Effects of Specimen Age on the Uniaxial Compressive Strength and 

Moisture Content of Weak Coal Measure Rocks
 

Timothy M. Barton, Research Engineer
 

Suresh K. Bhatt, Research Engineer
 

Gregory M. Molinda, Lead Research Geologist 

Christopher Mark, Principal Research Engineer 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA
 

ABSTRACT 

The uniaxial compressive strength (UCS) is the most 

fundamental measurement used in geotechnical rock 

characterization for mine design. While there are standardized 

procedures for how to conduct UCS tests, there are no firm 

guidelines as to when to conduct them. However, it is well known 

that the strengths of at least some rocks can change during the time 

between when the core first comes up out of the hole and when it 

is prepared and tested in the lab. 

The goal of this NIOSH (National Institute for Occupational 

Safety and Health) study was to evaluate UCS changes occurring 

in a broad range of weak coal measure rocks over a one-year time 

span. The study found the highest moisture contents were 

measured when the core was fresh, immediately after it was taken 

from the hole. The specimens then dried rapidly over the next few 

weeks. Subsequently, sample moisture contents decreased slightly 

in the winter and increased in the summer in response to the 

ambient changes in humidity. 

The measured UCS of the core also changed during the year, 

apparently in response to changes in the moisture content. The 

UCS values from the dry, winter months were, on average, 60% 

higher than the values obtained when the core was fresh, and the 

summer UCS was approximately 11% lower than the winter UCS. 

These findings have implications for the use of UCS as an input 

parameter for both empirical and numerical mine design methods. 

UCS values of unprotected core tested weeks to months after 

drilling can be significantly stronger and indicate stronger roof 

sequences than warranted. In order to obtain the most 

representative and reliable UCS value, it is necessary to test the 

core, or wrap and seal it, at the drill site shortly after recovery. 

INTRODUCTION 

The UCS is undoubtedly the geotechnical property that is most 

often used in rock engineering practice. It is widely understood as 

an index which gives a first approximation of the range of issues 

that are likely to be encountered in a variety of engineering 

problems including roof support, pillar design, and excavation 

technique (Hoek, 1977). The UCS is not a property that is intrinsic 

to a particular rock, however. Numerous researchers have shown 

that the measured UCS can be affected by a variety of 

environmental factors, including age and moisture content. 

Cummings, et al. (1983) emphasized the importance of 

obtaining fresh shale samples and testing them immediately. They 

also recommended special care while handling samples. They 

used specially prepared plastic bags, wax seal, and boxes to 

minimize moisture loss (2-4%) due to drying. They observed 

continued moisture loss from the core samples during the storage 

period of several months. 

Hoek, et al. (2005) advocated testing cores (for UCS) soon 

after drilling, right on the site. They noted that it is sometimes 

difficult to distinguish initially between sandstone and siltstone, 

but that after exposure siltstone can start to develop a fissile 

appearance. 

Unrug and Padgett (2003) found that the Rock Quality 

Designation (RQD) of some samples decreased by about 42% 

between the drill site and the laboratory. They felt that the freshly 

cut core was more representative of the rock behavior at the time 

of excavation, but that the change in RQD could be a better 

indicator of the excavated rock quality through time. 

A classic study conducted by Bauer (1980) showed that the 

UCS of coal measure shale is strongly correlated with its in-situ 

moisture content, with the weakest shales having the highest 

moisture contents. Oven-dried rocks were found to be two to three 

times stronger than rocks fully saturated with water. Matsui, et al. 

(1996) reported a reduction in mechanical strength properties in 

shales which are in contact with water. They also found more 

vertical roadway closure in wet areas (16 – 24 in) than dry areas 

(2 – 6 in). 

Bell and Jermy (2002) tested core samples obtained from a 

South African coal mine. After soaking in water for 72 hours, 

some sandstone samples showed reductions in their UCS values 

ranging from 29 to 58%, compared with their dry equivalents. 

Studies have shown that roof fall rates during humid summer 

months are significantly higher than they are in fall and winter. 

While most falls occur within 12 months of mining, they continue 

to occur up to six years after mining began (Mark et al., 2004; 

Molinda, et al., 2008). 

The goal of this NIOSH study was to evaluate UCS changes 

occurring in a broad range of weak coal measure rocks over a oneyear time span. Core was obtained from two boreholes drilled to 
 

 

the Pittsburgh coalbed in southwestern Pennsylvania. The core 

was never wrapped or otherwise protected so that the effects of 

time and season could be observed clearly. Point load testing 

(PLT) was conducted on rock from 57 different horizons within 

the overburden. A total of 19 Ferm rock types were represented 

within the 57 unit horizons. The tests were repeated at seven time 

intervals over the course of the year, beginning when the core was 

first recovered. In all, more than 1,500 axial and 1,100 diametral 

tests were conducted. Moisture content of the specimens was also 

measured at approximately each test interval. 

PROJECT DESCRIPTION 

Rock core was obtained from two drill holes located over the 

Pittsburgh coalbed in Greene Co., PA. The first hole was drilled in 

mid-August, and the second a little more than two months later. A 

generalized stratagraphic column for the boreholes is shown in 

figure 1. Figure 2 shows a segment of the actual geologic log, 

from one of the holes. This includes the location of some sample 

test horizons. The type of rock that was tested is also illustrated in 

the photograph of rock core runs shown in figure 3. 

