20210418_NCGS

Trip Report of Dr. Kenneth B. Taylor, PG (#1835)
State Geologist of North Carolina
to the Wake Stone Corporation
Triangle Quarry and the Knightdale Quarry
May 18, 2021
Arrived at 10:30 am to the Triangle Quarry office just west of Harrison Avenue in Cary and
southwest of William B. Umstead State Park. I met with three people at the office -- David F.
Lee, Wake Stone Corporation's Head Geologist & Environmental Supervisor; L. Cole Atkins,
Geologist and Environmental Specialist; and Hunter Bratton, Triangle Quarry Superintendent.
We went to the bottom of the pit which is at a depth of below sea -level. The rock inside the
quarry is highly jointed which results in easier removal and transport to the rock crushers. There
are several diabase dikes visible in the quarry face, including a large one that trends across
almost the entire quarry. The rocks being quarried are two types: CZrg — Reedy Creek
metagranodiorite, a metamorphically altered igneous rock on the eastern side of the quarry and
CZbr — Big Lake -Raven Rock schist, a metamorphosed mudstone. There are also pre -
metamorphic diabase dikes in the quarry, which demonstrates that these igneous rocks intruded
into rocks which were later metamorphically altered.
Following the quarry tour, David Lee, Cole Atkins, and I went to see drill core which is housed
at the Knightdale Quarry. Two sets of core were shown that were both drilled in the Odd
Fellows Track on either side of the creek. The rock alongside the creek is strongly foliated. In
order to demonstrate rock strength, two 180-foot cores were drilled. On one side of the creek,
the core was drilled parallel to the foliation. On the other side of the creek the hole was drilled
vertically, cutting across the foliation at about sixty degrees.
Eighteen boxes of core were laid out on two rows of tables. Each box contained 10 feet of core.
The total length of each core was 180 feet.
David Lee used a spray water bottle to allow the detail in the core to be observed. There were
many intrusives in these cores. Most were mobilized hot water intrusives of mineralized water.
Some were intrusive igneous rock which has crystalized at depth. Mineral crystals grow large
when the intrusives cool very slowly. In a fast cooling environment, the mineral grains will be
small. David also used an acid bottle to test for calcite which bubbles when acid is applied.
There were many places in the core where quartz and feldspar intrusives were parallel to
observed foliation. A photograph was shared which showed a 10-feet section of core that had
been recovered without breakage.
Foliations can be surfaces in the cores where there will be breakage. For the vertically drilled
core, the rock shattered along the foliations. Boxes of those cores showed discs of rock, where
breakage was along the foliation plane. In contrast, the core drilled on an incline, the foliation
was seen as a series of parallel zones perpendicular to the core barrel. Seeing the same rock
drilled vertically (with failure in the rock along the foliation plane) and little to no failure of the
inclined core where the foliation was perpendicular to the core barrel was an excellent example
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in understanding rock mechanics. One might think that one rock is weak and can be easily
broken, but that is not the case. Both cores are from the same rock. The orientation of the core
barrel demonstrates that the foliation in the rock controls the rock's strength. Mining a site in
lifts where the material lifts are parallel to the foliation will yields strong rock. In contrast,
mining the rock without regard to the orientation of the foliation could result in wasted rock due
to lots of fractures parallel to the foliation direction.
In places on the proposed expansion of the mine site, where there is a proposed bridge for heavy-
duty haul trucks over a creek, it would be a prudent idea to monitor the induced vibrations from
the trucks to see if surface rock begins to split along the foliation plane. If such fracturing were
to occur, there could be migration of creek water into the subsurface. Could such water
migration impact the bridge or the haul road? That would require additional geotechnical
analyses. The bottom line is that the two cores drilled on the creek demonstrated the rock is very
strong and competent, so long as it is mined parallel to the existing rock foliation.
This text was expanded on October 29, 2021 to provide a clearer explanation of the role
foliation plays in rock strength.
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