpected contaminants are encountered, immediately cease explorations. Initial actions may require demobilization from the site. Some signs of possible contamination:
- prior land use (e.g., old fill, landfills, gas stations,
- stained soil or rock
- apparent unnatural lack of vegetation or presence of dead vegetation
and trees in the local site context – while in some places this could indicate
contamination, in others, it is just normal desert
- odors – highly organic soils often could have a rotten egg odor that
should not be construed as evidence of contamination; however, this odor also
may be indicative of highly toxic hydrogen sulfide
- presence of liquids other than ground water
- marks of prior ground fires (at landfill sites)
- presence of visible elemental metals (i.e., mercury)
- low (<2.5) or high (>12.5) pH
This article is provided through the courtesy of the Nevada Department of Transportation’s Geotechnical Policies and Procedures Manual.” To view the complete document, visit www.nevadadot.com.