Safety of structures and humans is of crucial importance to engineers and engineering design. Where behaviour and properties of materials are not well understood, engineers will, of necessity, adopt a conservative approach to design to maintain this safety, sometimes at considerable financial cost to the public.

CIES researchers with industry support have recently completed a very successful ARC Linkage Project (LP140101078): Experimental investigation and constitutive modelling of weak rocks subject to mechanical and moisture degradation.

Weak rocks are geomaterials that fall in the boundary between soils and rocks in terms of mechanical properties. Such geomaterials constitute a major portion of the shallow rocks in Australia. Despite their prevalence, the characteristics of weak rocks have been  poorly understood mainly due to problematic sampling, lack of proper investigation measures - such as experimental equipment and methods, and the complex mechanical behaviour such materials show.

Indeed, the lack of understanding and predictive tools for the behaviour of weak rocks have over the years led engineers to adopt a conservative and often expensive approach to design.

The Project and Objective:

The aim of this ARC Linkage research project was to advance experimental, theoretical and computational bases for the mechanics of weak rocks, so as to provide scientists and engineers with much-needed predictive tools for quantitative evaluation and assessment of their behaviour in geological settings. 

The project used an extensive experimental program, combining rigorous fundamental studies  with robust and efficient numerical schemes. By incorporating previously neglected aspects in the behaviour of weak rocks such as mechanical, environmental as well as cyclic loading degradation, it aimed to increase practitioner confidence in the design methods, to the point where costly over designs can be avoided.

The project was led by CIES Professor Nasser Khalili with other investigators, Dr Arman Khoshghalb and John Rubsov, Director Technical Operations & Support at Roads and Maritime Services. Two CIES PhD students, Hamed Moghaddasi Kelishomi and Mohammad Khoshini, also closely collaborated with the industry partners of the project, TfNSW (formerly RMS), and Pells, Sullivan & Meynink (PSM) Consulting.

The research team conducted research on the development of procedures for cost-effective design of cut batters, soil nailing, retaining structures and bridge foundations in low strength rocks encountered in the Upper Blue Mountain region of NSW.

An extensive experimental program was devised and successfully conducted to completely characterise the material.

Rigorous fundamental studies were performed to understand the mechanisms behind the behaviour of the material and a complete set of constitutive laws for weak rocks describing the degradation effects of cyclic loading, moisture, and damage due to de-bonding of grains and the strongly non-linear deformation behaviour of the rock matrix were developed.  

Robust and efficient numerical schemes for the solution of the constitutive equations suitable for implementation into the numerical code FLAC were also developed.

Key Findings

Models, theories and relationships derived from this research will have direct and immediate impact on design, construction and management of geotechnical engineering projects including roads, tunnels, bridges, under- and over-ground excavations.

One of the immediate beneficiaries of the findings of this research is the current upgrade project of Great Western Highway in Upper Blue Mountains of NSW from Leura to Katoomba. The notorious weak rock encountered in the upper Blue Mountain region is no longer  an unpredictable difficult material to deal with for geotechnical engineers.