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ummary of Morwell Land Movement Survey and Report undertaken by Pells Sullivan Meynink (PSM) in August - September 2011

August 2012

Disclaimer: This document is an edited summary only of a technical report intended as a baseline study for future reference. The report does not take into account any historical data related to the study area. The information provided in this summary may be of assistance to you but it is provided on the basis that all persons accessing it undertake responsibility for assessing the relevance and accuracy of its content. The State of Victoria and its employees do not guarantee that the work is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this work.

Contents

  • 1. Introduction
  • 2. Aims and objectives
  • 3. Scope of work
  • 4. Observations and extent of cracking
    • 5. Discussion
    • 5.1 General
    • 5.2 Ground Related Movements
    • 5.3 Inferred Movement Direction
      • 5.3.1 Survey Prisms
      • 5.3.2 Site Observations
    • 5.4 Other Factors
  • 6. Conclusion and recommendation 

1. Introduction

There is a considerable body of documentation extending back over 40 years which records the history of movements and associated issues in the Morwell area.

This report presents a summary of the findings for the Morwell Land Movement Survey, conducted between 15 and 18 August 2011 in Morwell Township, Victoria.

The work was undertaken as requested by the former Department of Primary Industries (DPI) and in accordance with Contract 313377 (2 August 2011).

2. Aims and objectives

The monitoring and observation of the area since the recent movements in February 2011 has shown that some movements have also been detected in the Morwell Township.  Hence it was decided to undertake a reference survey within Morwell to allow effective ongoing management of potential future hazards, and to provide a reference for checking any significant changes.

The aims of the Morwell Land Movement Survey were to:

  • Provide a reference survey for future observation and monitoring; and
  • Provide a record of the existing ground movements within an area in Morwell ("project area"); and
  • Provide an overview of the movement types and magnitude that has impacted on the project area.

The extent of the project area, including the private residences inspected, is shown in Figure 1.

3. Scope of work

A geotechnical engineer undertook the survey from 15 to 18 August 2011.  The survey included both private and public areas along the western 200m of Wallace Street and southern 200m of Hazelwood Road, Morwell.  The scope of work has comprised:

  • Survey, inspection and investigation of the project area shown in Figure 1;
  • The project area included the bowling club and its grounds (bowling greens, car park), and the fenced tennis courts and adjacent grassed area;
  • Recording of the details of any openings and/or cracks for all exposed ground surfaces (driveways, concrete floor surfaces, pavements and fences);
  • Recording of any such features in written and photographic form;
  • Documenting and measurement, where access allowed, of all observable ground openings that were larger than 0.5mm;
  • Noting of any substantial cracks or joints on the exterior of buildings;
  • Provision of a plan showing all identified ground openings, cracks and joints; this has been overlaid onto the existing DPI plan; and
  • Provision of a report, summarising the above, commenting on what factors may have contributed to ground openings, cracks and damage and any sense of vertical or horizontal land movement.

4. Observations and extent of cracking

The most significant cracks and localised depressions were located to the southwest of the project area, within the fenced tennis courts.  Numerous linear continuous cracks were present along the southern fence, and some localised depressions in the ground were observed close to the western fence.  Horizontal separations up to 20mm and vertical separations up to 10mm were observed on the tennis courts.  Localised depressions up to 650mm in diameter and 150mm in depth were observed, several shallower localised depressions on the tennis courts were also observed as pooling water.

Outside the fenced tennis courts, there were also possibly more localised depressions that were not readily observable due to the extent and length of grass cover.

The bowling club building to the north of the tennis courts had minimal cracks or openings.

The asphaltic surfaces of both Hazelwood Road and Wallace Street within the project area showed minimal signs of movement.

Around the intersection of Hazelwood Road and Wallace Street, observable signs of land movement included a leaning electricity pole, a leaning "Bowling Club" billboard and misaligned kerbs. 

In the western 100m portion of Wallace Street, numerous "fresh" and "clean" cracks or openings were observed.  Significant clean separations were also observed at construction joints between concrete slabs and in the brickwork in this area.  The movements were in both vertical and horizontal directions. Similar cracks potentially related to normal wear and tear and/or poor workmanship were also present and at times it is difficult to determine whether the cracks were related to these different causes.

Possible signs of poor local drainage in the area were observed in the form of ponded water at a stormwater drain access point adjacent to the townhouses and localised water ponding at/near newly installed stormwater drains directly adjacent to old ones.  It is noted that poor local drainage may contribute to wear and tear or cracking of infrastructure.

Elsewhere on Wallace Street (eastern end) within the project area, the majority of cracks or openings were observed on the concrete pavements, footpath or driveways.  These cracks or openings were in most parts filled with soil, grass or moss and are interpreted as older features.

5. Discussion

5.1 General

Some of the likely processes that might contribute to ground surface movements within the project area include:

  1. Consolidation of subsurface materials due to long term regional dewatering of the deep aquifers.
  2. Stress relief effects from historical mine excavation.
  3. Small magnitude ‘episodic’ block movements of the ground, which have probably occurred throughout the past 50 years.
  4. Regional sinkhole development due to surface soils washing into cracks in the underlying coal.
  5. Shrink and swelling of the reactive soils which predominate in the area. 

At times it can be difficult to ascertain exactly whether an observed crack or opening is mine related or due to some other external factor or a combination of both.  This situation is also complicated because it is not known exactly when the cracks or openings occurred.

5.2 Ground Related Movements

A number of cracks or openings on the kerbs of Hazelwood Drive appear to be related to the land movement event that occurred in February 2011.

Along the western end of Wallace Street, a number of "clean" or "fresh" cracks or openings of the kerbing in the south side were observed; these run parallel to the Princes Freeway.  On the north side there were a number of pre-existing cracks or openings which exhibited signs of recent separation, i.e., partially filled with soil. The localised depressions and their associated cracking in the tennis courts, as well as the longitudinal cracks on the southern boundary of the project area were observed to have opened up further since February 2011.

Approximately 40% of the recorded cracks were observed to be "clean" or "fresh" cracks and recently opened cracks. 

These “clean” and “partially clean” cracks discussed above, were assessed to have opened up relatively recently.

5.3 Inferred Movement Direction

5.3.1 Survey Prisms

Survey prisms south of the Princes Freeway moved to the south prior to and after February 2011.  However within the project area, the prism vectors showed a northward movement direction initially, but changed to a southerly direction post February 2011. Time will allow a better quantification of the extent of any further movements.

5.3.2 Site Observations

Some evidence of the sense of movement is given by leaning power poles and signage, displaced kerbs (up to 5mm) on the road, and horizontal exaggerations (up to 50mm) of construction joints between slabs at the town houses. 

The movement direction is observed to be a North-South separation at the southwest corner of the project area.  The sense of this movement becomes less pronounced east of the town houses at 2 Wallace Street and north of the tennis courts, in these areas, the cracks were observed to be filled with soil/grass/moss and were typically less in magnitude (length, vertical and horizontal exaggeration).

This area of recent movements since February 2011 shows a change in the direction of movement from towards the north to southwards, towards the mine.

5.4 Other Factors

Cracks in concrete slabs in driveways or footpaths that were filled with soil, grass or moss were assessed to be pre-existing prior to February 2011. There is no evidence of these having recently opened up further.

Other external factors, most notably poor local drainage, reactive soils and inadequate design, construction or maintenance of buildings and infrastructure could contribute to the observed cracks and openings in the project area.

6. Conclusion and recommendation

There are a number of factors which may have contributed to the observed land movement in the project area. An ongoing monitoring and visual inspection program is recommended for this area.