Nazri Ali

Seasonal water uptake near trees: a numerical and experimental study

Issues related to the numerical simulation of moisture migration patterns in the unsaturated zone and in the vicinity of mature trees are explored in this paper. The research is based on the use of Richards equation for unsaturated moisture flow incorporating a sink term. A numerical solution has been achieved via the finite-element method for spatial discretization along with a finite-difference time-marching scheme. An axisymmetric solution is developed to represent water uptake near an established tree. The approach adopted utilizes radial symmetry and assumes a linear distribution of water extraction rates with both depth and radius. The model has been validated by direct comparison with field measurements recorded (by others) for a mature lime tree located on a boulder clay subsoil. Non-linear hydraulic properties have been obtained from independent published data. A good correlation between field data and simulated results has been achieved. The simulation covers a full annual cycle starting from field capacity in winter, extending through a full spring–summer drying period and subsequent autumn recharge. It is believed that this is the first attempt to simulate the behaviour of an established tree over such a time-scale. This relatively straightforward approach is thought to be suitable for development and application to a range of geo-engineering problems (e.g. slope stability, shrinkage/heave prediction, etc).

The effects of method of generating circular slip surfaces on determining the critical slip surface by particle swarm optimization

The main objectives of slope stability analysis are evaluating factor of safety for a given slip surface and determining the critical slip surface for a given slope. Factor of safety is usually calculated by limit equilibrium method. The main steps to determine the critical slip surface are generating trial slip surfaces as probable solutions and searching among them to determine the one with the lowest factor of safety. Although the process of searching the critical slip surface received much attention between researchers, the significance of method of generating slip surfaces is seldom addressed in the literature. The authors believe that this ignorance can affect the accuracy of the results of slope stability analysis even in the simplest problems with circular slip surfaces. Consequently, this paper focused on the method of generating circular trial slip surfaces as the simplest mechanism of sliding and considered its effect on determining the critical slip surface. A new method of generating circular slip surface was presented, which is more efficient and less restricted than the conventional method. A computer program was also developed to determine the critical slip surface of slopes by using particle swarm optimization. The performances of the proposed method and developed computer program were verified during comparative studies and sensitivity analysis. Based on the results, the effect of method of generating circular slip surfaces on determining the critical slip surface was confirmed successfully. In all considered problems, the proposed method of generating circular slip surfaces led to the lower values of factor of safety compare with the conventional method.

Tree induced soil suction and slope stability

This paper provides an investigation of moisture migration patterns in the vicinity of mature trees and soil slope stability. A numerical model is presented for unsaturated moisture flow incorporating a sink term to represent water uptake by a tree. The resulting variation in soil moisture content (or suction) is then included within an extended slope stability analysis that employs a method of estimating shear strength as a function of suction in addition to net mean stress. The moisture transfer equation is solved via application of a finite element method. The resulting water uptake predictions have been validated previously and were shown to be in reasonable agreement with field measured data. The current work provides an assessment of the importance of this aspect of the problem on slope stability. Results show that seasonal soil suctions generated by a tree can influence the facture of safety against soil slope failure in the order of 8%. The research provides a further contribution to the overall assessment of slope stability. It may be included in routine calculations where maintenance of rail/road embankments may involve alteration (or removal) of vegetation. It may also be applied to vegetated natural slopes subject to deforestation or development.

The contribution of particle swarm optimization to three-dimensional slope stability analysis.

Over the last few years, particle swarm optimization (PSO) has been extensively applied in various geotechnical engineering including slope stability analysis. However, this contribution was limited to two-dimensional (2D) slope stability analysis. This paper applied PSO in three-dimensional (3D) slope stability problem to determine the critical slip surface (CSS) of soil slopes. A detailed description of adopted PSO was presented to provide a good basis for more contribution of this technique to the field of 3D slope stability problems. A general rotating ellipsoid shape was introduced as the specific particle for 3D slope stability analysis. A detailed sensitivity analysis was designed and performed to find the optimum values of parameters of PSO. Example problems were used to evaluate the applicability of PSO in determining the CSS of 3D slopes. The first example presented a comparison between the results of PSO and PLAXI-3D finite element software and the second example compared the ability of PSO to determine the CSS of 3D slopes with other optimization methods from the literature. The results demonstrated the efficiency and effectiveness of PSO in determining the CSS of 3D soil slopes.

