低黏性砂土張力強度之探討

Abstract

由於土壤之張力強度遠比壓力強度小且欠缺一個確切性的試驗來求得，因此在大地工程中之設計與分析往往忽略土壤之張力強度。根據許多研究指出，土壤之張力強度有一定的重要性，尤其是應用在預測土壩的裂縫行為、機場跑道或公路的鋪面以及邊坡穩定分析中的張力裂縫。 在前人的研究中，有許多決定土壤張力強度的方法，主要分為直接法和間接法。直接法主要是以直接拉力試驗 (direct tensile test) 為主，而間接法常見的有劈裂試驗 (split tensile test) 與無圍壓貫入試驗 (unconfined penetration test)。 無圍壓貫入試驗之張力強度是由Chen在1975年所推導出的極限分析理論求得，其是將摩擦角、無圍壓縮強度、試體直徑、試體高度、貫入棒直徑、試驗所得最大軸向壓力及 'K值' 代入公式。Fang and Fernandez在1981年建議K值為固定值，但K值在理論中是由bH/a2、脆度 (qu/σt) 和摩擦角決定，故其量值應隨材料性質不同而改變，而非建議之固定值，因此本研究決定以疊代法找出各試驗之K值，再計算張力強度。 本研究以石英砂85 % 及高嶺土15 % 混合為試驗土樣，並進行兩部分的試驗，第一部分試驗以疊代法計算無圍壓貫入試驗之張力強度，並探討試體直徑、試體細長比及貫入棒直徑對無圍壓貫入試驗之影響。第二部分試驗以Lu等學者在2006和2010年的不飽和土壤有效應力架構，將各含水量在無圍壓貫入試驗之有效張力強度 (即張力強度之有效應力σpt') 與不飽和破壞包絡線之有效張力強度 (σft') 進行比較，以驗證無圍壓貫入試驗之準確性。 根據無圍壓貫入試驗的結果，試體直徑對無圍壓貫入試驗之影響不大，且試體細長比對無圍壓貫入試驗之影響也不大，但是當試體細長比為1.5時，無圍壓貫入試驗可能會受試體破裂行為而造成影響，而貫入棒直徑對無圍壓貫入試驗之影響很大。根據Lu等學者在2006和2010年的不飽和土壤有效應力架構，計算不同含水量在無圍壓貫入試驗之有效張力強度與在不飽和破壞包絡線下有效張力強度。結果顯示，σpt' 在不同含水量下之變異性很小，故不飽和土壤之有效張力強度並不會受含水量影響。在含水量為8.1、11.1 % 時，無圍壓貫入試驗與不飽和破壞包絡線之有效張力強度相當接近，代表無圍壓貫入試驗有很高之準確性。

Compared to the compressive or shear strength of soil, its tensile strength is generally assumed to be zero, or insignificant, in geotechnical engineering practice because of its relatively small value and lack of a satisfying laboratory technique. The tensile strength of soil is an important parameter in the design of geosystems, where tensile cracks contribute to progressive failure of landslides, stablility of dams, highway embankments, and other earth structures. In the literature, there are many methods of determining the tensile strength of soil. They can be categorized into direct and indirect methods. In the direct methods, the direct pull-out is used for tensile strength determination. In the indirect method, the split tensile test and the unconfined penetration test are often used for tensile strength determination. The tensile strength of the unconfined penetration test was developed within the theory of plastic limit analysis. Information including friction angle, unconfined compressive strength, the diameter of specimen, the height of specimen, the diameter of punch, maximum axial stress of the test and the 'K value' is required for calculating tensile strength in the developed formula. The K value was suggested to a fixed value by Fang and Fernandez’s research in 1981. However, the K value should be determined by testing configuration including the sizes of the specimen and punch (bH/a2), the ratio of the unconfined compressive strength to the tensile strength (qu/σt) and friction angle in the plastic limit analysis. In this study, compacted clayed sand specimens (with 85 % of sand and 15 % of clay by weight) were prepared for a series of the tests. The first part of the unconfined penetration tests is used to determine the tensile strength by iterative method. The effect of specimen diameter, the specimen slenderness ratio and the punch diameter are evaluated and discussed. The second part of the tests is based on the framework unsaturation soil mechanics, where the effective tensile strength, is respectively obtained by unconfined penetration tests (σpt') and unsaturated failure envelopes (σft'), at various water contents. Furthermore, it verifies the accuracy of unconfined penetration test. According to the result of the unconfined penetration tests, the diameters of the samples and the slenderness ratios of the samples do not affect significantly. However, the diameter of punches does influence the test. According to the result of comparing the effective tensile strength, which are calculated by unconfined penetration tests and the unsaturated failure envelopes, at various water contents, the σpt' varies slightly at different water contents, indicating the effective tensile strength of unsaturated soil will not be affected by the water contents. The σpt' and σft' are quite close when the water content is equal to 8.1 and 11.1 percent, which suggests that the unconfined penetration test be quite accurate in determining the tensile strength of lightly cemented sand.