The North-McKinnon-Centre (NMC) package involved replacing approximately 2.5 km of at grade track with three rail cuttings up to 8 m deep at each of the bridges which replaced the level crossings.  The cuttings are retained by piled retaining walls and during the main construction occupation approximately 250,000 cubic metres of soil was removed.  

Douglas Partners were the geotechnical consultant to the project alliance through a subcontract with KBR.  The alliance included representatives of the State Government together with John Holland and KBR.

Design and construction of the NMC package presented significant challenges due to its location in residential suburbs within a tight rail corridor and a short rail occupation timeframe.  As a result, careful design and innovative construction techniques were necessary to limit impacts on the community.  Douglas Partners addressed a range of geotechnical challenges which generally fell into one of three areas, earthworks, structures and groundwater. 

The structures comprised three bridges, stations, base slabs and around 4 km of piled retaining wall.  The piles included driven steel sheet piles and Continuous Flight Auger (CFA) piles.  Geotechnical challenges that Douglas Partners addressed included assessing the temporary and permanent stability of the walls, particularly as the lead time for the sheet-piles required locking in the design at a very early stage.  Pile driving involved the use of some innovative techniques to minimize vibrations.  

Douglas Partners were also deeply involved with assessing the structural base slab with a screw pile system being adopted for two of the three cuttings that extend below the groundwater table, to resist the long term hydrostatic uplift pressure and cyclic loading from trains.  Construction dewatering was also a major issue with two of the three cuttings extending below the water in a silty sand aquifer with the possibility of significantly affecting the wall stability. 

In response to the abovementioned ‘mining boom’ and in a reduced demand recently, Douglas Partners has been providing geotechnical advice for new mining projects and several proposed expansions of existing mine sites. A general description of Douglas Partners’ assistance on such projects is provided below.

Douglas Partners geotechnical specialists have been engaged on multiple occasions to undertake geotechnical investigations and provide construction supervision for new and expansions of mine sites and associated infrastructure. The geotechnical investigations typically included:

• Drilling of diamond core and shallow boreholes;
• Performance of cone penetration tests (CPTs);
• Excavation of test pits together with penetrometer testing such as Perth sand or dynamic cone penetrometers;
• Geophysical surveys; and
• Geological mapping.
  
  

Data from the abovementioned investigation technics were used to derive ground models for the study sites and to provide geotechnical advice regarding:

• Suitable foundation systems and site preparation (including ground improvement techniques to limit settlements to acceptable levels) for:
  
  
  • rail bridges (16 bridges in total) and communication towers along existing and proposed new railway lines;
  • railway crossing culverts;
  • multiple conveyor belts, crushers and other mining facilities; and
  • storage tanks up to 17 m in diameter.
    
    
• Ultimate and allowable bearing pressures and modulus of subgrade reaction, for foundation design of building and retaining structures;
• Bearing capacity, settlement and horizontal ground displacement analyses (using finite element modelling software), for train loading facilities, stackers, reclaimers and railway crossings;
• Raise bore drilling and curtain grouting design for the construction of a 750 m deep ventilation shaft;
• Pavement designs (including heavy duty and unsealed pavements and haul roads) and assessment of suitability of existing borrow materials for re-use in pavement construction; and
• Mapping and assessment of problematic soils such as collapsing and gilgai soils and their impact on proposed facilities such as a flash butt welding yard, several mining accommodation villages, a new LNG plant and pipeline.
  
  

Several technically suitable types of foundation systems (including shallow footings, ridged rafts, piled rafts or piled foundation systems) and site preparation techniques (including ground improvement techniques such as impact rolling) were suggested to our clients, to achieve their financial and constructability objectives. Earthworks supervision and piling supervision were provided.

Douglas Partners’ office includes several engineers with suitable expertise and successful experience in working within mining environment to assist our clients in developing practical, efficient and cost-effective recommendations for site preparation, foundations and excavation support associated with mining developments.

Douglas Partners’ capability also offer advanced FEM numerical analyses from a dedicated numerical modelling hub with a wealth of experience, directed by our Principal Analyst Dr. Richard Merifield.