PROJ00326

• Contract/PO #: C153498
• Contractor Name: U. S. Geological Survey
• Water District: 0
• County: Boulder
• Bill Number: SB 87-15
• Loan Projects - Doc Type: Contract Documents

grain-size characteristics with specific yield or specific
retention.

Data used in this study were obtained by labora-
tory analyses of undisturbed samples of consolidated-
rock core from hole CI or CIA. Core samples were
selected from carefully stored intact segments of drill
core (Raforth and Jehn, 1990), sealed injeweler's wax,
and transported to the laboratory where each sample
was sawed into three 3/4 in.-thick disks for analyses.
Core analyses consisted of porosity and specific-
retention determinations on three specimens from each
depth sampled and grain-size analyses on the samples
that were subsequently disaggregated. Mean porosity
and specific retention were calculated from the three
specimens for each sample, and specific yield was cal-
culated by use of equation 1. Porosity was determined
by use of a helium gas-expansion porosimeter and a
mercury-immersion displacement porometer; specific
retention was calculated from moisture-retention data
at 13.5 bars in a porous-plate apparatus (American
Society for Testing and Materials, 1977 ; McWhorter
and Garcia, 1990). A pore pressure of 13.5 bars was
chosen for the test because specific yields of clastic
sediments are very close to equilibrium at this and

higher pressures. Grain-size data were obtained from
mechanical sieve analyses.
Least-squares linear regression analysis of
specific-yield and specific-retention data on various
grain-size characteristics indicated that the best regres-
sion equations (those that had the largest coefficient of
correlation and smallest standard error of estimate) all
had specific retention as the dependent variable. These
regression equations were used in preference to the less
v.:ell-correlated regression eqllations that had specific
YIeld as the dependent variable because specific yield
can be calculated from specific retention by use of
equation I and porosity data obtained from geophysical
logs or laboratory analyses.
Five characteristics of a grain-size distribution
were found to correlate with specific retention.
These characteristics are the D50' D70 minus D50' and
D90 diameters and the P.0625 and P.125 percentages.
The D50 diameter is the 50th-percentile grain diameter
(diameter at which 50 percent of the sample is finer)
(fig. 3). The D70 and D90 diameters are the 70th- and
90th-percentile grain diameters. The P.0625 and
P.125 percentages are the percent of the sample that is
finer than 0.0625 and 0.125 mm, respectively.

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Figure 3. Grain.size distribution for sample 51.

SPECIFIC.YIELD AND SPECIFIC.RETENTlON ESTIMATES FROM GRAIN-SIZE DATA 5