Identification of bedding shears during site investigations is problematical. The paper describes the use of the observational approach and discusses its implications for design and construction.
L'identification des bandes de cisaillement dans des plans de stratification presente des difficultes au cours des etudes d'un terrain de construction. Cet article decrit I'usage des methodes base sur l'Observation et les implications pour les constructions.
ldentifizierung der Schichtungsseherebene wahrend der Bodenuntersuchung ist problematisch. Der Bericht beschreibt die Verwendung der Beobachtungsmethode und diskutiert deren Implikationen fuer die Bemessung und den Bauprozess.
The identification of rock failure mechanisms, and ipso facto, material parameters, is of prime importance in the investigation of rock cuttings since these are the most fundamental controls on overall design and provision of revetment. However, rock outcrops for direct assessment of rock conditions are usually limited. Even in areas of comparatively good exposure, experience has shown that often the most critical geological factors have been obliterated by weathering. Reliance must therefore be placed in the first instance on indirect techniques and geological prediction, based on an interpretation of the stratigraphy, structural history and hydrogeology. These can be supplemented by observations and controls within the construction process, but only if the appropriate contractual and financial provisions are in place. This paper discusses the problems and successes in the identification of potential failure mechanisms and subsequent design input for three case histories of deep motorway cuttings in diverse strata:-
700m long × 20m deep cutting in Thrace, Turkey, through very weak mudstones.
1000m long × 80m deep cutting in Anatolia, Turkey, through weakly indurated outwash fan deposits, consisting of sandy silts with gravel bands and thin layers of stiff clay,
600m long × 35m deep cutting in North Wales, UK, through strong banded mudstones and siltstones.
These three case histories illustrate very different geological environments but show many similarities. All were affected by tectonic activity, resulting in gently folded sequences and consistent dips over long distances. In all cases, an important control on stability was the presence of residual clay-coated shear surfaces, parallel to bedding planes. These developed by flexural slip as a result of folding. The extent to which theoretical considerations can be used effectively to predict whether a sequence is at risk from this mechanism is discussed.
Several sections of the Thrace Motorway in Turkey cross outcrops of the Sulemaniye Formation, Miocene strata often described as "Istanbul Green Clay", between 11 km and 47 km west of the centre of Istanbul. The stability problems encountered on this section of motorway are discussed in detail in Gordon, Lord and Statham (1991). In the mid 1980's, prior to motorway construction, this was an area of rolling farmland with few geological exposures. Hence little was known of the stratigraphic or structural details, although it was well known that the sequence contained heavily overconsolidated. The 25 motorway cuttings in these areas, had originally been schemed by others with slopes of 1 of 3 and no drainage measures. A maximum cutting depth of 20m occurred where the motorway crossed sidelong ground for a distance of some 700m. The geological complexity of this area had caused extensive landslides which, prior to motorway construction, were dormant and to some extent masked by a long history of intensive agriculture. Boreholes had been sunk at intervals along the route, mainly at the sites of structures, but these did not reveal the geological complexity and at feasibility stage, the motorway designers had not recognised the large areas of residual landslide. The scheme proceeded to construction without any further geotechnical investigation. Re-activation of landslides occurred during initial excavation of several of the cuttings, well before formation levels were reached. In the worst case, progressive displacement extended up to 80m beyond the crest of the original cutting. These difficulties required further investigations, re-appraisal of the design, significant delays and expensive remedial works.
