Geotechnical Field Testing

Site Preparation | Sampling | Field Testing

Site Preparation
Level Drill Pad – Prepare drilling sites before arrival of the drill crew. The kelly and mast of the drill rig are fixed to the truck bed and cannot swing, as some auger rigs can. Level the bed of the drill truck in order to drill a vertical hole. The truck is equipped with hydraulic jacks that can lift the front of the truck one foot off the ground and one foot on either side to accommodate uneven terrain. If the slope of the site is steeper than one foot, prepare a work pad 16 ft. wide and 70 ft. 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. Before extensive site work, consult the driller who performs the work for specific instructions. See the following figure for drill site requirements.

Overhead Clearance – Overhead must be clear of obstructions. Trees cannot block the raising of the mast. It is not safe to work within 25 ft. of an overhead power line. If necessary to work closer, contact the power company to cut the power or install insulating safety boots.

Underground Utility Locations – 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

The driller will be available to inspect locations and make recommendations on site preparation. Often it is possible to begin drilling easy sites while preparing more difficult sites.

Access – Ensure that permission to enter private property has been secured before drilling.

Barge Work – When a bridge must cross large bodies of water, barges are used to obtain foundation information. Barge work is complex and expensive, so coordination with the driller should begin well before start of drilling.

Sampling
Dry Barrel or Single-Wall Sampler – Use the dry barrel sampler to obtain core samples for visual soil and bedrock classification and logging. The core sample obtained is generally 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.

Diamond Core Barrel – 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.

Push Barrel or Shelby Tube Sampler – 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 to 36 in. 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:

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.

Wash Sampling or Fish-Tailing – 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.

Field Testing
Texas Cone Penetration (TCP) Test – See Tex-132-E in the 100-E, Soils, & Aggregates Test Procedures manual.

Standard Penetration Test (SPT) – The SPT uses a 2-in. diameter pipe (split spoon) driven with a 140-lb. hammer at a drop of 30 in. The test is described in ASTM procedure D 1586. This test is recommended mainly for granular soil but has been used in cohesive soil. It cannot be used in rock. It correlates roughly with the TCP test as follows:

• Clay: N_tcp = 1.5 N_spt
• Sand: N_tcp = 2 N_spt

Test correlations presented here are only for approximate evaluation of design adequacy from outside sources and not for normal foundation design work.

Observation Wells and Piezometers – Observation wells and piezometers are used to measure ground-water levels. Observation wells are essentially water wells and are sometimes pumped to determine the permeability of the soil to predict seepage volumes in excavations. Piezometers are instruments which 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. 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 groundwater 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, since they may be read from a remote location. Piezometers are also more sensitive to groundwater 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:

• Future depressed roadway sections. 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.

• Slope stability. 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.

Take a reading immediately and weekly thereafter until the water level stabilizes. Monthly readings thereafter are normally sufficient unless the site exhibits large fluctuations in readings.

Inclinometers – Inclinometers measure horizontal movements within a soil mass over time. The inclinometer is a sensitive device that measures deviations from vertical. Record these deviations at periodic intervals along a special casing grouted into a bore hole 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 is quite complicated. Consult Bridge Division geotechnical engineers if inclinometer measurements are required.