Figure 1. Generalized stratagraphic section of rock tested. 

Figure 2. Sample section of geologic log of borehole 2 

highlighting depth and length of sample horizon unit numbers. 

A total of 948 ft of core was logged and boxed from the two 

boreholes. Once the unwrapped core was returned from the field 

in closed coreboxes of wax-permeated cardboard, the closed 

coreboxes were stored in a building under normal room 

temperature and only opened to conduct UCS and moisture content 

measurements (figure 4). Within this core, 57 rock units were 

selected for testing. Each rock unit was classified using the Ferm 

rock classification system (Ferm, et al., 1981). Based on the Ferm 

code, the rocks were divided into four groups as shown in table 1. 

The moisture content measurement began with the initial 

weighing of the samples at the drill site. The samples were then 

bagged but left open and placed back into the core boxes. The 

same samples were then weighed periodically during the duration 

of the study, and finally oven-dried at the end of the study (figure 5) 

(ASTM, 2004; ISRM, 1985). The percentage moisture content 

(MC) was then back-calculated using the following formula: 
MC = [(Wm-Wd)/Wd] x 100 (1) 

where: Wm = Specimen weight at any given test date, and 

 Wd = Final oven-dried specimen weight. 

 

The initial series of point load tests (PLT) were conducted at 

the drill sites using a PLT apparatus and data acquisition system 

connected to a laptop computer (Brown, 1981). Once the core was 

returned from the field, each rock unit was divided into 7 time test 

units (figure 6). Between 3 and 8 specimens from each unit were 

tested at intervals of approximately 2, 4, 12, 24, 32, and 52 weeks 

after the core was extracted from the boreholes. At the end of the 

study year, the PLT apparatus was calibrated with a dead weight 

tester to verify its accuracy. 

Figure 3. Photograph of rock core from borehole 1. 

Figure 4. Pictorial overview of stages of fieldwork at both 

drill sites; clockwise from top left: (a) Diamond rock core 

drilling at site 2; (b) geologic logging of core runs at site 2; (c) 

axial PLT-UCS measurements of core specimen at site 1; (d) 

boxing of core runs in labeled core boxed made of wax 

permeated cardboard at site 2. 

Table 1. Rock type groupings included in this study, and 

their associated Ferm numbers. 

Rock type Ferm codes 

Black Shale 112, 113, 114, 117 

Grey Shale 122 124, 134 

Fireclay 127, 137, 157, 237, 327 337, 347, 427, 

437, 444 

Sandy Shale 322, 323, 324, 325 

Sandstone 543, 564, 742, 748 

Limy Rocks 787, 802, 804 

Figure 5. Pictorial overview of test stages of moisture content 

tests; clockwise from top left: (a) weighing of container and 

core specimen prior to precision oven drying; (b) post-dried 

core specimens in containers; (c) post-test measurements of 

bulk volume of core specimen; (d) vacuum desiccators with 

core specimen in weighing containers for specimen cooling 

without moisture loss prior to post-dried specimen weighing. 

The PLT data were used to determine the IS50 values using 

standard International Society for Rock Mechanics (ISRM) 

procedures. The values were in turn used to estimate the UCS 

using the equation developed by Rusnak and Mark (2000) for coal 

measure rock: 

UCS = 21 x IS50 (2) 

where IS50 = index of strength for 50 mm core 

Figure 6. Core separated into specimens for point load 

testing at different time intervals. Bag samples were tested 

for moisture loss. 
 

 

 

 

 

 

 

 

Following ISRM procedures, the highest 10% and the lowest 

10% of the test results from each test group were removed before 

the statistical analysis was conducted. 

RESULTS 

Figure 7 and table 2 shows the moisture content measurements 

relative to the time of the measurement. The initial moisture 

content of the fresh core, measured when the core was first 

recovered from the hole, varied from a low of approximately 1% 

up to a high of about 4%, with an average of about 2.5%. 

Statistical analysis shows that subsequent moisture content 

measurements averaged 40% lower, or about 1.5%. The difference 

between the fresh moisture content and the later moisture content 

is statistically significant. UCS, ksi 25 2015 10 50 UCS, ksi 50 

Fresh Summer Winter Fresh Summer Winter Fresh Summer Winter Fresh Summer Winter 

Black Shale Fireclay Gray Shale Sandy Shale 

Figure 8. Graph showing PLT UCS values for the four 

clay rich rock types studied. In each case, the strengths 

were lowest when the cores were fresh, and highest during 

the winter when the rocks were driest. The middle line 

represents the median value, and the upper and lower 

hinges of the box represent the 75th and 25th percentiles of 

the data. 

During the first winter following the drilling, the core 

continued to dry slowly, reaching an average moisture content of 

less than 1% at its driest point (3/8/06, table 2). Measurements 

made in the following two summers indicated that there was a 

statistically significant uptake of moisture of about 0.5% of the 

total sample weight. Table 2 provides more details on the moisture 

content measurements. 

