打樁對粉質砂土密度之影響

Abstract

本研究應用SPT或CPT試驗所獲得之貫入阻抗，檢測打樁前及打樁後土壤之密度變化，探討打樁對於粉質砂土密度之影響。為防止飽和疏鬆粉質砂土液化，亦可利用打樁方式進行地盤改良。打擊樁貫入土層時造成振動與樁身擠壓，使得樁周圍土壤孔隙體積減小、密度增加，因此造成土壤強度及承載力增加，液化潛能降低等效應。由於非凝聚性土壤不擾動取樣不易，通常以現地標準貫入試驗(SPT)或圓錐貫入試驗(CPT)之貫入阻抗值，來評估現地土壤改良成效及抗液化能力。 本研究建立一套流程，由打樁前之SPT-N值或CPT-qc值，運用等體積取代原理，預測打樁後土層各深度之N值或qc值，用以預估打樁後土壤液化潛能。上述預測流程將應用在日本新潟車站案例及台灣雲林麥寮REF與PP廠區案例，探討打樁在不同深度造成之土壤密度變化，及採用不同學者建議之經驗公式，所預測之土壤貫入阻抗值與實測貫入阻抗值之差異，並得到以下各項結論： 1.日本新潟車站案例： 打樁後土壤的相對密度約增加為打樁前土壤相對密度的1.1 ~ 1.7倍。以Meyerhof公式求出之預測N值與實測N值較Marcuson and Bieganousky 公式吻合。故依Meyerhof (1957)經驗公式所預測之效果較佳。 2.雲林麥寮REF廠區案例： 打樁後土壤之相對密度大致因打樁而增加，但也有例外情況，可能是因為打樁前土壤已經過DC工法之改良，土層之相對密度已經很高，而打樁過程因側向擠壓，反而造成土壤體積膨脹，相對密度降低，因此造成貫入阻抗值下降之現象。打樁後土壤的相對密度約增加為打樁前相對密度的1.0至 2.3倍。本論文所預測之qc值與實際qc值之標準誤差僅3.27 ~ 3.72 MPa，應屬可接受之結果。 3.雲林麥寮PP廠區案例： 打樁後土壤的相對密度約增加為打樁前相對密度的1.0至 2.2倍。本論文所預測出之qc值與實測qc值相比標準誤差僅2.13 ~ 3.14 MPa，亦應屬可接受之結果。

This paper studies the effects of pile driving on the density of silty sand. The variation of soil density was examined based on the SPT-N value and CPT-qc values before and after pile driving. Pile driving was used for ground improvement to be increase soil density and decrease liquefaction potential for saturated sandy soil. The piles were driven into the ground by hammering, vibration and insertion of the pile body decreased the volume of void surrounding the pile, hence increased the density of the soil. Due to the difficulty of sampling undisturbed cohesionless soils, the penetration resistance from the SPT and CPT tests is usually used to evaluate the effects of ground improvement. Based on the principle of constant volume replacement, a procedure was established in this study to predict SPT-N value and CPT-qc value after pile driving. The liquefaction potential within the group piles can be properly estimated. The prediction model was applied to the case of Niigata train station in Japan and the Mai-Liao REF and PP factory sites. The variation of soil density caused by pile diving is discussed in this study, and the following conclusions are made. 1.Case of Niigata train station: The relative density of soil after pile driving is approximately 1.1 ~ 1.7 times the Dr value before pile driving. The SPT-N value after pile driving predicted with the Meyerhof’s formula matched the measure SPT-N value quite well. 2.Case of Mai Liao REF factory: The relative density of soil increased after pile driving however some exceptions still exists. The relative density of soil after pile driving is approximately 1.0 ~ 2.3 times the relative density before pile driving. A standard deviation of 3.27 ~ 3.72 MPa exists between the predicted and measure qc after pile driving. The predicting procedure proposed in this study is considered acceptable. 3.Case of Mai Liao PP factory: The relative density of soil after pile driving is approximately 1.0 ~ 2.2 times that before pile driving. A difference of 2.13 ~ 3.14 MPa exists between the predicting and measured qc after pile driving.