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Correlation of liquefaction and settlement in windblown sands using the flat plate dilatometer
Author(s)
Neal, Patrick M.
Date Issued
2011
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
Dwellings in impoverished and upper class urban areas of the Western Cape have undergone serious
structural failure due to problematic underlying sand deposits, generally known as Cape Flats
windblown found in areas such as Delft, Blue Downs and the Atlantic Beach Golf Estate. The problem is
compounded further when moisture penetrates below the footings and reaches saturation in the winter
months. When poorly graded sand with a high fines content is coupled with vibration (through earth
tremors), liquefaction may occur and without proper precautions, this can lead to inadequate
foundation design, more than expected settlement and eventual structural failure. Some sand deposits
are highly contaminated with organic debris, leading to compressible and collapsible conditions. Little
knowledge is locally available of liquefiable conditions in windblown dune sand and what long-term
settlement can be expected. The Western Cape is not known as an area exposed to serious seismic
activity, but an updated (and more locally applicable set of data) is needed to eliminate possible
erroneous foundation design.
Samples have been extracted from typical sandy sites in the Western Cape where windblown dune sand
is evident. Laboratory tests have been carried out on representative samples for closer examination and
have been placed inside a purpose built calibration chamber that facilitates easy densification and probe
testing. A DMT (flat blade dilatometer) was used to hydraulically penetrate the chamber sand sample to
varying depths (up to 800 mm). The device can measure (with reduction formula) horizontal stress,
angle of friction, bearing capacity and settlement. An accelerometer was attached to the chamber wall
and vibration measured with the sand in varying states of moisture.
The DMT is an unexplored in-situ soil testing device in South Africa and so far the outcome indicates
favourably compared to other devices such as the Dynamic Cone Penetrometer (DCP. The DMT has the
ability to measure the in-situ stiffness, strength and stress history parameters of soil for better site
characterisation. Settlement within the chamber is easily measured. The DMT has, for example,
indicated that sand from the Philippi area are a problematic founding soil and should be treated with
special care at shallow founding levels. The horizontal stress index is low and according to the available
knowledge on soil stress history, these sand, coupled with low densities, can liquefy easily and result in
structural damage. The West Coast dune sand, being coarser and easily consolidated, poses less of a
problem under liquefiable conditions. A suitable terrain device for easy on-site manoeuvrability is
required to assist the DMT in further testing.
structural failure due to problematic underlying sand deposits, generally known as Cape Flats
windblown found in areas such as Delft, Blue Downs and the Atlantic Beach Golf Estate. The problem is
compounded further when moisture penetrates below the footings and reaches saturation in the winter
months. When poorly graded sand with a high fines content is coupled with vibration (through earth
tremors), liquefaction may occur and without proper precautions, this can lead to inadequate
foundation design, more than expected settlement and eventual structural failure. Some sand deposits
are highly contaminated with organic debris, leading to compressible and collapsible conditions. Little
knowledge is locally available of liquefiable conditions in windblown dune sand and what long-term
settlement can be expected. The Western Cape is not known as an area exposed to serious seismic
activity, but an updated (and more locally applicable set of data) is needed to eliminate possible
erroneous foundation design.
Samples have been extracted from typical sandy sites in the Western Cape where windblown dune sand
is evident. Laboratory tests have been carried out on representative samples for closer examination and
have been placed inside a purpose built calibration chamber that facilitates easy densification and probe
testing. A DMT (flat blade dilatometer) was used to hydraulically penetrate the chamber sand sample to
varying depths (up to 800 mm). The device can measure (with reduction formula) horizontal stress,
angle of friction, bearing capacity and settlement. An accelerometer was attached to the chamber wall
and vibration measured with the sand in varying states of moisture.
The DMT is an unexplored in-situ soil testing device in South Africa and so far the outcome indicates
favourably compared to other devices such as the Dynamic Cone Penetrometer (DCP. The DMT has the
ability to measure the in-situ stiffness, strength and stress history parameters of soil for better site
characterisation. Settlement within the chamber is easily measured. The DMT has, for example,
indicated that sand from the Philippi area are a problematic founding soil and should be treated with
special care at shallow founding levels. The horizontal stress index is low and according to the available
knowledge on soil stress history, these sand, coupled with low densities, can liquefy easily and result in
structural damage. The West Coast dune sand, being coarser and easily consolidated, poses less of a
problem under liquefiable conditions. A suitable terrain device for easy on-site manoeuvrability is
required to assist the DMT in further testing.
Additional information
Thesis (MTech (Civil Engineering))--Cape Peninsula Cape Town, 2011
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