Slide 1: Press Space Bar to Begin The Presentation Geotechnical investigations : Geotechnical investigations A Presentation By Mohammed Ahmed Khan
2007-MS-Geo-25 Geotechnical Engineering : Geotechnical Engineering “Geotechnical Engineering is the branch of civil engineering concerned with the engineering behavior of earth materials.” (wikipedia) Geotechnical Engineering : Geotechnical Engineering Geotechnical engineering includes:
Investigating existing subsurface conditions and materials
Assessing risks posed by site conditions
Designing earthworks and structure foundations
Monitoring site conditions, earthwork and foundation construction. Geotechnical Investigations : Geotechnical Investigations Geotechnical Investigations (G.I.), are performed by geotechnical engineers or engineering geologists to:
Obtain information on the physical properties of soil and rock at various locations and depths around a site
Design earthworks and foundations for proposed structures
Repair of distress to earthworks and structures caused by subsurface conditions. Information obtained from G.I. : Information obtained from G.I. G. I. of a site should provide information to:
Determine the type of foundation required
Stratigraphy of the Soil and Rock
Make a recommendation on the allowable load bearing capacity of the foundation
Sufficient data/laboratory tests to make settlement predictions
Location of the ground water table and its variation
Identify construction & environmental problems Information Required from a Geotechnical Engineer : Information Required from a Geotechnical Engineer The determination of a site’s classification for the application of standard designs for footings & concrete slabs.
Supervision of drilling and logging of boreholes / test pits.
Laboratory and site testing.
The provision of basic geotechnical parameters for detailed design purposes.
The inspection and identification of foundation strata for footings.
Inspections during construction works and related engineering advice Need of G.I. : Need of G.I. Geotechnical Investigation should be carried out whenever there are proposals to construct substantial structures and / or undertake earthworks
Whenever building works are planned in areas identified as having some ‘geotechnical hazard’ [e.g. landslip, rock-fall, shrink/swell soils, etc.] Need of G.I. : Need of G.I. The types of development that generally require geotechnical investigation include:
Roads, subdivisions, embankments & excavations
Retaining walls, including reinforced soil structures;
Tall Buildings, both residential and commercial;
Water supply systems and dams;
Effluent disposal systems in areas where land instability is suspected. Geotechnical issues that can arise on a site : Geotechnical issues that can arise on a site On ‘flat’ sites, the problems that relate to geotechnical issues are more commonly:
Construction stability issues, including excavations, shoring & retaining walls.
Movement of a ‘reactive clay’ [or shrink / swell] foundation under a building, with consequent damage the building elements. Geotechnical issues that can arise on a site : Geotechnical issues that can arise on a site Failure of footing or slab systems as a result of poor design / construction or inadequate earthworks [e.g. uncompacted fill].
Ponding of water on the site due to unsuitable slope of land for drainage.
Land subsidence due to poorly compacted fill or soft ground. Geotechnical issues that can arise on a site : Geotechnical issues that can arise on a site On sloping sites, important geotechnical matters relate to issues such as:
The depth and profile of the bedrock beneath the ground.
The amount of groundwater seepage and the paths of the seepage.
Possible instability of proposed excavations, fill and retaining walls, etc.
Possible long term ‘soil creep’ movements of the surface soils affecting footing design. Geotechnical issues that can arise on a site : Geotechnical issues that can arise on a site Rock falls and boulder rolls onto a site from slopes / cliffs above a site.
The locations of the various structures to minimize the risk of damage.
Drainage & effluent disposal.
Vehicular access & retaining walls. Ground-Related Problems Encountered During Construction : Ground-Related Problems Encountered During Construction ICE Publication “Managing Geotechnical Risk” (2001) Ground-Related Problems Encountered During Construction : Ground-Related Problems Encountered During Construction ICE Publication “Managing Geotechnical Risk” (2001) Expenditures involved in G.I. : Expenditures involved in G.I. Expenditure on geotechnical site investigations varies considerably
It can be as low as between 0.1–0.3% of the total project cost
Low levels of investigation result in large uncertainties (Whyte, 1995).
