BACKGROUND ART Longitudinal rail support is a recent innovation and needs to be briefly described before proceeding further in this specification.

The idea of the longitudinal rail support arose because of the expense and maintenance of transverse sleepers and in its simplest form beams of concrete are formed to lie under the individual rails so that they are supported along their full length. The beams may be prepared by pumping concrete into long bags under the rails and the top of the resulting beam curls around the edges of the rails. The beams may be free standing or may be formed in a trench.

Whether dealing with conventional transverse sleepers or longitudinal rails supports, the layers in a railway structure include the rails themselves, the sleepers or longitudinal supports and various layers of earth As this invention is primarily concerned with structures involving longitudinal support (although not necessarily restricted thereto) the various layers involved are illustrated below in the diagram. rack Stnucturs track Stmcture Rail Rtasilisnt& 1 Sector A Beam Batlast Mat. /Layer 1 Layer 2 Sector B /La c3 Layer3

It is an object of the present invention to provide a multilayered structure which requires minimum maintainance and rehabilitation.

DISCLOSURE OF THE INVENTION According to the invention a method of forming a multilayered railway structure includes the steps of forming each layer with respect to a predetermined allowable stress level having regard to the operating conditions, and with respect to its ability to accept the stress across the interface with its adjacent layer at an allowable level and to attenuate this stress level throughout the layer sufficiently to meet the allowable stress level criteria for the next adjacent layer and thence to the subgrade.

The term"allowable stress level"may be regarded as the relationship between the applied stress and the life of the structure under repeated loading. This relationship will vary between the layers due to the variation in the properties of the materials used and their construction. It may also be regarded as that stress below which fatigue failure will not occur irrespective of the number of load repetitions. Failure in turn could be defined as plastic strain beyond the specified limits such that the structure is no longer fit for the purpose for which it was designed.

As an example, if max and min are the upper and lower limits of stress during the cycle and say 4min = 8 MPa (subject to experimental verification!), then a = 4max-4min = 4max-8 and, average = (4maux + 8)/2 Substituting these into (1), we obtain (4max-8) = 24-l/54x (4maux+8) 2/4 This is a quadratic equation for tmax, the roots being 26.5 and =258. 5. Taking the positive root, the probable safe maximum stress is 26.5 MPa. This particular component must consequently be so designed to limit its maximum stress to 26.5 MPa (plus some factor of safety) to prevent fatigue failure.

In a preferred form of the invention the stress level at an interface at which a layer is loaded should be below the allowable stress level for the majority of the time.

The type of stress applied may be that which is most suitable as a performance criteria for the particular material, and the meaning of"allowable stress"should be related to the number of load applications that the particular layer has to sustain coupled with the properties of the layer; as well as a failure criterion which may be regarded as the extent of permanent deformation.

The permanent deformation that may be accumulated may be characterised by the resilience of the material and the design of the layers may have to balance such factors as resilience as well as the allowable stress.

The design strategy, particularly of the Sector B (see the diagram above) is to select a life that is commensurate with the project, with a 50 year life of a general project being aimed at.

The most important layers are those of Sector B where local material is used and which, if unsatisfactory, may have to be replaced or mixed with expensive imported

materials. Permanent settlement of the Sector is probably the main criterion coupled with a measurement of localised differential settlement.

Having regard to the concept of"loading"as a factor in the design of the structure, one has to take in to consideration a factor known as the load equivalence of traffic and this factor takes into account some extreme situations caused for example by flat wheels, rail corrugations as well as axle loads. Such adverse factors may be negated to a variable extent by the provision of resilience such as the introduction of resilient pads beneath the rails.

The concept of cumulative damage is also I All of the relevant factors may be listed as sensible values assigned thereto on the bases of results available and from the literature and with experience. For example in the following figure the basic load equivalency curve for a formation layer is shown and various curves including the effect of flat wheels and the further beneficial effect of the use of resilient pads and ballast mats.

Graph of Log P v Log N showing the affect of varying the exponent or the slope of the curve to accommodate different effects, ESAL type theory. a Log P P \ LveLog N Equivalent Load Derivation (ELD) with no effects. ELD with lrlat wheels

The prime concern may be regarded as the design of the Sector B allied to the quality of the in-situ subgrade and concentration should be given to the design for that sector. In looking at the performance of the different layers of the structure one should assess the performance of the layer around stress as opposed to load. All of the layers have to carry the load but it is the manner in which they do it which is important. The determinant of the performance of a layer is, as is discussed above, the stress in the layer and the transfer of stress across the interfaces. The design of the layers of the Sector B combined with its properties should be such that it can accept a stress at the allowable level and attenuate this stress level through the layer sufficiently to meet the allowable stress level criteria for the next layer. The process continues through the structure until the interface with the subgrade and the same basic criteria should be met.