近年來國內各項建設發展迅速，交通發展為當中重要一環，又以道路的鋪設為最常見的交通工程。台灣部分地區地質為含有高比例細粒料的軟弱土壤，淺基礎結構物興建時必然遭遇到如承載力不足與沉陷量過大及內部孔隙水壓影響等問題，所以有了運用加勁土壤的原理，來解決淺基礎構築於軟弱土壤上承載力不足及內部超額孔隙水壓產生問題。本研究以相同試體(南投眉溪砂)不同試驗條件(密度，乾濕度，加勁材有無，不同加載方式)，模擬無鋪面道路經輪胎底面快速加載之試驗，討論試體受利時內部的孔隙水壓、土壓力變化、基礎位移、加勁材受力情形，並藉由置入加勁材有無之影響，來觀察試體內孔隙水壓及承載力變化情形。比較加勁材對試體內部超額孔隙水壓之消散有無影響。

試驗結果，不同密度之試體，以不同加載方式測得之極限承載力與其對應密度試體所做之三軸試驗結果，得到一樣的承載力因數N_γ。加勁材用於淺基礎上，可使得淺基礎之承載力有明顯的提升，對於淺基礎之沉陷量的方面也有明顯的改善。此外，也發現於軟弱土壤上方加勁時，所得到之加勁效益較承載力較好之土壤高，所以地工加勁材非常適合解決淺基礎於軟弱土壤上方承載力不足之問題。飽和試體內置入加勁材，亦可以幫助超額孔隙水壓之消散，使試體內孔隙水壓越來越小，有效應力增大，改善土體承載力之效果，故使用加勁工法來改善土壤內孔隙水壓是可行的。

ABSTRACT

Effects of Porewater Pressure on the Failure Mechanism of Reinforced Ground

Advisor’s Name : Yo-Tang Liao
Ching-Chuan Huang
National Cheng Kung University Department of Civil Engineering
SUMMARY
Road is one of the essential infrastructures in the fast developing Taiwan. Lose sandy soil with high percentage of fine grain is found in some of the areas in Taiwan, and the problem of lacking bearing capacity, exceeding settlement, and the effect of internal pore pressure are issues to be solved in such areas. The theory of soil reinforcement is applied to solve the problems of shallow foundation building on soft soil and the problem caused by excess internal pore pressure. This study used the sand sample obtained from Meixi, Nantou, to perform tests under the conditions of different density, moisture content, with or without reinforcement material, and load types, to discuss the change of pore water pressure, soil pressure, foundation displacement, and force on reinforcement material in the case of fast applying load by tires on unpaved road. The effect of reinforcement material on the dissipation of pore water pressure within soil was also discussed.
From the results of tests performed using samples of different density, load types, and measured bearing capacity and its corresponding density, it shows that the obtained bearing capacity, N_γ, is the same. The reinforcement material was able to significantly increase the bearing capacity of shallow foundation, and at the same time reduce settlement. It is also found that the effect of reinforcement material is higher in soft soil. Moreover, reinforcement material was able to enhance the rate of dissipating pore water pressure and thus increased the effective stress and bearing capacity.

Key words: reinforced level ground、model test、 bearing capacity test、porewater pressure

INTRODUCTION
Loose sandy soils, especially when they saturated, can be a major problem in land development and highway engineering, because of the lacking of sufficient bearing capacity, or generating excessive settlements. A soil reinforcement technique called “reinforced earth slab” can be a potentially useful method for solving the problem. However, a majority of past researches focused on dry reinforced ground which provided very limited information for the applicability of reinforced earth slab to the case of saturated ground. To this end, a loading system consisting of a model sand box, a rigid footing and a diaphragm air cylinder (or a screw jack) is built to apply constant-loading-rate and constant-displacement-rate loads on level grounds formed by a sandy soil classified as SM. Factors investigated in the present study are soil densities, degrees of saturation, soil reinforcement, and cyclic loading frequencies.

MATERIALS AND METHODS
Loading tests are performed on reinforced and unreinforced sandy grounds to investigate various factors influencing the bearing capacity of strip footings. The model sandy ground (600 mm-deep, 500 mm-wide, and 1200 mm-long) consists of a sandy soil (classified as SM) with known physical and mechanical properties. The loading system consists of a rigid steel frame and a diaphragm air cylinder for applying constant-loading-rate loads (or a screw jack for applying constant-displacement-rate loads) on a 100 mm-wide strip rigid footing. Some wide-width tensile tests are performed to obtain the tensile properties of reinforcement and also to calibrate the output signals of strain gages. Two types of reinforcement materials are used: an extensible reinforcement which is a heat-bonded nonwoven geotextile with a breaking strength of 3.5 kN/m at a breaking strain of 13%, and an inextensible reinforcement which is an aluminum foil with a breaking strength of 1.6 kN/m and a breaking strain of 0.3%. Two types of strain gages with distinct strain limits compatible to the extensibility of reinforcement are used to measure the mobilized reinforcement forces during the tests. Three series of tests are performed:
Unreinforced dry ground with various densities and loading methods. This series of tests is to explore the performance of testing facility and also to gain fundamental understanding regarding the ultimate bearing capacity of footing.
Reinforced dry and saturated ground with monotonic and cyclic loading: This series of tests is to investigate the effect of ground saturation on the applicability of reinforced slab technique.
Reinforced saturated ground with various cyclic loading frequencies: This series of tests is to investigate the effect of excess pore water pressure on the soil reinforcement induced by various frequencies of cyclic loading.
Reinforced saturated ground with various extensibility of reinforcement: This series of tests is to investigate the influence of reinforcement extensibility on the behavior of reinforced saturated grounds.

RESULTS AND DISCUSSION
Major findings from the test results are discussed below:
The technique of reinforced earth slab is effective in increasing ultimate bearing capacity and stiffness of level grounds, both for dry and saturated conditions. This conclusion is regardless of the loading methods, i.e., for the cases of monotonic and cyclic loading, significant increases of bearing capacity are observed for dry and saturated level grounds.

The excess porewater pressure is also an important factor influencing the bearing capacity behavior of saturated level grounds. In the case of reinforced saturated ground, negative values of excessive porewater pressure were observed for the loaded ground. One the other hand, positive values of excessive porewater pressure were observed in the case of unreinforced ground. It is considered that the negative excessive porewater pressure generation is due to the confinement effect of reinforcement. The negative excess porewater pressure may increase the effective stress of soil which in-turn increases the bearing capacity of footings.
CONCLUSION
Based on the observations and analyses from the test results, the following conclusions are obtained:
(1) Experimental values of bearing capacity factor, N, agree well with the theoretical ones proposed by Meyerhof based on the internal friction angles obtained from the triaxial tests. This is true for the cases of dry and saturated unreinforced grounds, provided that the submerged unit weight is used for the case of saturated ground.
(2) Reinforced earth slab technique can be more effective for the case of saturated ground than the case of dry ground, in the sense that in the case of saturated ground, negative excess pore water pressures are generated I, due to the restraining of lateral soil deformation by the reinforcement.
(3) For the investigated range of cyclic loading frequencies, the effect of loading frequencies on the magnitude of excess pore pressures in the saturated unreinforced and reinforced grounds is not significant. That is, the magnitude of excess pore pressure under an undrained condition is regardless of the range of loading frequencies investigated here.
(4) The extensible (nonwoven geotextile) reinforcement seems to provide a better function of generating negative excess porewater pressures than the inextensible (aluminum) reinforcement. However, the influence of the negative excess pore water pressure on the ultimate bearing capacity is insignificant. This effect should be investigated further in the future.

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