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Jacked Piling in Australia
Dr Peter W Mitchell
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Why Are There Seashells In My Alluvial Valley? – The coastal geologist’s perspective of valley-fill sequences.
For engineers tasked with placing infrastructure across alluvial valleys in eastern Australia the mélange of unconsolidated sand and silt/clay deposits encountered beneath modern floodplains must often appear baffling, particularly the juxtaposition of river derived and marine influenced deposits. However, it is this apparent anomaly that immediately alerts coastal geologists, for whom modelling these valley-fill sequences is their stock and trade, that they are dealing with an ancient ‘drowned-valley estuarine’ depositional environment.
Valleys that ultimately connect to the coast have experienced numerous cycles of erosion and deposition as a consequence of major sea level fluctuations through geological time. The last major phase of fill has occurred in response to marine inundation of coastal valleys by rising sea level which stabilized at approximately its present level some 6,500 years ago.
With the exception of the alluvial capping layer that supports human habitation of these valleys, the greater proportion of silt/clay sediments deposited during this last phase have remained beyond the influence of pedogenic processes and are therefore ‘unripe’. This immaturity provides a number of challenging geomechanical and environmental aspects to working with these sediments.
Although local complexity may occur, stratigraphic models of ‘drowned-valley estuarine’ deposition provide a good general framework for understanding the distribution of both geomechanical and environmental properties of the valley- fill sequences.
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Examples of landslides in the Adelaide metropolitan area
This paper provides a brief overview of landslides in the Adelaide metropolitan area. Most landslides in this area occur in the Adelaide Hills or along the coastline, and those form the subject of the paper. Landslides associated with creek or river valleys on the plains or occurring in cuts and fills are not addressed.
Brief case histories are presented illustrating different types of landslide. The main purposes of the paper are to describe the types of landslides that happen and to provide some guidance on what to look for when assessing the stability of slopes in the Adelaide area.
For the purposes of this paper, the Adelaide metropolitan area is assumed to coincide with the 1984 Adelaide Metropolitan Planning Region shown in Figure 1. The area extends from Sellicks Beach in the south to Gawler in the north – about 90 km. Gulf St Vincent forms the western boundary and the area stretches around 35 km at its widest to an irregular eastern boundary running through the Adelaide Hills. The highest point is Mount Lofty (710 m) which is about 15 km south west of the city centre.
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Award winning construction at Curragh North coal mine
Collaboration between Thiess and Broons Hire has resulted in the development of a major innovation for levee construction and has contributed to the Curragh North Project receiving a High Commendation at the 2006 Engineering Excellence Awards.
Wesfarmers Curragh awarded Parsons Brinckerhoff and Thiess the design and construct civil infrastructure works at the Curragh North Project in October 2004. Located 20 km north of Wesfarmers’ existing Curragh mine in Queensland’s Bowen Basin, near Blackwater, the Project has received industry recognition for civil works undertaken, in particular the construction of an extensive 22 km levee bank adjacent to the Mackenzie River.
With an average height of approximately 5 m and a maximum height of 18 m, the levee bank took nearly 6 months to construct and materials used were locally sourced fine-grained alluvial soils. Central to the construction success was the implementation of a major innovation which involved the use of non-circular impact rollers to compact the levee to engineered fill standards.
Impact rollers are typically used for ground improvement, not creating engineered fills to act as a water-retaining structure. Thiess undertook extensive trials with different haul and spread techniques and altering moisture contents to find the most efficient construction technique.
The majority of the levee embankment was compacted using two Broons BH-1300 towed “square” impact rollers. Layers 700 mm thick required 15 passes with the impact roller travelling at approximately 12 km/hr to achieve 98% density ratio.
The result ensured that at full production 20,000 cubic metres of clay materials could be placed in a day, proving to be possibly the most cost and time efficient compaction method available in Australia. The innovation has set the benchmark for impact rolling to be used for engineered fills in dam and levee embankments with further endless opportunities.
Designed to exacting environmental conditions, the Curragh North mine exemplifies innovative, safe practices, infrastructure to reduce risk and meet demanding time and budget schedules in a constrained market.
Together the Wesfarmers Curragh, Parsons Brinckerhoff and Thiess team achieved outstanding results over the 14 month infrastructure construction period with the project completed to budget and ahead of schedule, providing a world-class asset which makes a very real contribution to the Queensland economy.
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The effectiveness of an impact roller on alluvial sandy clays
A new heavy haul railway embankment in the Pilbara region of Western Australia traversed an alluvial floodplain deposit that displayed sufficient dry strength but inadequate soil strength when saturated. The use of conventional ground improvement techniques for the design and construction of the embankment was assessed against the use of a dynamic impact roller. An impact roller trial was conducted to assess its effectiveness at increasing the soil density with the premise that an increase in density would increase the saturated strength of the soil. Trial results indicated that the impact roller generally increased the soil density to a depth of approximately 1m. However, when saturated, the strength of the modified soil remained unacceptably low as a heavy haul railway embankment foundation subgrade.
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An investigation into the effect of salt water on the geotechnical properties of a residual clay
During construction, water is utilised both as a dust suppressant and for modification of the water content to aid in the compaction process. With the current high level water restrictions in South East Queensland, measures that can be taken to reduce the quantity of potable water in construction are needed. In this case, the possibility of using saline water in earthworks should warrant proper consideration.
It has been known that the presence of salt affects the natural and built environment, including pavements, agriculture, reinforced concrete structures and underground services. The main issues being degradation of bituminous surfacing, impact on vegetation, attack of concrete, corrosion of steel and ground movement. However, some studies in the past indicated that salt water can be used in earthworks.
This paper presents the findings of a limited study, which looked into the effect of salt water on the geotechnical properties of a typical clay from Queensland. To investigate the effects of salt on the engineering properties of the clay, a number of laboratory tests were undertaken, these included Atterberg limits, standard compaction, electrical conductivity, dispersion, California Bearing Ratio, shear strength, soil suction and swell. Testing was carried out at three salt concentrations.
The results show that the addition of salt water, while increasing the electrical conductivity, did not greatly affect the Atterberg limits, compaction characteristics, dispersion, strength or shrinkage of the soil. The use of salt water has been found to increase the variability of testing results; this was particularly evident in the soil suction tests. There appears to be an increase in swell potential with the increase in salt content. Furthermore, salt content appears to affect the matrix suction.
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Ground Improvement Workshop
Serge Varaksin, Dr Chu Jian and Patrick Wong