Tuesday, January 22, 2013

Engineering Metamorphic Geology

My graduate school work involved tectonics and metamorphic rocks. As a consulting geologist my work has been more oriented toward surficial processes and engineering geology either learned through experience, mentors or self taught. It was nice to have a recent couple of projects where my metamorphic background combined with my engineering geology background. One project has been an ongoing project involving metamorphosed bedrock. I had an inspection visit recently and then I traveled to another project with metamorphosed bedrock on the same day.

The first ongoing project was to inspect the embedment of rebar into bedrock for the support of a couple of wing walls spanning between a concrete structure and a steep bedrock slope. The rock was drilled first and then the rebar was set into the rock with an epoxy. The foliations, lineations, variability in protolith lithology, and shear zones defined where the rebar supports had to be placed. Fortunately there was enough flexibility for the wall locations that the alignment worked even with the above described limiting factors. A few feet in another direction and the rock would likey no have held up to being drilled due to well developed foliations creating weekness in the rock. Metamophic mineralogy, foliation orientation and protolith matter!

The second site was to take a look at a steep slope underlain by metamorphosed bedrock. This site was within the same formation as the first site - the Darrington Phyllite and semischist of Mount Josephine. The two units are interlayered and in places sheared and faulted and include slivers and blocks of Shuksan Greenschist. The Darrington is .... phyllite. Most of the Darrington Phyllite is very soft graphitic phyllite derived from metamorphosed mud stone. Really bad stuff for getting solid embedment for rebar and rarely stands out as outcrops. However, there semischist and greenschist that include sandstone, volcaniclastic sandstone and lava protoliths are more solid depending on the degree of shearing and foliation development.

These rock units are part of the Easton Metamophic Suite within the Northwest Cascades. These rocks are a complicated melange of fault bounded and highly sheared rock. The semichist and phyllite typically contain at least two foliations along with multiple lineations and from the rock being stretched and tightly folded. These rocks have had a long hard life since being deposited on the ocean floor at least 165 million years ago. They got shoved deep into the subduction trench, thrust back out and then piled up along a series of other exotic terrains against and along the North American margin and then uplifted yet again before erosion brought them to or near the surface.

The first site with the rebar was located within an area that was predominantly coarse volcaniclastic protolith and even igneous protolith with a bit of phyllite and some ancient fault lines filled with talc to make things interesting. The second site was mostly siltstone and mudstone protolith. This second site had some nice exposures showing intense folding in the formation.

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