## VIBRATION MITIGATION FOR METRO LINE ON SOFT CLAY

Karin M. Rothschild1), Christian Madshus2) and Arid Brekke3)

1) Norwegian Geotechnical Institute, Oslo, Norway, karin.rothschild@ngi.no

2) Norwegian Geotechnical Institute, Oslo, Norway, christian.madshus@ngi.no

3) Brekke and Strand Acoustics, Oslo, Norway, arild.brekke@bs-akustikk.no

Soft clay ground conditions regularly lead to annoying vibration in buildings along railed traffic lines. This paper will present results from an actual case were an excising metro line is upgraded to modern standard. Prediction based on empirical models concluded that countermeasures would be needed to avoid unacceptable indoor low frequency vibration in neighbouring communities. Norwegian vibration acceptance criteria are formulated in a national standard which is based on extensive sociological investigations on human reaction to vibration in everyday living situations. Lime-cement pillars under the track were found to be the most beneficial countermeasure under the present conditions and would substantially reduce vibration in the low frequency range. Such pillars do also have the geotechnical side benefit of preventing settlements. However, introducing lime-cement pillars which constitutes a much stiffer element than the clay may have the disadvantage if increasing the higher frequency audible ground borne or structure borne noise level. The paper will describe how dynamic finite element analyses is used to find an optimum solution based on a combination of lime cement pillars and under-ballast mats to meet acceptance criteria for both low frequency whole body vibration and audible structure borne noise.