The Geotechnical Design of the Townline Road–Rail Tunnel: Discussion

Abstract

Conlon et al. ( 197 1 ) are to be congratulated for their concise presentationof t he geotechnical aspects of a major engineering project that will be of interest to both engineers and engineering geologists. A cursory examination of the physiography and surface geology of the tunnel site prior to geotechnical exploration and construction would have given little indication of the diversity and mag&ude of the geotechnical problems to be faced in the design and construction of the tunnel. Some indication of the geotechnical problems to come, however, was given by the previous occurrence of landslides along the existing canal and by the presence of the Salina formation. Throughout the Niagara peninsula the Salina formation is known for its content of soluble evaporite deposits. Further, in parts of upper New York state, the Camillus shale, a stratigraphic equivalent of the Salina formation, has caused foundation difficulties because of its solubility. Geotechnical considerations are seldom the sole criteria for site selection. Such was the case with the Townline road-rail tunnel where optimization of relocation of transportation systems was of primary importance. As a consequence, geographic restraints were imposed on the location of the tunnel. Since relocation of the tunnel to an area of more favorable geological conditions was not possible, there was no alternative to a thorough geotechnical evaluation of the site. The geotechnical investigations presented by the authors serve as an excellent example of a multifaceted approach to geotechnical engineering that required consideration of an area far greater than the site itself. In dealing with the problems of the stability of slopes the authors have recognized the significance of soil stratigraphy and structure to the strength characteristics of the unconsolidated sediments. Much of the geotechnical exploration and laboratory testing was directed therefore, toward identification of the relatively weak zones within the soil mass and to the determination of the degree of shear strength anisotropy. It is evident that a variety of sampling techniques, including 2, 3, and 5 in. (5.1, 7.6, and 12.7 cm) diameter thin-wall sampling tubes and hand-excavated block samples, were used to obtain samples with minimum disturbance that would permit laboratory strength tests to be made at various orientations. It is apparent from the data presented by Conlon and Isaacs (1970) that, in a general way, sample disturbance is inversely proportional to the size lor diameter) of the undisturbed sample obtained. However, a comment by the authors on optimum sampling techniques in consideration of cost and reliability of test results would be of interest as a guide to future geotechnical exploration in the Welland area. Considerable emphasis is placed by the authors on the lower stratified clav zone as J being of particular significance as the potential locus for the horizontal portion of a wedgetype failure. This emphasis is well justified, particularly for the deeper portions of the approach cuts where this zone will be closer to the bottom of the cuts. However, the upper stratified zone also possesses shear strengths not significantly different from those of the lower stratified zone and variations in shear strength occur in both zones. It is therefore assumed that the stability analyses and overall design of the approach cuts were such as to accommodate the low shear strengths of both the lower and upper stratified zones as well as the variations in shear strength within the zones. The author's comments on the magnitude of shear strength variations and the influence these variations had in the selection of design shear strengths would be of interest. The incorporation of sheet piling in the portal dikes as a second line of defence against cracking is a rather unusual design feature. The sheet piling itself, of course, cannot prevent differential settlement of the dike material and the possible consequent development of a ten-