Shear strength of dry tills from the southern half of Norway in relation to bedrock geology

Till is the dominant Quaternary sediment in Norway, where the till deposits mostly originate from local rocks with a transport distance less than five kilometres. Debris slides and flows are among the main geohazards on till-covered valley slopes, and in this context the shear strength of Norwegian tills is poorly studied. Therefore, a set of 33 near-surface, genetically independent till samples were collected from various locations in the southern half of Norway to investigate the shear strength and its relation to bedrock geology. The disturbed samples were sieved (<16 mm) and dried before they were tested in a large-scale direct shear box apparatus at normal stresses of 100, 200 and 300 kPa, and with a shear rate of 2.0 mm/min for approximately 40.0 mm horizontal displacement. Overall, the result regarding the average ‘initial dry testing density’ was 1.67 g/cm3, while the maximum shear stresses for the given normal stresses on average were 84.9 kPa, 163.2 kPa and 243.7 kPa, respectively. The average angle of friction and cohesion were 38.4° and 5.3 kPa. Furthermore, the samples were categorised into six regional rock provinces. The mapped rock type assumed to represent the origin of the till samples were based on their area of extent around the sample sites, and adjusted for by the late-Weichselian ice-flow directions. On average, the ‘Precambrian basement’ province locally affected by the Caledonian orogeny (mainly gneisses) had the highest angle of friction. In comparison, the ‘Caledonian’ province of overthrust sheets of sandstone and schist (mainly sandstones) had the lowest angle of friction. Although not conclusive, this study provides indications of a relationship between till shear strength and bedrock geology, suggesting that some provinces may, solely on the basis of their associated geological parameters, be more prone to debris slides and flows than others.

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