本文從地下水位的角度，切入動力夯實工法的改良效果討論。動力夯實工法(Dynamic Compaction, DC)之所以成為海埔地或疏鬆地層抗液化之主要的地盤改良工法。其主要原因如下: (1)DC機具簡單, 維修容易, (2)施工快速,工期較短 (3)工程費用低於其他工法。然而在台灣, 由於夯擊能量的限制(錘重、落距、地下水位等), 導致在一般能量在500 t-m之改良深度僅止於地表下10公尺至13公尺。對於潛在液化深度在13公尺以下之土壤, 則功效不彰。主要原因之ㄧ為部分夯擊能量被地下水體吸收且激發了超額孔隙水壓，降低了有效夯擊能量所致。
本研究自行研發室內控制排水條件之單擊試驗系統及自動圓錐貫入系統，探討浚填砂土在三種水位高度(高水位、中水位、低水位)及四種開孔率(0%、7%、15%、100%)之條件下，受夯擊後之下列各項物理參數:(1) 動態土壓力增量，(2) 內部所激發之超額孔隙水壓增量，(3) 孔隙水壓消散時間，(4) 殘餘水壓增量，(5) 夯擊後之土壤貫入阻抗等，並對設計之儀器及試體製作方式進行各項率定及貫入阻抗影響試驗。
結果顯示，乾土試體之改良成效均大於含水位試體，在相同水位高度條件下，高水位試體若開孔率大於7%，其改良成效將不再增加，中水位試體開孔率愈大其改良效果愈佳，低水位試體之各項開孔率其改良成效均接近乾土試體；在相同開孔率之條件下，低水位試體改良成效大於中水位者，又遠高於高水位者。由各項水壓參數分析可知，水位愈高且開孔率愈小之試體受夯擊後，所激發之超額孔隙水壓及殘餘水壓增量愈大，且水壓消散時間愈長，相對的改良成效愈低。本文引入改良率(improvement ratio, RI) 與貫入功 (penetration work, Wq) 之概念，針對以上各種水位與開孔條件進行整體性成效評估, 均顯示乾土試體之整體成效最佳. 可見，砂性地盤在動力夯實改良之前，若可預先降低地下水位，則應可增加效夯擊能量，間接增加了改良深度，同時亦可減少等待超額孔隙水壓消散的時間，而縮短工期，可謂一舉數得。

This paper presents an experiment study on the effectiveness of dynamic compaction on from the viewpoint of groundwater table. The major advantages of dynamic compaction to be the main soil improvement technique against soil liquefaction for reclaimed land or loose ground are addressed as follow, (1) simple machine and maintenance, (2) construction is fast, (3) construction cost is lower than other methods. However, due to the safety concern and mobility limit, the compaction energy is constrained at a level of 500 t-m (e.g. a tamper of 25 ton with a drop height of 20m), and the depth of soil improvement is limited at GL-10m to GL-13m. One of the major causes that a great part of compaction energy is absorbed by the groundwater which induces the excess pore water pressure and the effective compacting stress is reduced correspondingly.
An experimental device, namely Single-point Impact Test (SIT) was developed to perform the dynamic compaction under different three water levels and four drain conditions. Meanwhile, an automatic cone penetrometer also is developed to acquire the cone resistance in soils. During the tests, the following responses are obtained through the data acquisition system, (1) increments of dynamic stresses in soil, (2) the increments of excess pore water pressure in soil samples, (3) dissipation of excess pore water pressure, (4) residual water pressure, (5) cone resistance of soil sample after impacts, etc. Besides, many calibration and confirmation tests were performed for the experiment apparatus and soil samples.
The results indicate that the effectiveness of dry soils is higher than that with lower water level and much higher than that with high water level. Under the same water level, the cone resistance of soil sample with high water level reaches a maximum value as the opening ratio increasing to 7 %. For medium water level, the cone resistance of soil samples is increasing with the increasing of opening ratio. As for the low water levels, the cone resistances of all soil samples are similar to those of dry soil samples. Under the same opening ratio, the effectiveness of soil sample with low water level is higher than that of medium ones, and much higher that of ones with higher water levels. Besides, the induced pore water pressure and residual water pressure of the soil samples with higher water level and less opening ratio are higher than that of lower water level and consequently the dissipation time is longer and also the effectiveness is less. By introducing the terms of improvement ratio (RI) and penetration work (Wq) done for the above cases, the overall performance of improvement can be evaluated. The observations reveal that by lowering down the groundwater table prior to dynamic compaction at field, the soil resistance can be increased in depths and the depth of improvement could be extended accordingly.

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