2018 Young Geotechnical Professionals’ Night
Vera Li, Rebecca Szczurowski, Chamindi Jayasuriya and Douglas Warne
The Young Geotechnical Professionals Night is an opportunity to see four selected presentations and a selection of posters by young geotechnical professionals discussing interesting and challenging aspects of their work.
The following presentations will be given during the evening.
Controlled Modulus Columns (CMC) for NSW Transport Roads & Maritime Services (RMS) Bridge Works and Specification
Ground improvement packages in the nature of Controlled Modulus Columns (CMC), otherwise known as Rigid Inclusions or Concrete Injected Columns (CIC), is a commonly selected technique for government bridge abutment works nearing embankments. More recently, the constructability of CMCs has increasingly come into cross fire with the RMS R225 specification by which its construction is primarily governed by. The strict heaving tolerance outlined in the specification is often unachievable during production and is particularly apparent in challenging ground conditions and/ or in line with conservative designs. The aims of this paper were to describe CMCs as a displacement method for ground improvement and discuss the challenges faced in construction. Subsequent to this, the link between design and construction against the R225 specification was investigated. In this report, heaving above the 25mm tolerance was caused by the combination of installing CMCs through incompressible sub-strata and keying too deep into non-displaceable material. Design spacing and installation sequencing were also considered as contributing factors. The disregard for CMCs as a ground improvement technique often returns conservative designs that in turn, affects the capacity of its construction to adhere to specification. Ultimately creating an environment where we can lose out on the ability to deliver higher quality end products.
Using Parametric Modelling to Increase Efficiency in Design
In large multi-discipline projects, simplifaction and increased factors of safety in geotechnical design are often utilized for complex stratigraphy and geometry adding additional cost and resources to the design. Additionally, the geotechnical design needs to respond to rapid geometry changes from other disciplines. At all stages of the design, the geotechnical team needs to be able to provide clear easily trackable variations including sketches, hand calculations, and computational models.
Traditionally variations in design would be updated manually by an engineer. The work in updating sketches, calculating new geometry, updating calculation spreadsheets and computational models is laborious work that is very time-consuming. This type of ‘rework’ is prone to human error and can be hard to track as revisions develop quickly. The laborious work hinders design optimisation and typical dimensions are often adopted over large sections of the design with high factors of safety.
Generic parametric scripts to model 2D and 3D geometry can be built using Rhino 3D, a CAD application software, in conjunction with Grasshopper, a plugin that provides a visual interface for building algorithms. The scripts can be created to take typical input values such as dimensions, RL’s and eastings and northings and output 2D sketches and 3D geometry. Ground models, excavations and design details such as basements, tunnels, retention systems and foundations can be automatically produced as scaled and dimensioned 2D sketches and 3D models. The scripts can also output automatic length, area and volume calculations.
These scripts streamline the design process from concept to final design and greatly reduce the amount of manual editing as a project evolves. The scripts become particularly effective in multi-discipline projects where the design has to respond to geometry changes from other disciplines. This system of modeling offers significant time saving, easy optimization of design, simple methods of tracking and verifying the design and automatic area and volume calculations based on 2D and 3D stratigraphy.
These scripts have been used and developed for three geotechnical applications including automating numerical modeling, specifically building and importing 3D geometry to plaxis, and automating pile and anchor embedment calculations in complex stratigraphy. Using these tools engineers can optimise their design and manage uncertainty and risk enabling geotechnical designs to become more economically and environmentally efficient.
University of Wollongong
Use of Under Sleeper Pads (USP) to Improve Rail track Performance under cyclic loading
In recent years, with the increasing demand for passenger and good transportation faster and heavier rail traffic has been introduced by many countries. Thus, railway geometry and safety of ballasted rail tracks are adversely affected. Increased stresses in granular foundation leads to progressive degradation and this resulted in excessive vertical and lateral deformation, fouling, and poor drainage etc. in conventional ballasted railways. Year by year track maintenance cost is increasing rapidly due to efforts taking by railway authorities to maintain track geometry to ensure the safety and passenger comfortability. Ballast degradation is one of the major reason which attributes highest percentage of track geometry deterioration. Increasing axel loads and higher loading frequency generated due to higher train speeds accelerate the ballast degradation and rate of plastic strain accumulation. This situation is more critical at isolated locations like bridges, tunnels, railway crossings due to stiff subgrade. Finding an economical solution to mitigate ballast degradation was challenging task for the researchers and use of elastic pads seems to be an attractive and innovative solution.
There for a study was conducted in university of Wollongong, to improve track performances and reduce fast degradation of foundation granular layers using elastic elements in track substructure. In this research, performance of ballasted rail track on stiffer subgrade has been evaluated with the presence of rubber pad under the concrete sleeper (Under Sleeper Pad –USP). Series of Large Scale process simulated triaxial tests were carried out to investigate the behaviour of ballast with and without USP and to evaluate the effect of properties of USP. Laboratory experiments results have shown a significant improvement in ballast performance with the introduction of under sleeper pads and proven that the stress level has been reduced in the granular layer as rubber mats have absorbed more energy due to it damping characteristics. Reduction in ballast degradation causes decrease in vertical settlement and lateral ballast movements. This can be proposed as an environment friendly solution as increase of ballast life requires less aggregate quarrying and waste tires can be recycled to make under sleeper pads which provides a solution to waste tire accumulation all over the country.
FEM Modelling of Unsaturated Soil Behaviour for Design of Retaining Structures in Adelaide Clays
The Torrens Rail Junction (TRJ) Project is a grade separation of the rail lines by lowering the Outer Harbor rail line below both the interstate rail line and the adjacent Inner Ring Route (Park Terrace). The lowered Outer Harbor rail line extends for a length of approximately 1.5 kilometres supported by an approx. 8m deep cantilever pile wall.
The project is underlying with River Torrens alluvium. The River Torrens alluvial deposits transition to alluvial fan sediments of the Adelaide Lower Outwash Plain. These materials typically comprise of stiff to hard clay soils with local interbedded sandier horizons. These clays are typically unsaturated above the groundwater table and are highly reactive.
The technical solution proposed the use of an economical, practical and compliant design. To achieve this, the retaining wall design approach proposed differed from normal design practice to consider effects of suction and unsaturated soil mechanics principles.
Numerical methods were employed to analyse soil-structure interaction. The analysis considered staged construction, effects of volume change in clay soils due to unsaturated condition including variations from an initial equilibrium condition to a new equilibrium suction (wetting), effects of swell pressures and the beneficial effects of suction in the shear strength of clays when assessing lateral loads against retention structures and local/global instability.
As this design approach was novel, the proposed methodology was compared to a full scale pile wall trail, under taken by DPTI (SA) to verify the design approach. The results using the proposed design methodology indicate the numerical models were able to reliably predict both deflection and structural reaction at all construction stages.
Engineers Australia members participating in AGS technical sessions can record attendance on their personal CPD logs. Members should refer to Engineers Australia CPD policy for details on CPD types, requirements and auditing guidelines.