黏土層內撐開挖之攪拌樁改良土等值不排水剪力強度探討

Undrained Shear Strength of Grout Piles Improved Clay for Braced Excavation

廖洪鈞(H. J. Liao) ； 杜明昇(M. S. Du) ； 蘇世豐(S. F. Su)

《地工技術》 93期 (2002/10) Pp. 23-32

https://doi.org/10.30140/SG.200210.0002

摘要

為增加黏土層開挖底部之穩定性，常使用攪拌樁改良方式來增加底部土壤之強度。可是目前工程界對攪拌樁改良後土壤之等值剪力強度決定，仍是停留在經驗公式之層級。有鑑於此，本文透過模型試驗的方式，模擬攪拌樁改良後之開挖行為，並評估攪拌樁改良土之等值剪力強度。為減少因縮小模型尺寸所產生之尺寸效應問題，本模型試驗將保持原型與模型之穩定數（γH/Su）相等。因此在模型試驗過程中，黏土強度將依現場尺寸和模型試體尺寸之縮小比值而減少。攪拌樁改良土等值剪力強度之求取，是藉由比較攪拌樁改良和全面改良後之開挖底部隆起量來決定。試驗結果顯示，在增加黏土層開挖底部穩定性方面，攪拌樁體強度對攪拌樁改良土剪力強度發揮的貢獻度，只有攪拌樁體強度之一小部份而已，而且其貢獻值要小於慣用經驗公式所採用之數值。

關鍵字

開挖工程 ； 軟弱黏土 ； 攪拌樁 ； 不排水剪力強度 ； 模型試驗

並列摘要

To increase the base stability of braced excavations in soft clay, grout piles are frequently used to improve the base soil. Although the strength of base soil has crucial effect on the base stability of excavation, it is mostly determined by means of empirical equations so far. To rationalize the empirical equation commonly used, this paper will evaluate the apparent shear strength of grout piles improved clay based on the results of a model test simulating deep excavation in soft clay. The size effect of the model test was minimized by keeping the base stability numbers (= γH/Su) of the model test and the field excavation being equal. The undrained shear strength Su of soil used in the model test was reduced according to the size reduced from the prototype. The composite undrained shear strength of grout piles improved clay is evaluated by comparing the basal heave behavior of soil with grout piles improvement and that of soil with 100% ground improvement. Test results indicate that the contribution of grout piles to the undrained shear strength of the base soil underlying a deep excavation is only a small fraction of the grout pile strength. The contributed grout pile strength is smaller than the value which is usually adopted in the empirical equation for the undrained shear strength of grout piles improved clay.

並列關鍵字

excavation ； soft clay ； grout pile ； undrained shear strength ； model test