ABSTRACT
Pakistan has several regions of seismic activity, having low to high seismic potential. Similarly, a variety of soil deposits exist in the country; some of which are known to have significant liquefaction potential. Liquefaction potential of a certain site is generally a trade-off between the state of denseness of saturated sand deposits and the peak ground acceleration (PGA) at that site. River Ravi flowing through the Punjab province, has its flood plain in the upper Punjab. The soil deposits in its flood plain consist of fine sand with variable silt content (upto to 15%). The PGA in these plains ranges from 0.16 to 0.24g with an average of 0.20g for 475 years return period earthquake having 10% probability of exceedance in 50 years. At an important project site located in the upper reaches of river Ravi, recently loose Sand deposits were encountered up to 10-12m depth below the ground level. Light to moderately heavy structures were planned over the site. The evaluation of free-field liquefaction potential using the Cyclic Stress Approach based on SPT blows (Youd, Seed et al., 2001) with due adjustment for the moment magnitude of 6, indicated sands to be liquefiable up to 12 m depth. The mitigation of liquefaction risk at this site consisted of providing long R.C. drilled shafts (ignoring skin friction in the upper 10 to 12 m zone) under the moderately heavy loads and providing earthen platforms (raising the ground level) under the light structures. A Factor of Safety of 1.1 was taken as the threshold of liquefaction in terms of ratio of the cyclic stress ratio required to cause liquefaction to the one induced by earthquake of average PGA of 0.22g. The project structures stand constructed safely at the site. This paper provides a complete account of the process of evaluation of liquefaction potential and its effective mitigation using simple means. These type of mitigation measures can be adopted for the structures in similar ground and seismic conditions.
