Comparison of Two Ground Modification Techniques to Reduce Settlement

At the La Rosita Power Plant site in Mexicali, Mexico, a 3-m thick layer of loose very fine sandy silt was encountered at about 7.5 m depth. Settlement calculations performed for the heavier (150 to 200 kPa) settlement sensitive structures indicated values of up to 65 mm, compared to an allowable settlement of 50 mm. During the design of the base plant, various options for limiting settlement were reviewed. Based on a cost and schedule review, jet grouting of the loose silt layer was chosen. The predicted post-grouting foundation settlement was approximately 35 to 40 mm. During construction of the base plant, the owner elected to add an additional unit on an adjacent site that had very similar subsurface conditions. The same ground modification options were again considered for this expansion unit. However, this time removal of a portion of the upper clayey soils and replacement with compacted structural fill materials was chosen as the best option. Computations indicated that removal to a depth of 3 m would reduce the predicted settlement to less than 50mm. Measured settlements on both sites (base plant with jet grouting and expansion plant with soil replacement) were well within predicted limits. A comparison of the settlements showed that the soil replacement was slightly more effective than the jet grouting in these circumstances. One meter of structural fill was required under all structures to prevent shrink-swell of the upper clayey soils. During the jet grouting operation at the base plant, the jet grout spoil materials were used in lieu of structural fill under the cooling tower foundation. At the expansion site, the cooling towers were supported on 1 m of structural fill since no jet grout spoils were available. The measured settlements of these two structures were almost identical, confirming that the jet grouting spoils produced an excellent fill material when properly handled