3276
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Substances that could potentially leach from tyres are
already present in groundwater in developed areas. Studies
suggest that leachate levels generally fall well below allowable
regulatory limits and have negligible impact on water quality in
close proximity to tyres (Hylands & Shulman, 2003) and that
rates of release decrease with time (Collins et al. 2002).
Similarly there is no evidence of significant deterioration of
tyres buried in the ground for decades (Zornberg et al. 2004).
Figure 1. A typical tyre bale with dimensions.
Spontaneous fires in whole tyre dumps are not known to the
author. In the USA, while combustion due to sparks from
agricultural machinery and lightning have been reported, most
observers suspect some form of arson in almost all cases. Baling
whole uncompressed tyres reduces the volume by a factor of
four to five, greatly reducing the available oxygen as well as the
exposed rubber surface area as tyre-to-tyre contacts are formed,
without exposing any steel reinforcing in the tyres. The
exothermic oxidation reaction potential is significantly lower
than for whole tyres and the risk of spontaneous combustion
from tyre bales is viewed as extremely low. A modelled storage
condition for a 17.5m by 6.0m by 3.0m volume of bales needed
to reach and maintain a temperature of 188
o
C for 39 days before
spontaneous combustion became possible (Simm et al. 2005). In
contrast reports have been made of internal heating of tyre shred
and of apparently spontaneously combusted fires in large
volumes in the USA (Sonti et al. 2000). Further details of tyre
bale properties and behaviours are available (Anon. 2007).
Tyre bale use reflects positively on the sustainable use of
materials and energy and other factors. In the last decade the
application level has moved from domestic works/river bank
erosion projects to slope failure repairs adjacent to a major
Interstate Highway in the USA (Winter et al. 2009) and the
construction of a lightweight embankment as part of the A421
A1-M1 link road construction which won the British
Geotechnical Association’s prestigious Fleming Award.
An adequate supply of tyres, and the resources to turn them
into bales must be secured prior to the commencement of a
project. As bales are around ¼ to
⅕
the volume of whole tyres it
can be particularly difficult to gauge the volume of bales that
will result from a stockpile of tyres. A series of nomograms was
developed by Winter et al. (2006) and further refined (Anon.
2007) to rapidly describe the number of bales required to fill a
given volume, the number of tyres likely to be used in their
manufacture, and the number of eight hour (two man) shifts
required to manufacture those tyre bales.
Tyre bales costs are similar to those of other road foundation
materials (e.g. UK Type 1 Sub-Base). However, the main
advantages of tyre bales are the much reduced plant and labour
costs resulting from their rapid placement (Winter et al. 2006).
2.2
British Standard
The tyre baling industry in the UK reached a level of maturity
with the production of a British Standard Publicly Available
Specification (PAS) for tyre bales (Anon. 2007). It assists
manufacturers to produce high quality, consistent and traceable
products for use in construction by responsible and competent
organizations, and demonstrate high and consistent quality via a
Factory Production Control process. It covers activities and
aspects of tyre bale manufacture, storage and use in
construction, including: receipt, inspection and cleaning of
tyres; handling and storage of tyres; production of bales
(including a system for measuring and labelling bales to ensure
traceability); handling and storage of the bales; transport,
storage on site and placement of the bales; and factory
production control.
Guidance is given to construction professionals in
formulating preliminary design and construction proposals. Not
all aspects of design are covered but information not available
from other engineering documents is given. This includes: the
measurement of properties; engineering properties and
behaviours associated with tyre bale use in construction;
example applications; and end of service life options.
3 METHODS OF CONSTRUCTION
There are two main approaches to road construction over soft
ground: above ground (floating); and below ground (buried).
Both use large volumes of granular fill.
It is important to decide whether or not a crust in, for
example, peat may be breached or whether it must remain
intact. Figure 2 illustrates advantages and disadvantages of
floating and buried construction. The crust in peat will often be
formed from fibrous vegetation. Similarly, many normally
consolidated lowland clays in parts of Scotland and many
Scandinavian ‘quick clays’ will have a stiffer crust. In general
terms it is inadvisable to breach the crust of these materials and
thus floating construction is preferred to buried construction.
3.1
Floating construction
In areas of deep soft soil, replacement techniques are
unattractive as large volumes of material must be excavated,
transported and disposed at both monetary and environmental
cost. The surrounding soft material may create difficulties
related to excavation support, basal heave and other factors,
making the works uneconomic. Where the natural surface
‘crust’ is stiffer than the lower layers due to the presence of
vegetation, desiccation, compaction and other factors, the
surface may form the subgrade. Care is needed to ensure that
the crust is not compromised during construction and that as the
road is built the imposed loads are spread over a wide area.
In the past construction often utilised bundles of twigs
(fascines), usually two layers orientated at 90
o
, at subgrade level
to resist differential movement. For greater loads logs were used
on the fascines, working best for materials with stiff crusts (e.g.
fibrous peat overlying softer amorphous, or humified, peat). The
modern equivalent is a geosynthetic material; the use of tyre
bales or other lightweight fill on the geosynthetic/sand layer
lessens the applied load. The success of temporary surcharging
is often limited in very soft soils such as peat due to the
potential for long-term secondary and/or tertiary consolidation
and the potential to breach any overlying stiffer layer.
3.2
Buried construction
The removal and replacement of in-situ materials with new,
lightweight, fill is a costly option and may involve excavation
below the water table. However, sidewall lateral restraint adds
durable construction stiffness. The key to construction is to
ensure that the fill adds minimal load.
Buried construction may be preferred in more competent
materials, or in thinner layers of less competent materials for
which removal is an option. Such materials include normally
