Abstract:

The critical unloading void ratio of preloading method is obtained base on Bjerrum’s equitime curve and Yin and Graham’s 1-D elastic visco-plastic model. This critical unloading void ratio decided by runtime duration and allowable settlement takes account into both primary consolidation and secondary consolidation for soft clay, so farther do the critical unloading void ratios for surcharge preloading method and undersurcharge preloading method. The critical settlement for unloading time is determined by the critical unloading void ratios. It overcomes the flaws that the classical surcharge preloading method based on primary consolidation theory could not reflect mechanism where surcharge preloading reduce secondary consolidation compression deformation. The minimum preload can also be decided by the formulae of the critical void ratio for undersurcharge preloading method. In the process of deriving the critical void ratio for undersurcharge preloading method, the new modified hypothesis A is suggested The condition is proved that the modified hypothesis A is approximately equal to hypothesis B. The examples show that both the actual total settlements and the actual critical settlement at unloading time are bigger than those calculated by effective stress area method.

Key words: surcharge preloading method, creep, EVP model, critical settlement