To reduce the level of ground uncertainty, the National Research Council (1984) recommends that at least 3% of the total project cost be dedicated to site investigation Expenditures involved in G.I. : Expenditures involved in G.I. However, there is no universal yardstick that gives the cost of inadequate site investigation as a percentage of construction cost. (Littlejohn et al., 1994)
The extent of a site investigation varies according to the spatial variability of the ground
Uncertainty in ground conditions can cause significant cost overruns and time delays for both client and contractor.
As the variability increases, so does the extent and cost of the site investigation Quality of Investigation : Quality of Investigation Ground is an extremely variable and, hence, uncertain material.
The potential for the loss of time and money is great if the properties of the soil and rock are not adequately quantified.
However, money is not the whole picture. Quality of Investigation : Quality of Investigation An appropriate site investigation must be carried out with adequate time resources
The quality of the investigation is perhaps more important than the cost
The risk of a foundation failure is heavily dependent on the quantity and quality of information Causes of Inadequate G.I. : Causes of Inadequate G.I. Lack of awareness of the hazards associated with the ground
A minimalist approach to geotechnical engineering investigation and testing
Inadequate focus of finance
Insufficient time given to G.I. Causes of Inadequate G.I. : Causes of Inadequate G.I. Not keeping the geotechnical engineering consultant involved in all stages of the project
Not seeking additional geotechnical engineering advice if the building design/location had altered
Not enough attention is paid, by the geotechnical engineering profession as a whole
Not enough attention, by the geotechnical consultant, to emphasizing the key issues in the report Effect of Inadequate G.I. : Effect of Inadequate G.I. Site investigation forms a vital part of a building design
Scope of such an investigation is constrained by finance and time
A limited site investigation will result in gross over-designs
The foundation may not meet the design criteria
It can cause large cost overruns and time delays causing the project to come to a standstill. Effect of Inadequate G.I. : Effect of Inadequate G.I. High factor of safety may result in uneconomical and very expensive foundations
The structure can not be laid out with confidence
Limited predictions of the behavior of the soil can be made Examples of Inadequate Geotechnical Investigations : Examples of Inadequate Geotechnical Investigations Khalifa City : Khalifa City Khalifa City is a residential suburb of the city of Abu Dhabi in the United Arab Emirates.
It is divided into 3 areas: Khalifa City A, Khalifa City B, and New Khalifa City.
Khalifa City A is on the main highway to Dubai and lies about 30 kilometers from Abu Dhabi. Slide 27: 2m height x 2m wide cave was encountered during construction work of ADSS Contract 212. The cave was a serious obstacle for the construction of sewer pipeline. Slide 28: 650mm micro tunneling machine collapsed between manholes KB39 and KB40 (ADSS Contract 212B), 10m below ground surface.
The photo shows traces of inaccurate grouting, due to non-precise identification of the cavity location and size led to collapse of tunneling machine. Slide 29: Large cavity, 2m height x 10m wide, with heavy water flow encountered in gypsum layer (ADSS Contract 218B). Without geophysical surveys, this subsurface hazards could remain undetected, to threaten future constructions. Slide 30: Large cavity, 2m height x 5m wide, with high water flow formed under weak sandstone layer 6m below existing ground surface (ADSS Contract 219B). Al Falah Housing Scheme : Al Falah Housing Scheme Project Name: Al Falah - Abu Dhabi, UAE
Project Type: Urban Design and Structure Planning
Completion Date: October 2005
Size: 4730 plots on 11.9 sq km Slide 32: Al Falah Area – massive cavity in sandstone, encountered during site leveling Slide 33: Very deep cavity exposed in a weak layer of sandstone. It runs into under ground channels in different directions. The extent of this cavity is not known. Ground surface collapse occurred under a tractor while excavating for development of a farm at Al Bahia – North of Shahama Town. Slide 34: Al Falah Area – D8 excavator slipped hanging into huge cavity Slide 35: Khalifa City (B) – Ground collapse under machine while drilling to confirm cavity detected by Resistivity Concluding Remark : Concluding Remark As a final concluding remark stated by Institution of Civil Engineers (1991): “You pay for a site investigation whether you have one or not.” Slide 37: THANK YOU Questions ? : Questions ? References : References http://www.fhwa.dot.gov/engineering/geotech/pubs/reviewguide
Whyte, I. L. (1995). The Financial Benefit From Site Investigation Strategy. Ground Engineering
Bowles, J. E. Foundation Analysis and Design, Appendix A, McGraw-Hill, NY.