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Explaining Geotechnical Risks
Tim Chapman
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An Engineering Assessment Of The Strength And Deformation Properties Of Brisbane Rocks
The use of standard relationships between point load index (Is(50) ), Uniaxial Compressive Strength (UCS) and elastic modulus values is commonly used within the geotechnical engineering community. The validity of these relationships is examined using data from several major projects carried out in Brisbane recently. Additionally the methods of Point Load Index testing is examined, particularly the impact of the direction of testing. It is established that in extremely low to low strength rocks, axial tests are likely to produce a higher point load index value. Based on our data set, the use of the standard multiplier of 24 (Broch and Franklin, 1972) to obtain UCS from Is(50) values is shown to be unconservative for several rock types. In terms of modulus values, in comparison with published recommendations (eg Deere, 1968), the established ratios are generally within the 100-500 times UCS ratio quoted, however the value for the DW metasediments is considerably higher.
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Proposed Revisions To The Piling Standard The Effect Of Geotechnical Investigations And Test Loads
Proposed revisions to the current version of the Australian Piling Standard will affect the way that the geotechnical and structural engineering professions and the pile construction industry will consider the geotechnical capacity of piled foundations. Significant changes are to the manner in which geotechnical investigations and test load programmes affect the geotechnical strength reduction factor, which are generally lower than the current standard. The proposed changes also reduce the permissible settlement under test load Special consideration will be required for the structural capacity of piles which are to be test loaded to prove ultimate geotechnical capacity.
This paper discusses some of the proposed changes and compares them to the current requirements. The general emphasis will be for piles for normal industrial, commercial and residential structures. The developers of large infrastructure and heavy industrial projects usually recognize the importance of comprehensive investigation and test programmes and require little incentive to implement them.
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The hidden cost of geotechnical investigations
This paper presents the results of an analysis of 201 geotechnical reports prepared by most of the geotechnical consultants in Sydney. The analysis considers the recommendations for several important design parameters in the most common soils and rocks in the Sydney Basin and compares them against a level of Geotechnical Investigation Risk. The results of the analysis are used to demonstrate that investigations with a small scope of works result in conservative design and expensive construction – the hidden cost of low levels of investigation.
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Disposal of stormwater runoff by soakage in Perth, Western Australia
Extensive use is made in Perth of soakage basins (referred to locally as sumps) for the disposal of runoff from roads in urban areas and soak wells for the disposal of roof runoff. Software packages such as PCSUMP, INFIL and MODRET as well as “rules of thumb” have been used to design the soakage basins. This paper discusses the advantages and disadvantages of these design software packages as well as presenting a new simplified design method. Methods of assessing soil permeability and other considerations in the design of sumps and soak wells are presented together with advice on related issues including cracking of houses near sumps and proximity of sumps to bridge foundations.
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Groundwater Drawdown Related Settlement and Pump Text Interpretation
Settlement associated with groundwater drawdown can be an important design factor for construction below the watertable. As a result, prediction of the extent and magnitude of groundwater drawdown is an essential part of the evaluation of potential impacts of deep structures. Pump test analysis is routinely employed to for assessment of hydraulic properties of aquifer units affected by deep excavation. Interpretation of pump tests makes use of the rate of groundwater extraction and the drawdown measured in the pumping bores and at other locations. Settlement is not usually considered during interpretation of pump tests. This paper discusses settlement due to dewatering and proposes use of settlement monitoring for interpretation of pump test results.
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Pavement materials and surfacing aggregates used in road construction in Perth
The principal materials used for base, sub base and surfacing in the construction of roads in Perth are bitumen stabilised limestone, crushed limestone, crushed granite, crushed dolerite, hydrated cement treated crushed rock base, lateritic gravel and crushed massive laterite (ferricrete). The selection of the appropriate materials for a particular project depends on factors such as the drainage environment, traffic loading, traffic speed and cost. This paper presents typical specifications, properties and applications of the materials used in road construction in Perth.
“The size of stones for a road has been described in contracts in several different ways, sometimes as the size of a hen’s egg, sometimes as at half a pound weight.” John Louden McAdam 1811.
“It is well known that the more clean and free from dirt the broken stone laid on roads is, the better”. Major-Gen Sir J Burgoyne 1844.
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Computer Design Of Pile Foundation Systems For High-Rise Buildings
High rise buildings are usually founded on some form of piled foundation which is subjected to a combination of vertical, lateral and overturning forces. Conventional methods of assessing foundation stability may not be adequate when designing such foundations because they tend to focus primarily on foundation resistance under vertical loading.