Tree Induce Suction for Slope Sustainability

Many slope areas in Malaysia whether it is man made or natural slope are particularly vulnerable to soil erosion and shallow failures due to the increasing of soil moisture and porewater pressure. This study will determined the active root zone of the tree near the toe of the slope which suctions have been generated within this section. Hence, the effect it only focusing on hydrological aspect with soil moisture pattern near vicinity of the tree. The mechanical aspect of tree root such as tensile strength and bonding between root and soil which can led to increasing soil strength are assumed negligible in this study. From preliminary result of field monitoring show significant increase of total suction at near vicinity of tree which can lead to stability analysis on slope. The comparison of FOS will be presented between at the toe of slope with and without the tree. Furthermore, for slope studies area the reresult indicates that tree induced suction can be related to cause the factor of safety against slope failure improve up to 33.07%.

Tree Water Uptake And Suction Distribution On Tropical Residual Soil Slope

This paper present an exploration of soil matric suction effected by water uptake via tree root at toe of slope on various condition between wet condition (high rainfall) and dry condition (prolonged no rainfall). Matric suction generated by active root tree has substantial influence soil moisture content on residual soil slope. A field monitoring was carried out to collect matric suction data at slope in two conditions; with a tree located at toe of slope and absent of a tree. The installations of instruments particularly at slope with tree at toe were placed within vicinity of the tree with certain depths and distances. The matric suction data from field monitoring was influence by the rainfall events that lead to the instability of soils slope. Analysis of soil matric suction distribution pattern indicates that the highest matric suction value was at shallower depth and proximity of tree. The matric suction profiles obtained from field monitoring are applied as an input data to develop soil matric suction contour. The effect of transpiration driven by active root zone generated matric suction on soil at vicinity of tree may create dry soil to increase soil shear strength.

Utjecaj temperature na stišljivost i potencijal kolapsibilnosti rezidualnog granitnog tla

U ovom se radu istražuje utjecaj temperaturnih promjena na indeks stišljivosti i potencijal kolapsibilnosti rezidualnog granitnog tla na koje se često nailazi u Maleziji. Zbijeno tlo analizirano je pomoću modificiranog edometra s kontrolom temperature. Dvije serije uzoraka tla s raznim vrijednostima suhe gustoće ispitane su pri temperaturama od 27, 40 i 60°C. Rezultati ispitivanja pokazuju da vrijednost indeksa stišljivosti ne ovisi o temperaturi. S druge strane, grijanje dovodi do smanjenja potencijala kolapsibilnosti, a ta je pojava izraženija pri niskim vrijednostima suhe gustoće.

Lateral stress induced due root-water-uptake in unsaturated soils

A two-dimensional (2-D) lateral stress was modeled as a result of matric suction change caused by vegetative induced moisture transfer. The negative pore-water pressures are estimated through governing partial differential equations for unsaturated soils. The results of the of simulated root water uptake are used as an input for the prediction of 2-D lateral stress in a stressdeformation analysis in an uncoupled manner. The soil is allow to expand and contract free laterally, as the as water is being abstracted from the soil. A mature Lime tree located on a Boulder clay subsoil for period covering a full spring/summer drying period was used as a case study. The result shows interdependence of lateral and vertical stress generated resulting from root water-uptake.

Vegetative induced ground displacements: A comparison of numerical and experimental study

A volume change was modeled as a result of matric suction change caused by vegetative induced moisture transfer. Theory of unsaturated soil mechanics and two stress state variables principles approach was used in the stress-deformation model. The negative pore-water pressures are estimated through governing partial differential equations for unsaturated soils. The results of the root wateruptake analysis are then used as an input for the prediction of ground displacements in a stressdeformation analysis in an uncoupled manner. A mature Leyland Cypress tree located on Gault clay was analyzed, covering a full spring/summer drying period for the first case study. The second and third cases were analysis on Mature Lime tree located on a Boulder clay sub-soil for period covering a full spring/summer drying period; while the third case considered field capacity in winter and extends through a full spring/summer drying period and subsequent autumn recharge. Time varying boundary conditions have also been considered in the third case and the sink term was activated to represent water uptake by transpiration, during spring/summer soil-drying phase and deactivated during the autumn/winter recharge phase. The stress-deformation model has been validated by direct comparison to field measurements recorded for three cases. A good overall correlation between field data and simulated results has been achieved where the difference between the two set of results was less than 5% for all cases considered.