Geotechnical Field Testing
January 2, 2008
Proper site preparation is a critical aspect of any geotechnical field test, beginning with a level drill pad. If the slope of the site is steeper than 1 foot, prepare a work pad 16 feet wide and 70 feet long for leveling the rig and providing a safe place for the crew to handle the drill stem. For safety reasons, the crew is not allowed to block up the jacks to accommodate greater slope angles. The mud pan must be level or slightly down slope.
Overhead must be clear of obstructions. Trees cannot block the raising of the mast. It is not safe to work within 25 feet of an overhead power line. If it is necessary to work closer, contact the power company to cut the power or install insulating safety boots.
You must know the exact location of underground utilities including the following:
- high pressure gas lines
- water lines
- sewer and storm lines
- electrical and telephone conduits and cables
Ensure that permission to enter private property has been secured before drilling. Often, it is possible to begin drilling easy sites while preparing more difficult sites.
Geotech SamplingUse the dry-barrel sampler to obtain core samples for visual soil and bedrock classification and logging. The core sample obtained generally is in a disturbed condition due to the pressure applied when cutting the core and packing it into the barrel for recovery. The core is extracted from the barrel by water pressure. When used for sampling in practically all foundation materials except very soft clay (muck) and cohesionless sand, the dry barrel sampler obtains a sample containing all components in the original formation. The amount and degree of disturbance depends upon the consistency and density of the material. Although this method is called the dry-barrel method, circulating water is used. In hard formations, a smaller volume of water is circulated while cutting the core.
Use diamond-core barrels to obtain intact rock samples for field or laboratory tests and classification. The diamond-barrel sampler has an inner and outer barrel. The inner barrel is slightly oversized with a spring-loaded core retainer at the bottom.
Use the push-barrel sampler to obtain relatively undisturbed soil samples for field and laboratory tests and soil classification. The device consists of a thin-walled tube 24 inches to 36 inches long, with one end sharpened to a cutting edge, and the other end reinforced and designed for easy attachment to the drill stem coupling. The thin-walled tube is steadily pushed into the formation with the hydraulic pull-down of the drill rig. This sampler recovers good undisturbed samples where it is adaptable, but its usefulness is limited to materials that it can be forced into, and that have sufficient cohesion to remain in the barrel while the sampler is being withdrawn from the hole. Use the device as follows:
1. Force the sampler into the formation with a slow, steady push to within 3 inches to 4 inches of length.
2. Rotate the sampler several times to shear off the core at the bottom before withdrawing.
3. Bring the push-barrel to the
4. Detach the barrel from the
5. Mount the barrel on a hydraulic sampler extruder and extrude the core.
6. Cut the core into 6-inch lengths, and wrap them in thin plastic to retain moisture content.
7. Place the samples in cartons for transport to the testing lab.
For samples of soft soil, sample disturbance can be a problem during transport to the testing location. To ensure minimum disturbance, support soft samples in their cartons. Fine, dry sand poured around the sample in the carton provides excellent support during transport. Store samples that are not immediately tested in a moist room.
Of the many methods for penetrating overburden soil, consider only those that offer an opportunity for sampling and testing the foundation materials without excessive disturbance. Do not use wash sampling or fishtail drilling unless absolutely necessary. Attempts to classify the soil materials by watching the wash water may lead to erroneous conclusions about the subsurface soil being penetrated.
Ground Water LevelsObservation wells and piezometers are used to measure ground water levels. Observation wells essentially are water wells, and sometimes are pumped to determine the permeability of the soil to predict seepage volumes in excavations. Piezometers are instruments that measure water pressure at the elevation of the installed sensor.
For short-term observations of water levels, leave exploration core holes open for several hours to several days to monitor the ground water level and note the depth to water in the hole. Then, cover the hole to protect people or livestock from injury.
For long-term observations, install either observation wells or piezometers. Observation wells are most useful where the ground water conditions are fairly stable, and in relatively porous soils or rock. They are simple to install and read, however, they must be placed in a location where the top of the well is accessible. Piezometers are useful where access is difficult, because they may be read from a remote location. Piezometers also are more sensitive to ground water changes in fine-grained soils. Many types of piezometers are available, with each having advantages and disadvantages. Consult with the designer regarding selection and installation of piezometers.
Some typical applications for piezometers are to evaluate ground water levels in future depressed roadway sections and ground water effects on slope stability. The construction and long-term performance of depressed roadway sections can be affected adversely by ground water. The final installation may need special drainage features to control water inflows and provide a stable pavement section. Ground water affects slope stability by reducing the effective stresses in the soil through buoyancy. This applies to both side slope stability and bearing capacity of embankments and retaining walls.
The basics of the process:
1. Drill the hole with no water, if possible. If that’s not possible, drill with clear water. If hole stability continues to be a problem, add small amounts of drilling mud to the water.
2. Place the assembled observation well piping into the hole. Use either a slotted screen or drill holes in a section of the pipe, and then wrap them with filter fabric. The upper sections of the pipe are not perforated.
3. Place a granular media in all but the upper 5 feet to 10 feet of the hole. Use a fairly coarse sand or pea gravel to allow easy placement through water.
4. Seal the remaining upper portion of the hole with grout or bentonite pellets. When using bentonite pellets in a dry hole, pour several gallons of water over the pellets for 10 minutes to 15 minutes to start expanding the pellets to seal the hole.
5. Finish the well in such a manner as to not be a hazard to the public. Utilize a locking cover if vandalism is possible.
Take a reading immediately and weekly thereafter until the water level stabilizes. Monthly readings thereafter normally are sufficient unless the site exhibits large fluctuations in readings.
Inclinometer UseWe can measure horizontal movements within a soil mass over time with an inclinometer, which is a sensitive device that measures deviations from vertical. Record these deviations at periodic intervals along a special casing grouted into a borehole to determine the horizontal deviation of the casing from the bottom of the casing to the top.
The most common application is for monitoring slope failures to determine the failure plane depth. Install inclinometer casing at several points in and adjacent to the slope failure, and use information from inclinometers in stability analyses. In order to be effective, the bottom of the inclinometer casing must extend well below the failure plane.
Take an initial set of readings immediately after casing installation to establish the baseline reading. Compare all subsequent readings to the baseline to determine direction and amount of movement. Base frequency of readings on the rate of failure of the slope. The installation of casing, operation of the inclinometer, and data reduction are rather complicated; it may be a good idea to consult geotechnical engineers in these cases.
This article is provided through the courtesy of the Texas Department of Transportation.