Table 2. Results of the moisture content tests. 

Test date n Mean, % Standard 

deviation, % Standard error, % 

95 % Confidence 

interval (lower limits), 

% 

95 % Confidence 

interval (upper limits), 

% 

10/18/2005 22 2.46 0.77 0.16 2.14 2.79 

2/13/2005 30 1.49 0.59 0.11 1.28 1.71 

1/17/2006 30 1.29 0.46 0.08 1.12 1.45 

3/8/2006 30 0.99 0.36 0.07 0.86 1.12 

7/13/2006 37 1.50 0.47 0.08 1.35 1.65 

9/15/2006 31 1.66 0.51 0.09 1.48 1.84 

10/12/2006 44 1.58 0.51 0.08 1.43 1.74 

9/20/2007 60 1.53 0.45 0.06 1.42 1.65 

Results of the analysis of the PLT UCS data followed similar 

trends with time (figure 8). Initial statistical analyses indicated 

that rock type, core freshness/moisture content, and season were 

all highly statistically significant when regressed against PLT UCS. 

Table 3 shows the correlation between UCS and season for the 

four main rock types tested. The data show that, for all rocks, the 

average UCS of the air-dried core during the winter was about 

60% greater than the average UCS of the original, fresh core. This 

result is most pronounced for gray shale, but it is consistent across 

all the clay-rich rocks tested. This result is statistically significant, 

with a t-value of more than 7 for the data set as whole. 

In summer, the measured strength of the core was reduced by 

an average of about 11% when compared with its peak, wintertime strength. This finding, while significant at greater than the 

99.9% confidence level for the entire data set, is not as robust 

statistically as the finding about the strength of fresh core. Moisture Content, % Moisture Content, % 

Figure 7. Moisture content of the core samples. The middle 

line represents the median value, and the upper and lower 

hinges of the box represent the 75th and 25th percentiles of 

the data. The T-line indicates the data range. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 3. Results of the PLT UCS tests. The t-values (and associated probability values) are relative to the base case of the
 

PLT UCS measured for the fresh core.
 

Rock Type Season n 

Mean 

UCS 

(psi) 

Standard 

deviation 

(psi) 

Standard 

error 

(psi) 

95 % 

confidence 

interval (lower 

limits) (psi) 

95 % 

confidence 

interval (upper 

limits) (psi) 

t-test p-value 

All Rocks Fresh 132 4,580 3,040 260 4,060 5,110 

Winter 559 7,350 4,180 180 7,000 7,690 7.17 0.000 

Summer 368 6,380 3,770 200 5,990 6,770 4.93 0.000 

Black Shale Fresh 7 2,350 740 280 1,810 2,900 

Winter 24 4,010 1,570 320 3,380 4,640 2.68 0.006 

Summer 44 3,650 1,770 270 3,120 4,170 1.90 0.032 

Grey Shale Fresh 35 2,520 900 150 2,220 2,820 

Winter 177 6,450 2,380 180 6,100 6,800 9.63 0.000 

Summer 107 4,780 1,590 150 4,480 5,090 8.01 0.000 

Fireclay Fresh 44 3,390 1,410 210 2,980 3,810 

Winter 206 4,600 2,460 170 4,270 4,930 3.15 0.001 

Summer 112 4,580 2,470 230 4,130 5,040 3.01 0.001 

Sandy Shale Fresh 46 7,630 3,060 450 6,740 8,520 

Winter 152 12,640 2,850 230 12,190 13,090 10.3 0.000 

Summer 105 11,060 2,910 280 10,500 11,620 6.56 0.000 

CONCLUSIONS 

The results of this study confirm that the strength of 

unprotected rock specimens can change dramatically over a 

relatively short time after core drilling is complete. When testing 

was conducted two or more weeks after drilling, the UCS 

increased by an average of 60% compared with the fresh core 

values obtained at the drill site. It seems likely that this 

augmentation in strength is associated with the approximately 40% 

decrease in moisture content that occurred over the same time 

period. Additional, smaller changes in strength later appeared to 

be associated with seasonal changes in atmospheric humidity. The 

specimens were found to be slightly stronger in the winter than 

they were in the more humid summer months. Therefore, it seems 

from the results in this study that there is an inverse trend between 

UCS and moisture content of weak coal measure rocks. 

Procedures for testing and storing rock core can vary widely. 

Sometimes it is weeks or longer before core can be logged, 

samples selected, and UCS testing completed. The results from 

this study strongly suggest that significant strength changes are 

possible in unprotected core, and that the first few weeks can be 

critical. One way to prevent the changes in UCS, standardize the 

preservation of rock samples, and instill confidence in the 

measured strength values, is to wrap and seal the core at the drill 

site in order to preserve the original moisture content. The study 

also illustrates the advantages of conducting numerous point load 

tests on fresh core right at the drill site. Such procedures are 

necessary if accurate strength values are to be obtained for use in 

geotechnical rock characterization for mine design. 

Disclaimer 

The findings and conclusions in this report have not been 

formally disseminated by the National Institute for Occupational 

Safety and Health and should not be construed to represent any 

agency determination or policy. 

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