滨海含软土夹层粉细砂地基高能级强夯加固试验研究

滨海粉细砂场地地基常分布有软土夹层或淤泥包且地下水位较高，地基处理难度大。目前采用高能级强夯加固滨海粉细砂场地的工程案例较少。结合具体工程研究了某地下水位较高且含软土夹层的滨海粉细砂场地上开展的5、8、12、15MN·m能级强夯加固试验。除5MN·m能级强夯试验区外，其余试验区均先采取高能级点夯加固深层土体，然后采用中等能级点夯加固夯点间土，最后利用低能级满夯加固地基浅层。对比分析了夯沉量和强夯前后的旁压、静力触探测试数据，发现夯击7~8击后夯沉量变化明显减小，每遍的单点夯击击数宜控制在8~9；在有效加固深度范围内，土体的旁压模量和静力触探锥尖阻力均明显提升，高能级强夯能有效消除滨海粉细砂的液化势。试验场地内上述各个能级的有效加固深度分别为7.5、9、10.5、10m，在有效加固深度范围内，表征土体相对加固程度的提升系数沿深度大致呈直线下降。现场试验数据还表明，将地下水位降低到距地表以下2.5m有助于提高加固效果；软土夹层的存在会明显影响加固效果及限制有效加固深度的发展，因受软土夹层的影响，场地15MN·m能级强夯的有效加固深度明显偏小。建议在级配不良的滨海粉细砂场地上按照规范JGJ 79—2012中细颗粒土的标准来确定高能级强夯的有效加固深度。

Field test study of high energy dynamic compaction on marine silty fine sand deposits with soft interlayers

The marine silty fine sand ground often includes soft interlayers, silt balling and water table at shallow depth, which results in the difficulty to reinforce the ground. High energy dynamic compactions on this type of soil is rarely reported up to now. Field test of high energy dynamic compactions with 5, 8, 12， 15MN·m energy levels on a silty sand ground with soft interlayers and water table at shallow depth were carried out in this paper. Except for the test zone of 5MN·m dynamic compaction, the following test procedure are conducted in all test zones: high energy dynamic compactions were firstly used on deep soils, followed by dynamic compactions with medium energy level conducted on soils between compaction points, and finally the dynamic compactions with low energy level was used to reinforce shallow soils. By analyzing the compaction subsidence and the pressure data before and after dynamic compactions, it is found that the change of compacted subsidence becomes very small after 7 to 8 blows, thus the number of blows used at a compaction point for each compaction phase should be 8 to 9 blows; the modulus of side pressure and the tip resistance of soils within the reinforcement depth have significant increase after compactions, indicating the high energy dynamic compaction is very effective in eliminating the liquefaction potential of sand. Effective reinforcement depths at the above energy levels are 7.5, 9, 10.5, 10m respectively. The relative improvement effect of dynamic compactions exhibits a linear decreasing trend with the depth within the effective reinforcement range. The field data also suggest that lowering water table to 2.5m below surface can improve the reinforcement effect and soft interlayers hinder the developing of effective reinforcement depth. Because of the existing of soft interlayers, the effective reinforcement depth of the 15MN·m energy level dynamic compaction is smaller than expected. It is recommended that the reinforcement depth of dynamic compaction on marine silty sand ground with uniformly distributed grain size should be determined using the criteria from the standard JGJ 79—2012 for fine grained soils.