軟弱地層中砂石樁的應用，可增加地層承載力、減低沈陷與加速壓密。然而，極軟弱粘土中的砂 石樁，卻因近地表處的側向圍束應力不足，易導致腫脹破壞；因此，近年掀起對砂石樁加勁，使其應 用更為廣泛的研究與應用趨勢。外包具抗張特性的地工合成材於砂石樁的方式，提升砂石樁受載時的 圍束應力，避免砂石樁於地表附近的腫脹破壞。 當軸向受載壓縮而產生側(徑)向變形，外包的加勁材即逐漸藉由伸張而發揮張力，因而增大對砂 石樁的圍束應力；同時，周圍粘土受擠壓所引致的側向增壓，都將提升砂石樁的承載能力。而砂石材 料的力學行為又與圍壓密切相關；因此，正確掌握砂石材料受力時的剪脹行為，及圍束應力遞增下砂 石材料力學行為的模式，是成功探討外包加勁砂石樁承載行為的基石。 本計畫由模型試驗著手，探討極軟弱粘土中外包加勁砂石樁的載重-變形行為；進而，建立可考 量變圍壓下砂石力學性質之數值分析模型，剖析樁體受載側向擴張擠壓周圍粘土與拉伸外包加勁材， 且反施於樁體之漸進式增壓的力學機制；並應用至擬現場之外包加勁單樁，以及群樁效應行為。研究 內容規劃分三階段、三年期進行。 Granular columns were introduced into engineering practice to improve bearing capacity and reduce settlement in a weak or soft soil. This method is considered one of the most versatile and cost effective techniques for improving in-situ ground conditions. When vertical and coincident lateral deformations of the granular column occur under a vertical load, the surrounding soil stratum generates additional confining stress to the column. The lateral confining pressure of the in-situ soil typically increases with an increase in depth. Therefore, most granular columns fail from bulging near the top due to insufficient lateral support. The bulging failure consequently restrains the column load capacity to a lower value irrespective of the stiffness and strength of the constituent materials. Therefore skirting the granular column by wrapping a geosynthetic liner around the top section of the column and enveloping the granular column in a membrane were proposed to strengthen the column. This study starts with laboratory model tests and follows by numerically analyzing the behavior of geosynthetic-encased sand columns embedded in ultra-soft clay. The proposed method employs the elastic-plastic constitutive law with non-associated flow rule for filled material to characterize the prominent expansive behavior of medium to dense sands. The proposed method was validated through laboratory experimental model tests on un-reinforced and encased sand columns embedded in ultra-soft clay. A series of parametric studies also provides elaborate work on the influence factors for encased columns.