Several exposures of mica schist rock supporting loadings of 20 tons per sq. ft. (20kg/sq. cm.) show loss of crystalline structure and of elastic properties. When lateral support is removed, serious loss in bearing capacity becomes possible. Records of actual observations and experiences are included.
Differentes tranchees dans des roches de mica-schiste, supportant des charges de 20 kg/cm2, montrent des pertes de structure cristalline et une reduction des proprietes elastiques. Quand la pression laterale est supprimee, une diminution serieuse de la capacite de support devient possible. Des resultats d'observations et d'experiences reelles sont inclus.
Verschiedene Aufschluesse von Glimmer- Schiefer unter Traglasten von 20 kp/cm2 zeigen Änderungen der kristallinischen Struktur und Verrninderung der elastischen Eigenschaften. Entfernung der seitlichen Einspannung ermöglicht wesentlichen Veriust der Tragfahigkeit. Der Bericht enthalt gemessene Zahlwerte und Beobachtungen aus der Praxis.
The greater part of Manhattan Island on which is founded the center of New York City is covered by a coarse quartzmica schist, identified as Manhattan schist. Of pre-Cambrian age, it is a schistose blackish rock, essentially biotite and quartz, the greatly metamorphosed equivalent of the Hudson slates and shales found 50 miles (80 km) north of New York City. Manhattan schist is often imbedded with hornblende schist and some altered basic intrusions with bands of pegmatite made up of microline, quartz and biotite. This area was covered by glaciers and deposits of dense sand-gravel as well as loose uniform grained sands are often found. The strike is N30° E (parallel to the axis of Manhattan Island) with folds along the same axis; axial planes dip eastward at 70° or more. In many of the submerged valleys are found deep layers of fine silts, sand and clays, in varved formation, of extremely variable density and bearing value. Fracture of the rock is easy but along uncertain planes. Drilling is not too definite, holes often deviating from straight lines, and care must be taken to prevent the drill steel from binding in the hole. Most of the rock exposures can be wedged and peeled. Preparation of a level surface is almost impossible. Split faces are slippery because of the large amount of exposed mica. Under traffic, the surface pulverizes into a dust which becomes a slippery mud when saturated. The New York City Building Code classifies the material either: Class 2 - Medium hard, presumptive bearing capacity 40 tons per sq. ft. (40 kg/sq. cm.), with some cracks and slight weathering along cracks. Class 5 - Soft, presumptive bearing capacity 8 tons per sq. ft. (8 kg/sq. cm.), when the rock is decomposed, with some disintegration and softening and with considerable cracks allowed. Except for very large structures where pit excavations may be required to considerable depths to expose satisfactory exposure, the medium hard rock is usually expected to carry 20 tons per sq. ft. The Code makes the assumption that the exposure of rock in foundations, covered with concrete of the footings, is unaltered with age. The percentage of rock, especially in the schist areas, which can be classified as better than «soft rock» is extremely small. This is fortunate, because of the tendency of the schist to soften and disintegrate even after being sealed by concrete. The time required for such change, in the built-up areas of the city, is surprisingly small. Exposure of footings supporting buildings is not uncommon. Development of adjacent areas for new buildings and for subway structures is frequent, and a part of such construction is the frequent exposure of footings founded on rock. Normally, it is assumed that these footings had been placed on unsound rock even if the records indicate that it was intended to found on sound rock. During the construction of the 6th Avenue Subway, several buildings were underpinned to lower levels through disintegrated rock. In one case, the supervising engineer of the construction of the foundations (in 1917) personally examined the exposure in 1936. His original field records were still available and his notations of «hard, ringing rock» for each of the footings left no doubt that the rock had altered completely in 20 years. The disintegrated rock under the footings was removed by hand shovels and picks, after temporary shoring of the columns, to as much as a 25 foot depth before sound rock was again encountered. The records of the old 6th Avenue Elevated Line, then over 50 years old, indicated sound rock levels much higher than were actually found when the footings were removed. Even the rock under the footings of some monumental structures along 6th Avenue, less than 10 years old (in 1936–8) could easily he removed by one's fingers. There is definite indication that mica schist, under superimposed loadings of approximately 25 tons per square foot, changes physically in a matter of 10 to 20 years.