Chapter 5 The influence of soil saturation on the stability of abandoned agricultural hillslope terraces under Mediterranean climatic conditions

The strength of two residual soils, terra rossa and rendzina, which constitute most of the abandoned agricultural hillslope terraces near Yirka village, Western Galilee, Israel, was determined by two methods: shear by a basal direct field shear apparatus and probing by a dynamic penetrometer. The first apparatus was used to determine the cohesion and the angle of internal friction of the top soil and the second to inspect the change of the undrained shear strength of the above soils with depth. The strength parameters with midsummer water content (16.91% for terra rossa and 11.07% for rendzina) were found to be: terra rossa: c≈10.57 kN/m², =41.24°; rendzina: c≈10.72 kN/m², =42.34°. The strength parameters of the same soils, with midwinter water content were: terra rossa: c′≈2.18 kN/m², ′=40.75°; rendzina: c′≈5.24 kN/m², ′=39.03°. Stability calculations confirmed by field investigations showed that the drop of cohesion of both soils in consequence of their saturation during the rainy season can cause terrace landsliding unless the terraces are retained by sufficiently strong enough retaining walls.