Rock Cores

(3) Below Groundwater Table. When sampling below groundwater table,

maintain borehole full of water or drilling fluid during cleanout, during

sampling and sample withdrawal, and while removing cleanout tools. Where

continuous samples are required, casing should remain full for the entire

drilling and sampling operation.

(4) Soft or Loose Soil. Sampling of a soft or loose soil directly

below a stiff or compact soil in the same tube should be avoided.

Discontinue driving of sample tube when a sudden decrease in resistance

occurs.

3. UNDISTURBED SAMPLES FROM TEST PITS. Hand trimmed samples may be

obtained in test pits, in test trenches, or in surface exposures. Samples

so obtained are potentially the least disturbed of all types of samples.

The basic procedure consists of trimming out a column of soil the same size

or slightly smaller than the container to be used in transportation, sliding

the container over the sample, and surrounding the sample with wax. Tight,

stiff containers that can be sealed, and are not readily distorted, should

be used.

4. ROCK CORES. Rock is sampled with core barrels having either

tungstencarbide or diamond core bits as listed or described in Table 9 and

Figure 2.

The suitability of cores for structural property tests depends on the

quality of individual samples. Specify double or triple tube core barrel

for maximum core recovery in weathered, soft, or fractured rock. The

percentage of core recovery is an indication of soundness and degree of

weathering of rock. Carefully examine core section for reasons for low

recovery. More details on rock recovery can be found in Chapter 1.

5. SAMPLING OF DISINTEGRATED ROCK TRANSITION ZONES. General guidance on

sampling of rock with various degrees of disintegration is given in Table 11

(modified from Reference 7, Sampling of Residual Soils in Hong Kong, by

Brenner).

6. OFFSHORE SAMPLING. For water depths less than about 60 feet, land

type soil boring equipment can be used on small jack-up platforms, small

barges or barrel floats. Floating equipment requires suitable anchoring and

is limited to fairly calm sea conditions. For deeper water or more extreme

seas, larger drill ships are required to obtain quality undisturbed samples.

See Table 12 for common underwater samplers. Numerous types of

oceanographic samplers, both open-tube and piston types, are available for

use from shipboard. These depend upon free-fall penetration and thus are

limited in depth of exploration. The quality of samples obtained by most

oceanographic samplers is not high because of their large length to diameter

ratio. For detailed information on underwater sampling equipment see

Reference 8, Underwater Soil Sampling, Testing and Construction Control,

ASTM STP 501, and Reference 9, Seafloor Soil Sampling and Geotechnical

Parameter Determination - Handbook, by Lee and Clausner.

7.1-80