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Previously published studies of the mechanical impedance of granular media to root growth have shown that the rate of elongation of barley roots was halved by a lateral cell pressure of 0.02 MPa, applied externally to the growing media. It was incorrectly assumed that this lateral pressure was always the pressure on the growing root. In this paper, the stress distribution around a growing root was modelled both theoretically and experimentally by a thin cylindrical rubber tube, which was expanded radially in two granular materials in a modified triaxial cell. The theoretical model which simulated the deformation of the granular material around the expanding rubber root analogue was controlled by the elastic stiffness parameters, bulk and shear modulus in the early stages and later, at larger diameters, by the plastic yield parameters, cohesion and friction angle. This theoretical stress-strain model was validated experimentally by the good agreement with the results obtained using the thin rubber tube in the two granular materials. Using the theoretical model with root diameters similar to those for barley roots, it predicted soil pressures on the 'root' surface of 0.2-0.3 MPa for lateral triaxial pressures of the order of 0.02 MPa. This result was similar to the predictions made using earlier analytical models. The model also predicts lower mechanical impedance for finer roots, and strongly suggests that cylindrical expansion of the root behind the tip is effective in relieving soil pressure ahead of the elongating root.

Soil movement is of great importance in Australia because about 20% of the total area consists of expansive soils, most of which occur in the semi-arid climatic zone. The first measurements of soil movement were made in Adelaide under natural grassland. From this limited information, it was deduced that there was little soil movement below 1·6m. Recently, movements have been measured in clay layers at much greater depths. These movements have been attributed to shrinkage due to desiccation by deep rooted vegetation, and swelling due to ponding of water. Several case histories are discussed where the effects of vegetation have caused movements of buildings of up to 150mm settlement and 100mm heave. To calculate potential soil movement caused by vegetation, the extremes of total soil suction and a quantitative value of the volume change properties of the soil are required. The instability index, Ip has been suggested as a measure of this soil property and was measured on two of the most expansive and widespread soil types in Adelaide. Values for the black earths were 0·06 and for the underlying dispersive clay, 0·10, compared with 0·02 for the red-brown earths down to 5 m. Changes in total soil suction were measured in these two soils both near and away from several species of trees. Near eucalyptus trees, the total suction of red-brown earths attained a maximum value of 2MPa to depths exceeding 5m. Under natural grassland, the profile was uniformly wetted to a suction of 0·3 MPa. In the saline dispersing clay underlying the black earths, the corresponding suctions were of the order of 3 MPa and 1 MPa respectively. However, in this latter case, the effect of Ip and the depth of the clay, causes much greater movements in spite of the lower suction changes. At present, soil movements due to the effects of vegetation and its associated watering regime, calculated from measurements of Ip are only a guide. Past experience of the movements of buildings on similar soil profiles in similar circumstances is still essential. Le mouvement du sol est très important en Australie, parce que 20% environ du continent entier comporte des sols expansibles, dont la plupart se trouvent dans la zone climatique demi-aride. Les premières mesures du mouvement des sols furent prises à Adelaide sous des herbages naturels. Sur la base de ces données limitées on a déduit qu'il n'y avait que peu de mouvement du sol à une profondeur supérieure à 1·6m. Récemment des mouvements ont été mesurés dans des couches d'argile à des profondeurs beaucoup plus grandes. Ces mouvements ont été attribués au retrait causé par la dessiccation due à la végétation profondément enracinée, tandis qu'une autre cause possible est le gonflement produit par la formation de mares. L'article discute plusieurs exemples où les effets de la végétation ont causé des tassements jusqu'à 150mm et des soulèvements jusqu'à 100mm de bâtiments. Pour calculer le mouvement potentiel du sol produit par la végétation, il faut connaître les valeurs extrêmes de la succion totale du sol et une valeur quantitative de ses propriétés de changement de volume. L'indice d'instabilité Ip a été proposé comme mesure de cette propriété des sols, et cette valeur a été déterminée sur deux des sols les plus expansibles et les plus usuels à Adelaide. Les valeurs pour les terres noires ont été de 0·06, la valeur pour l'argile sousjacente dispersive étant de 0·10, à comparer avec 0·02 dans le cas des terres brunes-rougeâtres jusqu'à une profondeur de 5m. Des changements dans la succion totale du sol ont été mesurés dans ces deux sols dans le voisinage d'arbres de plusieurs essences, ainsi qu'à quelque distance. Dans le voisinage des eucalyptus on a trouvé une valeur maximale de 4 MPa pour la succion totale des terres brunes-rougeâtres, jusqu'à des profondeurs dépassant 7 m. Sous des herbages naturels le profil a été humidié de façon uniforme à une succion de 0·3 MPa. Dans l'argile dispersive saline sous les terres noires les succions correspondantes étaient repectivement de l'ordre de 1 MPa et 3 MPa. Cependant, dans le dernier cas l'effet de Ip et la profondeur de l'argile causent des mouvements beaucoup plus marqués, malgré les changements réduits dans la suction. A present les mouvements du sol dus aux effets de la végétation et au régime d'eau associé calculé à partir des mesures du Ip ne donnent qu'une indication générale. On a encore essentiellement besoin de données concernant les mouvements des bâtiments dans le passé sur des prols de sol similaire dans des circonstances analogues.

Since the commencement of mining at the strip coal mine at Goonyella, Queensland, Australia, serious stability problems have been experienced in both the spoil piles and the highwalls. A collaborative investigation was commenced in 1974 and the significant factors influencing stability have now been defined. This paper outlines the results of field and laboratory studies associated with the investigation and describes the various numerical methods used to analyse stability. The main conclusions, which are similar for both the spoil piles and the highwalls, are that the failures generally occur along two or three planar surfaces; that the shear strength in the basal areas is controlled by the sensitivity of the materials to moisture and shear strain prior to failure; and that tensile cracking due to stress relief and blasting is the dominant factor in the upper or escarpment areas. Possible control and remedial measures are discussed briefly.

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