As many wise geotechnical engineers have said, “Good quality test results start with a good quality sample!”
It is vital for geotechnical engineers and project managers to think twice about how the samples for their project will be handled from extraction in the field to arrival at the laboratory, as ultimately, the information from these samples will be turned into pertinent data for use in project design. The resulting quality of that data, and its impact on design and project costs, can be highly dependent on how well the samples were cared for.
Before the number crunching begins by the geotechnical design team, geotechnical exploration must be completed, and samples must be obtained for analysis. The expense and complexity of obtaining those samples increase exponentially when the site is hard to access. Drilling in dense woods requires clearing, a slow and painstaking process. Drilling in remote or limited access areas often puts the drill rig into dangerous zones. Offshore exploration can mean drilling off ships or barges where the costs and logistics to mobilize are prohibitive and “sampling do-overs” are virtually impossible. Squeezing a drill rig into a busy urban environment can be a logistical nightmare. Bottom line…it takes A LOT to gather those subsurface samples, and how they are handled from the moment they leave the ground to the time a technician extrudes them in the laboratory for processing and testing is of PARAMOUNT importance to the test results and the final design.
A lone 4-inch soil sample (never mind a box of them!) can yield a wealth of information on which a quality geotechnical design depends on. Therefore, careful planning and consideration can help deliver the samples to the laboratory in as close to “in-situ” conditions as possible. An excellent reference that is often used is the standard ASTM D4220/D4220M-14 (Standard Practices for Preserving and Transporting Soil Samples), but other resources are available. While some level of sample disturbance must be expected, the totality of it is very dependent on soil type, sampling methods, handling and storing of the samples, the know-how of the drilling crew, as well as laboratory staff. The best geotechnical teams will work in tandem with drilling crews and laboratory staff to utilize the best sampling methods appropriate for the encountered soil conditions and be prepared to properly extract samples from the ground and store them in the field utilizing containers suitable for the encountered soil types.
There are a few things about proper handling of soil samples that are most important for geotechnical engineers to keep in mind:
Soil Matrix Integrity
Once a sample is extracted from the ground, it should be housed in such a container that it stays intact until extruded in the laboratory. The sample should be protected from vibration and disturbance that could potentially alter the matrix of the sample through shaking, bouncing, dropping, etc. Padded containers with bubble wrap, foam, or other means of absorbing vibration should be used for all samples but particularly on loose and sandy samples prone to disturbance.
It is just as critical to maintain the in-situ moisture content. Containers that seal the sample and prevent air from entering or escaping are recommended to maintain the sample’s state close to what it experienced underground. If the time of transport or time in storage is of concern, wax or similar seals should be implemented on the samples for added protection.
Storing and transporting soil samples to the laboratory while maintaining the sample’s true orientation is very important. Containers that store samples upright, such as the GeoTesting Express (GTX) Shelby tube shipping container, take care of this while also minimizing vibration of the sample during transport.
A state-of-the-art geotechnical laboratory can produce some of the most valuable data about the classification, compressibility, and strength behavior of soils. Geotechnical laboratory technicians and geotechnical engineers can aid designers in the most complex geotechnical projects. However, their work depends heavily on the quality of the soil sample that arrives in the laboratory. One further step that can be considered for sensitive or limited samples is to X-ray the Shelby tube, which can be very helpful to understand the state internally to allow choosing the best candidate for testing. Every team member that takes part in the initial phases of a geotechnical program should put the highest emphasis on treating each geotechnical field sample as if they were precious, one-of-a-kind, answers to subsurface mysteries that only that sample can unlock.
Just as it matters to a new bride if her diamond measures at 1.01 or 1.11 carat (as does the cost), it matters to a geotechnical engineer if the φ angle is 30° instead of 27° . A few degrees of difference in friction angle can translate to enormous cost differences in design when hundreds of deep piles are to be installed. Be it a friction angle, φ, or cohesion value of a soil, or any of the tens of engineering parameters a laboratory delivers to a design engineer, it can be a difference in thousands of dollars and significant labor on a large construction project. This “elevated” treatment of all subsurface samples will result in the most accurate testing results and better, safer engineering design.
Post by: Anna Kotas, PE, Geotechnical Engineer at Geocomp/GeoTesting Express. Anna has been with GTX for 7 years doing business development throughout the Mid-Atlantic region and beyond. Throughout her career, she managed geotechnical projects from the earliest phases, including site recon, drilling, lab testing, analysis, and report preparation. She believes her hands on experience in the early years of her career combined with her management roles were invaluable to her current role as a representative of a world-class geotechnical laboratory.