Road structure – process – or apparatus – Pavement – Including bed
Patent
1996-04-10
1998-10-20
Lisehora, James
Road structure, process, or apparatus
Pavement
Including bed
404 31, 404 82, 4283044, E01B 968, E01C 306
Patent
active
058237062
DESCRIPTION:
BRIEF SUMMARY
This invention relates to pavements generally and is particularly, but not exclusively, concerned with highway and railway pavements. By "pavement" is meant any surface which is laid on the ground and which is intended to bear loads, in particular cyclic/dynamic loads, in service.
In highway pavements, a layer of a graded granular material (commonly known as a subbase) forming part of the pavement construction is usually placed on top of the natural soil (commonly known as the subgrade) to spread the stress that is transmitted through upper layers of the pavement over the subgrade surface to a permissible value, to act as an isolating layer to protect the subgrade soil from frost action, and to provide a working platform for construction of the upper layers of the pavement. In order to fulfil these functions, the subbase must operate under drained conditions. When it is not clean (i.e, when it contains a large quantity of fines), undrained conditions develop which ultimately lead to a failure to perform acceptably. It is therefore necessary to protect the subbase/ballast during its lifetime from contamination by soil fines. Contamination by soil fines can occur as a result of "pumping" of fines from the underlying subgrade into the subbase and the sinking of subbase particles into the subgrade usually occurs when the following combination of conditions arise:
(1) The subgrade is cohesive,
(2) The pavement layer above the subgrade (typically the unbound granular subbase) lacks fine particles (medium to fine sand),
(3) Free water exists at the subgrade/subbase interface and/or sufficient water is contained within the subgrade material, and
(4) The pavement is subjected to cyclic/dynamic loading.
The result of this is to reduce the efficiency of the subbase and cause the stress which is transmitted to the subgrade to increase, with a consequent reduction in the performance of the pavement.
In railway pavements, an essentially similar situation arises with open-graded railway ballast.
In order to prevent the pumping of fines, it is known to employ a layer of sand as a separator between the subgrade and the subbase/ballast. Although the sand layer appears to work efficiently in separating the two layers, it is sometimes inconvenient to use where there is a lack of local availability of sand, skilled labour is needed for placement, difficulties arise in the control of the thickness of the layer of sand and the occasional mixing with underlying cohesive subbase/ballast. Thus, the use of sand is an expensive and time-consuming operation. Attempts have also been made to prevent the pumping of fines into the subbase by the use of a textile sheet material to act as a separator. The advantages of these materials are that they are light in weight so that transport is not a problem, they are easy to place without the need for skilled labour and they are less expensive than sand. Such textile sheet materials act efficiently in preventing the coarse aggregates of the subbase/ballast layer from penetrating the cohesive subgrade. However, their action in the reverse direction (namely to stop migration of the fines from penetrating the coarse aggregate subbase/ballast) is doubtful. It is believed that, with existing textile sheet materials, success in preventing "pumping" of fines into the subbase/ballast has been achieved only with certain types of subgrades which contain relatively high percentages of sand or where the upper sub-layers of the pavement are well graded with a high percentage of sand.
It is therefore an object of the present invention to obviate or mitigate the problem of "pumping" of fines into the subbase/ballast even in cases where the subgrade is cohesive and the upper layers of the pavement are of less than ideal composition in terms of grading and sand content.
In one of its aspects, the present invention resides in the use of a multi-layer structure at the interface between the pavement subbase/ballast and the subgrade, said multi-layer structure comprising upper and lower flexible sheet materials and
REFERENCES:
patent: 3832263 (1974-08-01), Cleveland
patent: 4235371 (1980-11-01), Kohler
patent: 4720043 (1988-01-01), Ortwein
Translation of Swiss Patent Document No. 513302, Nov. 15, 1971.
Translation of French Patent Document No. 2030559, Nov. 13, 1970.
Alobaidi Imad M
Ghataora Gurmel Singh
Hoare David J
Lisehora James
The University of Birmingham
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