Notes - Typical Geotechnical Problems in Melbourne PDF

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Geology of Melbourne...

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As has been discussed previously, there is a significant quantity of information available about the distribution and characteristics of the soils and rocks in Melbourne and its surrounding areas. It follows that there has been considerable experienced gained with many different forms of construction in these materials. This experience has led to a relatively detailed understanding of what can be expected of particular materials, what can go wrong and, of course, what should be done to avoid these problems. This section will consider a selection of these different soil and rock types and discuss some of the problems likely to be encountered along with some of the solutions which would normally be expected. The soils to be discussed are the 6 soils listed in Table 1 of the notes on the Engineering Geology of Melbourne. These topics are discussed in much more detail in the Blue Book (Engineering Geology of Melbourne1 ).

The Coode Island Silt extends over most of the area identified by the light green colour shown on the 1:31,680 geological map, either at or relatively close to the surface. In actual fact, the Coode Island Silt is rarely encountered at the surface because over the years, fill has been placed for a number of reasons, usually related to access and trafficability. This material is a silty clay, generally soft at the surface increasing in strength with depth. Where it extends to depths approaching 30m, it is usually stiff because of the consolidating effect of the overlying materials. Close to the surface it is generally very compressible giving rise to large settlements even with relatively modest applied loads. In addition, the material exhibits considerable creep properties which mean that settlement can continue for many years after primary consolidation has been completed. Figures 1 and 2 show the Coode Island Silt as encountered during the construction of the casino on Southbank. With respect to engineering foundations, it should be avoided as much as possible simply because of its low bearing capacity and tendency to cause major ongoing settlements for anything other than the lightest of loads. It can also pose significant difficulties when excavations are placed in it with instability resulting because of its low strength even for modest excavations. There are of course, occasions where some reasonable sized construction is required either on this material or below its upper surface. An example of construction on the Coode Island Silt is the ground supported approach ramps for the pile supported elevated section of the Westgate Freeway. Had the several metres high approach ramps been placed just before the start of operation, they would have settled away from the effectively rigid pile supported freeway structure leaving a considerable gap where the two join. This would have been particularly noticeable with the average 3.5m of fill settling about 700mm in 3 years with a further 500mm occurring mainly as creep settlement over longer periods. The importance of placing the fill for the ramps several years before they were required was very important so that the differential settlements between the two types of structure could occur before the roads were completed. It was also important that vertical sand drains were installed in the Coode Island Silt below the ramps so that the rates of settlement could be accelerated. 1

Engineering Geology of Melbourne, edited by W.A. Peck, J.L. Neilson, R.J Olds and K.D. Seddon. Published by A.A. Balkema, Rotterdam, 1992

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Where excavations are required into the Coode Island Silt, as a result of the soil’s low strength and high watertable, side stability can become a considerable issue with even modest sloping batters becoming quite unstable (see Figure 3). For excavations of more than a metre or two, it is normally necessary to provide a retaining wall of some form, which because of the soil and water characteristics, may need to extend significant depths into the Coode Island Silt for only a modest depth of excavation.

Figure 1 Coode Island Silt from a surface test pit (photo courtesy of Golder Associates)

Figure 2 An excavation in Coode Island Silt (photo courtesy of Golder Associates)

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Figure 3 Instability on the Coode Island Silt One of the other major problems with a material such as the Coode Island Silt is that should the water table level be reduced for any reason, such as with the construction and operation of any tunnel or any deep excavation, the increase in effective pressure can cause major settlements in surrounding areas possibly resulting in major structural damage. This has been a major factor in numerous projects including the construction of the Arts Centre and the interchange and tunnels of CityLink. Design and construction problems associated with the Coode Island Silt have probably been responsible for more legal activities than any other material in Melbourne.

