Engineering geologic factors in the design of a large underground structure in a tuff sequence in Cappadocia

Abstract

The Cappadocia region of Inner Anatolia has produced very distinct cultures since the Bronze Age (6500 BC). The settlers were accustomed to hew their house in massive to thickly-bedded tuffs. A thick succession of lapillistone, welded tuff, silicified tuff, clayey tuff, agglomerate, and lava flows constitutes the majority of the Late Tertiary-Quaternary-aged volcanic units of this region. Most of the historical structures hewn into the thick to massive layers have preserved their original integrity; whereas most of the structures carved into a medium- to thinly-bedded sequence have failed. Bedding planes are the primary discontinuity type in the region, and must be accommodated in geotechnical designs. Joints and normal faults present the second prominent discontinuity types, which are very widely spaced. Bedding planes beyond the structural influence of major fault zones dip around 10°, due to syndepositional deformation during regional uplift. In addition to the thickness of layers, the engineering characteristics of the persistent discontinuities have a great influence on the orientation and dimension of au underground cavern in Cappadocia. Construction of this cavern, the International Underground Congress Hall, commenced in January 1993 in Cappadocia, south of the town Avanos. The initially proposed location and orientation of the structure were completely changed and readjusted according to the geotechnical model prepared in September 1993. The geotechnical re-design of the structure was then accomplished through a series of numerical analyses carried out using a Boundary Element technique. The structure in the former proposal was found susceptible to kinematic collapse of joint-bounded rock blocks, possible breakup of thinly bedded and very weakly cemented tuff layers, and quick erosion by groundwater flowing along persistent discontinuities. Moreover, groundwater percolating along discontinuities requires sophisticated engineering measures to maintain stability wherever cleft-water pressure builds up.