The effects of vegetation on the swelling and shrinking of soils in Australia

Abstract

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.

Drought-induced building damages from simulations at regional scale

“…These studies have shown variations in the sensitivity of building infrastructure to drought-induced soil subsidence. In particular, the fraction and size of vulnerable structures depends on the properties of the underground (Popescu et al, 1998), the presence of trees (Richards et al, 1983;Navarro et al, 2009) and the type of buildings (Crilly, 2001).…”

Comparative Study of Rainfall Infiltration into a Bare and a Grassed Unsaturated Expansive Soil Slope

“…The transpiration of plants requires an increased demand for water and hence it can result in more signiˆcant soil moisture depletion in the ground (Hillel, 1998;Holtz, 1983). The soil moisture depletion in turn leads to an increase in soil suctions in the soil near the root system as well as in the zone beyond the root system, i.e., the in‰uence zone, (Richard et al, 1983;Driscoll, 1983). The average height of grass (H) on the slope surface was about 0.5 m. According to the so-called`1H' rule derived from Ward (1953), the eŠec-tive root systems spread a depth equal to the height of the grass.…”

“…The average height of grass (H) on the slope surface was about 0.5 m. According to the so-called`1H' rule derived from Ward (1953), the eŠec-tive root systems spread a depth equal to the height of the grass. In addition, the in‰uence zone may extend 1.5 times the eŠective root zone (Richard et al, 1983). Thus, the transpiration impact of the grass in the grassed area may aŠect the soil suctions in the soil layer as deep as 1.2 m or more.…”

A comparison of sap flux and water relations of leaves of various isolated trees with special reference to foundation movement in clay soil

“…This problem may be caused by water extraction by trees located in the vicinity of buildings (Bozozuk and Burn, 1960;Biddle, 1983;Driscoll, 1983;Holtz, 1983;Longworth et al, 1984;Ravina, 1983;Richards et al, 1983;Williams and Pidgeon, 1983). There are situations where decisions have to be made to remove existing trees from the vicinity of damaged buildings if the trees are considered to be the principal cause of building damage.…”

“…Richards et al, 1983) where a certain amount of competition from nearby lawn or shrubs is expected. Transpiration of isolated trees can be inferred from the estimates of sap flux in stems either by the heat pulse velocity method (Green and Clothier, 1988;Lassoie et al, 1977;Swanson et al, 1979) or by the heat balance method (Cermak et al, 1976).…”