Case Studies of Filling Movements under Buildings

2.5 Settlement Monitoring

On completion of earthworks, nine settlement plates were set into the upper 1 m depth of the filling surface at locations overlying intermediate and deep filling depths, as well as locations overlying buried inclined quarry side walls. Settlements were monitored by a registered surveyor using precise levelling techniques. Survey frequency was initially fortnightly, then monthly. Monitoring was continued for between 15 and 22 months.

Settlement data was analysed to determine monthly and ongoing settlements and strains, and was plotted against time. The results were used to calculate an individual creep coefficient Cά for each monitoring point, which was then used to estimate an overall Cά for the site (typically 0.2% to 0.3%).

The results of the analyses and settlement plots were then used to forecast long term total and differential settlements over a range of design life terms. A plot of monthly strain as a percentage of filling depth versus time for all settlement monitoring points within the higher, western half of the site where filling depths were greatest is presented in Figure 1. The graph shows the line of best fit representing diminishing monthly strains as measured in terms of their percentage of filling depth.

Figure 1: Percentage Strain Versus Time for Higher Western Half of Site (Area of Deepest Filling)



Considering the number of source sites that were required to backfill the quarry, the nature of the filling obviously varied. Together with the variations in filling depth, variations in filling performance across the backfilled quarry might have been expected. However, as the results of settlement monitoring clearly showed, the performance of the filling was reasonably consistent across all monitoring points across the site.

It was concluded that filling performance can be managed by implementing strict controls within earthworks specifications. In this case, controls were introduced for material types, compaction levels were linked to material types and moisture contents during compaction were tightly controlled. In addition, the specification and subsequent inspection and testing programme required all aspects of the various controls to be regularly checked by way of compaction and materials compliance tests. The site is being developed and settlement monitoring has ceased. The result is a filling volume that continues to perform well and within expected limits.

3. Case Study 2 – Sports Club in Western Sydney


3.1 Background Information

Case study 2 involves the construction of a sports club on a filled bench. As there were engineering problems with the site, the locations and names of the parties involved have been withheld.

One of the authors of the paper was asked to investigate the sports club building as it was showing signs of structural and movement distress. The building had large, floor to ceiling windows which were located between the tilt-up frame of the building and on the ground bearing slabs. The large windows allowed patrons of the club to enjoy the view of the sport fields and garden beds. Movements of 20 mm to 30 mm were observed across joints between the external tilt-up panels and internal walls.

A geotechnical investigation was carried out by others on the site in 1996 and a site classification of Class M provided in the report. Starting in June 1997, up to 2 m thickness of filling from the adjacent car parking areas was placed on the gently sloping site to provide a level building platform. The

clubhouse was constructed in 1998 and comprised tilt up reinforced concrete external walls founded on piers with a ground bearing slab. The tilt up walls had spaces for the windows to be placed between the panels and the ground bearing slab.

By November 1998, a number of defects which included outward bowing windows and relative movement between internal and external walls had been observed in the building. A structural engineer, in October 2000, considered that the cracking was a result of differential settlement likely to be the result of inadequate founding of the piers, poor filling compaction and moisture changes of the filling. Douglas Partners was asked to further investigate the cause.

3.2 Investigation Findings

Two pits excavated to expose the piers found that the piers were greater than 2 m deep and were founded in natural material. The pits also confirmed the presence of clayey filling to about 2 m depth.

Laboratory testing on the clayey filling provided the following results given in Table 3.

The results of the tests indicated that the filling was moderately reactive to changes in moisture content and was relatively “wet” compared to optimum moisture content.

There were 12 density tests carried out on the filling during its placement and these were reviewed as part of the investigation. The density test results are summarised in Table 4.

Using the civil engineer’s specification, all tests passed. If the geotechnical engineer’s recommendations were adopted, 15 out of 22 density tests would have failed.

An industrial building was constructed over the filling in late 2002. By 2006, the building was showing signs of distress with relative movement of up to 50 mm between the piled building structure and ground bearing slabs (refer Figure 2).

4.3 Post Construction Investigations

There were at least three investigations carried out by different geotechnical engineers after construction. The investigations confirmed the subsurface profile of filling over residual silty clay and shale and found that the filling had a general consistency of at least stiff. A summary of selected laboratory test results from the investigations is presented in Table 6.

The post construction investigations indicated that the moisture content of the reactive clay filling had generally increased since the time of placement.


As the piles did not appear to have moved, it was concluded that the relative movement of the ground bearing slabs was mainly caused by the reactive clay filling swelling as a result of an increase in moisture content. The situation was exacerbated by the filling being over-compacted with a moisture content dry of optimum.

The movement is to be expected given that reactive clays swell significantly when compacted at high densities and low moisture contents (Chen 1975; Cox 1978).

As the concrete slabs were to be replaced for other non-geotechnical reasons, the recommendations for repairing the damage due to the swelling clays on site was to remove the concrete slabs, remove and replace the top 1 m depth of filling and install subsurface drainage. The replacement of the filling was to be carried out using good engineering practice and strict density and moisture control of any reactive clay used as the replacement filling.

5. Conclusion

The case studies have shown that new filling, when placed and compacted using good engineering practice can be used for founding new buildings, even when placed to considerable depths. Similarly, the performance of filling can be managed by developing specifications that account for site specific conditions, including fillings that are constructed through importation of large filling volumes. When managed appropriately, the short term settlement performance of well-constructed filling can be used to estimate long term total and differential settlements, which has significant benefits when undertaking civil and structural design.

Moderately to highly reactive clay filling must be placed and compacted under strict guidelines as well as providing appropriate drainage, otherwise there may be long term problems as demonstrated by Case Studies 2 and 3.


Cox, D. W., “Volume Change of Compacted Clay Fill“, Proceedings of the Conference held at Institute of Civil Engineers, London, November 1979.

Chen, F. H., “Foundations on Expannsive Clay“, Amsterdam, 1975.

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