超載歷史影響粘土二次壓縮之實驗研究

超載工法常被用來改良軟弱土壤，此工法對於軟弱粘土層的沉陷控制有相當大的成效，可避免建物完成後所產生不均勻沉陷而衍生的問題，亦可增加土壤的強度。過去研究多僅觀察超載載重移除後之二次壓縮情況，然而工程實作時除將超載載重移除外，也可能移除部分或全部之預壓載重，隨後再將結構物載重施加地盤之上。本研究為探討上述兩種不同加載方式之超載預壓，利用傳統單向度壓密試驗對七種土樣分別進行不同超載比之單向度超載壓密試驗，針對試驗結果進行分析，評估超載後二次壓縮指數及超載後再加載二次壓縮指數Cαs。根據分析結果，超載比越大則超載後二次壓縮指數越小，但隨著時間的增加而增加，且超載比越大則二次壓縮再現所需的時間越長。超載後再加載二次壓縮指數Cαs隨著超載比增加而減少，且在超載較小時Cαs有隨時間之增加而增加的趨勢，而當超載比增加至1時趨於定值。超載後再加載的二次壓縮應變速率之初始值較大，因此隨著時間的增加所造成的沉陷量較超載後沉陷量大。而兩種不同加載方式各別引致之應變速率與時間的關係曲線有一個交點，表示於相同二次壓縮時間達到相同應變速率，因此兩種不同加載方式之超載沉陷差異將只在交點之前。

關鍵字

地盤改良；後超載割線二次壓縮指數；壓密；超載工法；後超載二次壓縮應變速率

並列摘要

Surcharging is often being used to improve soft soil, and it has great effects on controlling the settlement of soft soil layer. It not only prevents the problems resulted from the differential settlement, but also increases the strength of the soil. In the past, most researches mainly focus on observing the secondary compression after removing the surcharge loading; however, when it comes to real construction, it might remove the whole preloads, followed by reloading due to construction works. In order to discuss these two different surcharging methods mentioned above, this research uses seven different soils to conduct one-dimentional surcharging tests with different surcharge ratios. The test results are analyzed to evaluate the post-surcharge secondary compression index and the recompression secondary compression index Cαs.. According to the result, the higher the surcharge ratio is, the smaller the post-surcharge secondary compression index will be；however, increases with increasing time and it takes more time for the secondary compression to show up again. The index Cαs decreases with increasing the surcharge ratio. When it’s less surcharged, Cαs will increase with increasing time and it gets steady when the surcharging ratio is 1. The initial strain rate is higher for the recompression secondary compression, therefore, the secondary settlement is also greater. There is an intersection point between the strain rate and time relationships by these two surcharge loading methods, which shows the same rate at the same secondary compression time; in other words, the difference in settlement of the two surcharging methods will appear only before the intersection point.