Research is being carried out at Technion in Israel to study the in¯uence of plant roots on the stability of slopes. The present paper describes the part of the investigation concerned with the determination of the additional shear strength contributed to soil by roots. Speci®cally, results of the following studies are presented: tension tests on roots, pull-out tests of roots from the soil, and direct shear tests on soil and root-reinforced soil. The quantitative results obtained in these investigations provide data which may be used in calculations of slope stability, although this should be done with caution, as pointed out in the paper.

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SUMMARYIn this paper, time-harmonic axisymmetric vibration of an incompressible viscoelastic half-space having shear modulus linearly increasing with depth is studied. The half-space is subjected to a vertical time-harmonic load on its surface. Numerical results concerning surface displacements due to a point force are given for the case of non-zero shear modulus at the surface. Hankel's transforms of the solutions have an infinite number of poles lying on the real axis of the complex plane in the non-dissipative case. A suitable contour of integration is used to avoid all the singularities. Calculations are performed for the dynamic as well as for the static case. In addition, vertical vibrations of a rigid disk on the considered half-space are investigated, and the influence of the non-homogeneity on the dynamic stiffness of the loaded area is demonstrated.