驗證動力夯實夯錘夯擊效益之研究

A Study of Verifying the Effectiveness of Tamping in Dynamic Compaction

葛致中(Chih-Chung Ku)

指導教授 ： 馮道偉

中原大學/工學院/土木工程研究所/碩士(2002年)

https://doi.org/10.6840/cycu200200391

摘要

摘 要

動力夯實工法為一種以重錘夯擊地表來改善地下土壤相對密度之改良工法。當地層受重錘夯擊時，產生出三種型式之彈性波，其中表面波對於改良效果並無助益，剪力波使顆粒產生側向位移，而壓力波使土壤顆粒組構解體，促使土壤顆粒排列趨於緊密，因此增加剪力波之能量應可獲致較佳之改良效果。前人研究曾以多種同重但不同錘形之夯錘，進行單一細顆粒含量(5％)之麥寮砂試體室內動力夯實試驗，並採用不同之初始相對密度及夯錘落距等條件，探討不同錘頭形狀夯擊下所造成之貫入阻抗值、改良成效、陷坑深度、表層高程及孔隙水壓變化，進而評估夯錘之錘頭形狀對動力夯實改良效果之影響。
基於上述之考量，本研究針對三種不同細顆粒含量（即3.4％、8.0％、14.0％）之麥寮砂，搭配三種落距（即30 cm、60 cm、120 cm），及三種不同錘底形狀（即平底、錐尖角90°、錐尖角120°）之夯錘進行實驗室動力夯實試驗，以探討錘底形狀對改良範圍之影響。同時在麥寮進行動力夯實現地試驗，使用錐尖角90∘夯錘及平底夯錘，比較平底與非平底夯錘在現場所造成的改良成效，以驗證室內試驗結果。
室內試驗結果顯示，相較於平底夯錘，非平底夯錘夯擊後造成陷坑體積約增加5〜20％。而細顆粒含量愈高，無論以何種錐底形狀之夯錘改良後，試樣之圓錐貫入阻抗值皆愈高。而夯擊後影響範圍方面，細顆粒含量愈高則改良影響範圍愈廣，夯錘形狀之影響則為錐尖角120°夯錘 > 錐尖角90°夯錘 > 平底夯錘。而現地試驗之結果顯示，於第一階段夯擊完成後，錐底夯錘夯擊後造成之陷坑體積相較於平底夯錘約增加13％，而在第三階段夯擊完成後兩種夯錘造成的陷坑體積並無明顯的差異，原因在於錐底夯錘於第一階段夯擊所產生之剪力波應已影響第三階段夯擊點下方之土層。在圓錐貫入阻抗值方面，於第一階段夯擊完成後，夯擊點中心周圍下方2〜10 m處之土壤，錐底夯錘改良效果優於平底夯錘；而10〜15 m處之土壤，平底夯錘之改良效果較佳，但部分之試驗點呈現夯擊後貫入阻抗值小於原始值。於第三階段夯擊完成後，夯擊點中心周圍下方2〜5 m處之土壤，錐底夯錘改良效果優於平底夯錘；而5〜15 m處之土壤，平底夯錘之改良效果較佳。就此點可研判，平底夯錘主要改良區域範圍集中於夯擊點下方；而錐底夯錘影響深度較淺，主要改良區域範圍涵蓋夯擊點下方及其兩側。而在對照台朔重工針對動力夯實改良工法阻抗值之規範要求，兩種夯錘所增加之貫入阻抗值皆遠大於規範之要求。因此總體而論，非平底夯錘在陷坑體積及影響範圍所呈現的改良效果皆優於平底夯錘。

關鍵字

動力夯實

並列摘要

Abstract

Dynamic compaction is a ground improvement method of increasing soil relative density by drop weights. When the drop weight impact the ground surface, there are three types of elastic waves generated. Surface waves are not useful for soil improvement, shear waves make soil particles move in the lateral direction and compression waves destruct soil structure, so that soil fabric gets denser. Therefore, if the partition energy of compression waves and shear waves are increased, the effect of improvement will be better. Previous studies carried out laboratory dynamic compaction tests by using identical weights, different end-shape of drop weights and Mai-Liao sand of 5% fine content. These studies considered different conditions of relative density and drop weights distances to probe into cone resistance, volume of crator, the settlement of specimen surface, and variation of pore water pressure, to find the effectiveness of end-shape of drop weights in soil improvement.
This study is mainly to focused on the effects of end-shape of the drop weights on the effectiveness of soil improvement by using three Mai-Liao sands of different fine contents (3.4%, 8.0% and14.0%), three droping distances (30 cm, 60 cm and 120 cm) and three drop weights (flat-bottom, conical-bottom with 90 apex angle and conical with 120 apex angle). In addition, in-situ dynamic compaction tests are carried out with flat-bottom and conical-bottom drop weights to verify the results of the laboratory tests.
Compared with the flat-bottom drop weight, the laboratory test results show that the conical-bottom drop weight produced a 5〜20% more of crator volume. By using either type of drop weights, the cone resistance is higher when the fine content is more. Higher fine content can improve the effect range greatly. In-situ tests show that, after first step of tamping, the conical drop weight produced 13% of crator volume more than the flat-bottom drop weight. But the difference is none after the third step. It may be explained that the shear wave after first step of tamping have affected the lower soil layer. When comparing cone resistance, the conical drop weight shows greater effect than flat-bottom drop weight for depths 2〜10 m below the center of tamping point after first step punching. But for depths 10〜15 m below the center, the flat-bottom drop weight shows greater effect. Part of the test results show less cone resistance than original value. After third step of tamping, the conical drop weight shows greater effect than the flat-bottom drop weight for depths 2〜5 m below the center, but the flat-bottom drop weight show greater cone resistance for depths 5〜15 m below the center. Judging from these results, the major range of improvement is concentrated below the tamping point, when using the flat-bottom drop weight. The major range of improvement for the conical tamper include the center and side below the tamping point. Compared to the acceptable value defined by Formosa Heavy Industries Company, the cone resistance of these two type of drop weights are far more than acceptable. In brief, the effects of conical-bottom drop weight on the improvement zone and degree are better than the flat-bottom drop weight.

並列關鍵字

dynamic compaction

參考文獻

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