These sands are generally fine to medium grained with little if any fines content. The upper sands tend to be relatively loose but generally become medium dense and dense with depth. While foundations can generally be adequately founded on the denser sands, the main engineering problems with these materials are: The sands will not stand unsupported for any length of time and therefore any major excavations will either require side retention or, if there is adequate space, moderate slopes down to excavation bases. The sands tend to be water bearing and, therefore, any excavations below the water table can introduce significant drainage, dewatering and stability problems.

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The Newer Volcanics are generally encountered as a few to several metres of weathered basaltic clay overlying basalt of varying degrees of weathering. The transition from clay to relatively hard rock can be quite rapid and it is not unusual to encounter “floaters” within the clay matrix. These “floaters” can be a problem when piles are to be installed to basement rock and a “floater” is encountered to restrict the progress of a pile but not add substantially to its capacity or performance. They can also pose problems with excavations particularly with large “floaters”. Similar problems can be encountered where the basalt rock body is relatively fresh and the individual blocks of basalt are large. Figure 4 shows a mix of clay and boulders in the highly weathered Newer Volcanics profile. Figure 5 shows the fresher basalt as exposed in the Old Burnley Quarry.

Figure 4 Mix of clay and boulders in the weathered Newer Volcanics profile (courtesy T Flintoff)

Figure 5 Basalt in Old Burnley Quarry 4

While the residual clay (and clearly the fresher basalt) is a relatively good material for locating foundations from a bearing capacity and settlement viewpoint, the clay is highly reactive. This means that it can shrink and swell in accordance with how much water is available for absorption within the clay. The residual basaltic clay contains significant quantities of the clay mineral montmorillonite which can absorb large quantities of water to cause swelling. Similarly, when high suction forces are present due to high ambient temperatures or vegetation, the same soil can shrink significantly. This generally means that when water is plentiful (such as in wet springs or winters where drainage is poor), the soils will swell but when water is in short supply (such as in summer and/or in the presence of large trees), these soils will shrink. This leads to the problem, depending on the water which is available, of the clays swelling significantly in the winter/spring period and then shrinking in the summer/autumn period. This can cause large seasonal movements under foundations which, in the case of lightly loaded foundations can cause major structural problems, particularly when there are areas of large potential differences in water content. Such a difference would occur between the underside of a large house ground slab where the slab on ground does not allow water to evaporate from the soil and is relatively wet, and the edge of the slab which can wet and dry according to the seasons and the availability of water. Typical differential movements experienced by house foundations on basaltic soils can be as much as 70mm or more. Figure 6 shows shrinkage cracks in a basaltic clay. Figures 7, 8 and 9 show the sort of damage which can occur as a result of differential movements on basaltic clays.

Figure 6 Cracking in basaltic clays (the instrument is the shaft of a golf driver)

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Figure 7 Typical damage as a result of differential movement on basaltic clays in Burnley

Figure 8 Typical damage as a result of differential movement on basaltic clays in North Carlton

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Figure 9 Typical damage as a result of differential movement on basaltic clays in Richmond

Figure 10 Stiffened Raft Design for Differing Class Sites (from AS 2870 – 1996)

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The way to overcome these shrink/swell problems is generally a combination of placing foundations deep enough to limit shrinkage and swelling due to climatic changes from the soils below the foundations and linking the foundations in a relatively stiff manner so that differential movement is spread more evenly by the stiffness of the foundation. It is also important to make sure there are no major trees close to the foundations. Figure 10 shows the recommendations contained in AS2870 – 1996 Residential slabs and footings for stiffened raft foundations. Note the increase in the depth (D) of the beams as the soil becomes more reactive. Class A is sand or rock which has virtually no ground movement, Classes S, M and H are for slightly, moderately and highly reactive sites respectively. It should also be noted that the more rigid the type of construction (and therefore greater likelihood of cracking), the deeper the beams become to produce less movement as well as making the raft structure more rigid.

