Gneiss Cavern at Peachtree Center EarthCache
Hidden : 11/4/2011
Difficulty:
2 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

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Geocache Description:


PEACHTREE CENTER STATION is an underground Train station on the Red and Gold lines of the Metropolitan Atlanta Rapid Transit Authority (MARTA) rail system. It is the deepest station in the MARTA rail system, at 120 feet (37 m) below Peachtree Street, which was opened in 1982.
It serves the Peachtree Center area of downtown Atlanta, and is the first station north-northeast of the rail system hub at Five Points. This is one of the busiest stations on the Red/Gold Lines handling over 15,000 people per weekday.
The Peachtree Center station has an island platform and was built by tunneling through solid gneiss, a grainitelike rock formed of layers of quartz and mica. This rock provides underground support for the station.

This station is only one of a few tunnels in the world where the walls and the ceiling were carved from solid rock.

For visiting this place you will need the Breeze Card which can be purchased for $2.50 and can be reloaded on a bus (cash only) or at a Breeze vending machine (BVM).
The one-way flat-rate fare for MARTA will cost US $2.50 (October, 2011).
Train Service Hours
Weekdays: 4:45 a.m. to 1 a.m.
Weekends: 6:00 a.m. to 1 a.m.


GEOLOGY LOCATION OF ATLANTA

Atlanta (GA) is situated in the Southern Piedmont.

Piedmont geologic region is composed of igneous and metamorphic rocks resulting from ancient (300 to 600 million year old) sediments that were subjected to high temperatures and pressures and re-exposed about 250 to 300 million years ago. Rocks typical for the region include schist, amphibolite, gneiss, migmatite, and granite. This region is more hilly than mountainous and is marked by lower elevations than the Blue Ridge. Nevertheless, the Piedmont is home to prominent features like Stone Mountain and the Brevard Fault zone, an ancient fault zone that last moved about 185 million years ago.

ATLANTA REGIONAL GEOLOGY

Much of the Atlanta area is underlain by medium to high grade metamorphic rocks of the Piedmont geologic province. The metamorphic rocks which make up the Piedmont are derived from sediments deposited 400 to 600 million years ago during the late Precambrian to early Paleozoic eras. These sediments were subjected to several periods of intense heat and pressure resulting in widespread deformation, metamorphism, and granitic intrusions. The most recent and most intense of these periods occurred approximately 250 million years ago, late in the Paleozoic era. Recumbent folding is the predominant structural style in this area of the Piedmont. Orientation of fold axes vary, but they primarily plunge gently to the northeast. The regional foliation in the Atlanta area strikes northeast and dips gently to the southeast. Local variations in attitude are caused by folding (both open and recumbent) on a variety of scales. Granitic intrusions occurred in the Southeast Piedmont between 600 to 250 million years ago.

GEOLOGY AT THE PEACHTREE CENTER STATION

The bedrock consists primarily of biotite gneiss and granitic gneiss.

The biotite gneiss encountered during subsurface explorations contained abundant biotite dark mica that produces a strong foliation. The granitic gneiss contained less mica and thus shows less foliation.


ROCK SUPPORT

Several ground support types were specified for construction based on expected rock mass conditions, as well as Rock Quality Designations values along the cavern alignment.
Ground support for the station consisted of patterned rock bolts ranging from 10 to 20 feet (3.0 to 6.1 meters) in length, with alternating offset patterns ranging from four to 8 feet (1.2 to 2.4 meters).

Unconfined Compressive Strength (UCS) tests were performed on representative core samples of rock and UCS values obtained during laboratory testing ranged from 6,700 to 12,270 psi (46 to 85MPa).
Project designers were able to rely on excellent rock quality, as well as documented local construction experience, to develop a successful and cost effective solution for the project owner.

THE CACHE

To claim this cache you must fulfil following tasks and send answers to my profile GOGO CZ.

a) What type of gneiss predominates on the walls of stations? You can see it from the platform.
b) Estimate volume of gneiss (in CY or m3) which had to be excavated from the station. You can measure/guess it from the platform.
c) Append a photo with your GPS at the platform - this task is OPTIONAL.


You can log the cache without my confirmation but you should send me answers before the logging. If there is anything wrong with your answers I will let you know. Logs without sending answers will be deleted.




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