This paper sets out the principles of using computer programs for pile group response to design a pile foundation system according to the Australian Piling Code AS2159-1995. The approach involves three sets of analyses:
- An overall stability analysis;
- A serviceability analysis;
- An analysis to obtain the structural actions for structural design of the raft and the piles.
The approach is illustrated via its application to a simple example problem and then to a high-rise building in the Middle East. The computer program DEFPIG has been used to assess overall foundation stability while the program GARP has been used for the serviceability analysis.
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Does emerging evapotranspiration (ET) cover technology offer a suitable alternative for landfill covers in the Hunter Region?
The NSW Environment Protection Authority (EPA) Environmental Guidelines: Solid Waste Landfills state that “the site capping (of a landfill) should ensure that the final surface provides a barrier to the migration of water into the waste, controls emissions to water and atmosphere, promotes sound land management and conservation, and prevents hazards and protects amenity”. The Environmental Guideline identifies benchmark techniques to achieve these goals. The EPA recommends that the final capping should have five parts including a seal bearing layer, a gas drainage layer, a sealing layer, an infiltration drainage layer and a revegetation layer. This traditional approach to capping of the landfill aims to seal the surface so that rainfall cannot infiltrate the waste and landfill gas cannot escape to the atmosphere. This is a highly engineered and costly means of achieving the desired environmental goals. Experience in the Hunter Region is that final covers created with a compacted clay sealing layer as recommended by the NSW EPA do not always achieve these required environmental goals. Problems relating to conventional covers include the availability of suitable materials, particularly for the low permeability sealing layer, the potential for cracking of the sealing layer particularly during extended dry conditions, the response to differential settlement within the waste and problems with revegetation. Thus opportunities exist to explore alternative options for the final cover of landfills.
One type of alternative cover that is of increasing popularity in the United States is the evapotranspiration (ET) cover. In the United States, ET covers are in place or on trial at a variety of different types of landfill including those used for hazardous and municipal waste. Evidence suggests that these can perform well in a variety of climatic conditions.
The principle behind ET covers is the effective management of water balance in the cover medium. Precipitation is balanced by evapotranspiration, water storage in the cover medium and influx to the underlying waste. In areas where the potential for evapotranspiration approaches or exceeds rainfall, the influx of water through the ET cover can be very low or neutral and can effectively control migration of water into the waste.
This paper considers the potential for the use of ET covers in the Hunter Region.
There are two key considerations:
- Climatic factors – is evaporation greater than rainfall? Are there any critical climatic factors such as prolonged periods of high rainfall and low evaporation? Sample water balances are presented to demonstrate the applicability of ET covers in the Hunter Region.
- Cover media factors – the water holding capacity of the cover medium, availability of materials and ability of the media to support vegetation. A variety of materials available in the Hunter, including naturally occurring soils and waste materials from power generation, coal mining and the waste management industries are assessed for their potential for use in ET covers.
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Some notes on the design of pile foundations in sedimentary rock
This paper presents some technical notes on the design of pile foundations in sedimentary rock based on the author’s thirty-year research and consulting experiences. Firstly, the fundamental difference between the superseded working stress method and the current ultimate limit state (ULS) approach within the practicing codes will be discussed from a geotechnical engineer’s perspective. Then some delusion and confusion encountered by practicing geotechnical engineers such as rock classification and the characteristics of each rock class will be highlighted. The importance of soil and structure interaction and establishment of design criteria for structures and substructures will be emphasized. An overview of the published methods for assessing the pile end bearing capacity and lateral resistance will be carried out to appreciate some issues that practicing engineers are often required to deal with. For typical bridge pile foundations and piled deep excavation retention or retaining structures, both serviceability limit state and ultimate limit state assessments are required to satisfy the requirements set out in current codes of practice. For vertically loaded piles in sedimentary rock it is found that the serviceability limit state is governing the design rather than the ultimate limit state condition for most road and railway projects, based on the commonly accepted design parameters. For laterally loaded piles in rock it is noted that the method based on the lateral force and bending moment equilibrium such as described in Hong Kong Geoguide 1 is frequently used to determine the pile socket length. The critical input parameter required by this method is the ultimate lateral resistance of the rock mass, which is often arbitrary with little guidance provided, and a degree of confusion is often noted by the author. It is proposed to undertake a lateral equilibrium assessment under ULS conditions for a piled wall along with analysis of the deformation characteristic of the rock mass to come up with the “mobilised” rather than the ULS lateral pressure for pile socket design. Worked examples will be given to demonstrate how the pile socket in sedimentary rock can be determined with reasonable confidence for a cantilever piled wall for tunnel projects, and for a bridge structure.