Mitigation of railway-induced vibration by using subgrade stiffening

Abstract

Railway-induced ground vibration is often associated with sites with soft ground. Stiffening of the subgrade beneath the railway track is one particular measure that has potential to reduce the vibration level at such sites. However, the mechanisms behind this reduction are not well understood. Here, the effects are examined in the context of two alternative approaches: (i) subgrade stiffening, where the soil directly under the track is stiffened, and (ii) stiff inclusions introduced at some depth beneath the track, sometimes known as 'wave impeding blocks'. The efficacy of the measures is considered for different ground types in a parametric study carried out using a 2.5D coupled finite-element / boundary-element methodology. The soil is considered to consist of a soft upper layer over a stiffer substratum; corresponding homogeneous grounds are also considered. With a 6 m wide, 1 m thick, concrete block directly under the track, the vibration between 16 and 50 Hz was found to be reduced by between 4 and 10 dB for ground with a 3 m deep soft upper layer. For a deeper soft layer the reductions were greater whereas, for a stiffer ground without the soft upper layer, the reductions in vibration from this block were negligible. Slightly smaller reductions in a similar frequency region were observed when the block was positioned 1 m below the 2 surface, suggesting that, as with stiffening directly under the track, the reduction in vibration was primarily due to the increase of the effective stiffness of the soil beneath the track rather than the effective creation of a new, thinner soil layer. Jet grouting is considered as an alternative to concrete and, although it is found to be less effective due to its comparatively low stiffness, it may still be considered as a practical measure for existing tracks on soft soil sites. The reduction in vibration from this form of soil improvement with a depth of 3 m is similar to that for a 1 m thick concrete block. Finally, results are presented for three example sites with different soil properties which show similar trends.

Reducing railway-induced ground-borne vibration by using open trenches and soft-filled barriers

“…This frequency (20 Hz here) will be referred to in the remainder of the paper as the 'cut-on' frequency of the upper layer. For the deeper surface layer, not shown here, the same phenomena occur but at lower frequencies, with the first surface wave becoming localised in the upper layer from around 10 Hz [7]. vibration by using subgrade stiffening 89-103, with permission from Elsevier) [7].…”

“…The same ground parameters were considered in [7] to study the effect of subgrade stiffening. These properties are typical of sites investigated during the EU FP7 project RIVAS (Railway Induced Vibration Abatement Solutions) [53] and are based loosely on sites identified at Rubigen in Switzerland where an upper layer of silty sand with clay and gravel, with a thickness of 2-10 m, is located over a stiffer substratum of silty gravel with sand.…”

“…As discussed in [7], the characteristics of the layered soil have a large impact on the wave propagation. Dispersion plots for the soil with an upper layer of depth 3 m are reproduced in Fig.…”

“…Dispersion plots for the soil with an upper layer of depth 3 m are reproduced in Fig. 3 [7]. These were obtained using an analytical model of the layered soil and indicate the wavenumbers of free surface waves that can occur in the soil at each frequency.…”

“…In principle there are a number of possible ways to reduce railway-induced vibration [1,2,3], including changes to the vehicle [4], modifying the track [5] or the ground beneath it [6,7] or introducing a barrier of some form beside the track. A stiff barrier in the ground beside the track, constructed for example from concrete, can give effective shielding of vibration [8][9][10].…”