The Brighton Group of materials covers extensive parts of the south eastern suburbs of Melbourne. The upper parts of the Brighton Group are commonly referred to as the Red Bluff sands and comprise clayey and silty sands with varying layers of sandy clays (see Figure 11). While there have been several reported cases of foundation movements because of the often moderately reactive nature of these clays, because of their generally high strength and low compressibility, they are generally very good foundation soils for a wide range of small to moderately large developments. It is very common to see multistorey developments in the Brighton group with 1 or 2 basements. Where basements are used, it is important to assess the presence of water in the more permeable sand layers which can cause significant drainage problems on excavation.

Figure 11 Excavation in the Brighton Group 8

The lower parts of the Brighton group comprise the Black Rock Sandstone which while termed a sandstone is more a clayey and silty sand not unlike the upper Red Bluff sands, with some gravel layers and calcareous sands or weak limestone. Where the Brighton group is encountered further south in the Mornington Peninsula, it is generally referred to as the Baxter Sandstone, not too dissimilar to the Black Rock Sandstone. The Baxter Sandstone is often associated with cliff instability in many of the relatively steep bay-side slopes encountered from Frankston through to Portsea. This instability is caused mainly by strength loss through weathering although it can be associated with erosion of underlying reactive clays such as the Balcombe Clay. Figure 12 shows a slope failure in Davey’s Bay, Mt Eliza, caused by major erosion of the Balcombe Clay and the overlying Baxter Formation.

Figure 12 Instability in the Baxter Sandstone at Davey’s Bay, Mt Eliza.

Apart from their greater age and considerably reduced surface exposure, from an engineering point of view, the Older Volcanics have characteristics very similar to the Newer Volcanics.

The Melbourne mudstone is the common name given to the sequence of sedimentary marine silty and sandy rocks of the Silurian and Lower Devonian periods which make up the bedrock of the whole Melbourne region. The mudstone comprises interbedded claystones, siltstones and sandstones in which the siltstones predominate. These rocks can be found in a wide range of different weathered states from the residual soil which is a yellow/brown silty or sandy clay of a generally stiff or better consistency. The mudstone then progresses through the yellow/red/brown/grey/pink extremely weathered and highly weathered conditions where the rock is generally soft (in rock terms which is similar in strength to blackboard chalk), through moderately weathered to the dark grey/dark blue slightly weathered and fresh conditions. The fresh rock has a strength which would

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be similar to concrete. The lighter coloured, extremely and highly weathered mudstone is frequently encountered in the CBD and eastern suburbs but the darker coloured, stronger mudstone is usually only encountered at depth, usually with deep foundations below shallower formations to the west of the CBD and beyond. Although there are areas where significant folding and faulting have taken place to produce a highly fractured mudstone mass with rough, irregular, and often clay filled joints, the majority of the mudstone is quite massive with defect spacings of the order of metres with clean, tight and planar joints. Figures 13 to 16 show the mudstone at various locations.

Figure 13 Highly weathered mudstone cuts on the Eastern Freeway near Kew

Figure 14 Highly weathered mudstone in a cut on EastLink near Mitcham 10

Figure 15 Highly weathered mudstone in a cut on EastLink also near Mitcham

Figure 16 Highly to moderately weathered mudstone at the base of the Queen Vic Building

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Figure 17 Slightly weathered mudstone in a pile socket from 50m depth near Spencer Street The Melbourne mudstone generally provides an excellent material for structural foundations with high strength and low compressibility. Where the mudstone is encountered close to the ground surface, it is usual to use shallow foundations, strip and pads for the lighter buildings with rafts used for multi-storey construction. To the west of the CBD and further south and west, the mudstone encountered at increasing depth usually provides the founding stratum of most major buildings, although the Moray Street Gravels and occasionally the Newer and Older Volcanics where they are thick enough, are sometimes used. Where it is the mudstone and because of the depths involved, foundations usually take the form of large diameter, bored, rock socketed piles drilled several metres into the mudstone. Figures 18 shows two such piles although these were formed close to the ground surface to examine the effects of a number of construction practices on integrity.

Figure 18 Near-surface excavated, rock socketed piles 12...