การทำนายการทรุดตัวของคอสะพานในบริเวณดินอ่อนโดยใช้หน่วยแบริ่ง / ทินกร โรจนธารา = Settlement prediction of bridge approach on a soft foundation using Bearing Unit / Tinnakorn Rojanathara
The major problems of highway construction on the soft foundation over Bangkok area, which have the heavy traffic volumes, are slope stability and settlement of an embankment. The slope stability problem can be solved by effective methods, while the settlement is usually ignored in solving, especially, the differential settlement between bridge and its approach. The differential settlement occurs because the bridge is on a pile foundation which has a little settlement, but its approach, is on a shallow foundation, so it has more settlement. This problem causes an interrupted smooth grade, surface irregularities., uncomfortable ride and costly maintenance. The objectives of this research are to study the settlement characteristic of bridge approach on a soft clay using bearing unit and to analyze the performances of settlement prediction methods. The study will benefit engineer for the future road design, construction and maintenance. The Klong Palat Praing bridge approach, station 6+402.755 of the Bang Na-Bang Pakong highway improvement section I, was studied. There were 13 settlement plates and 11 open stand pipe piezometers along the bearing unit to observe settlement and excess pore pressure respectively. Undisturbed samples to 29 meters below mean sea level were collected for Laboratory testing and seven field vane shear testes were performed in order to obtained parameters for settlement analysis. The settlement predictions of pile foundation are used for analysis. There are two methods as followed. 1. Conventional method ; 1.1 Terzaghi's method 1.2 Tomlinson's method 2. Modified theory of elasticity called Poulos's method For checking the results of hand computation, the author also used program of settlement analysis of pile foundation (SAPF) developed by Pongchai (1985)for IBM personal computer under disk operation system (DOS) in version 3.0. The reliability of settlement prediction by each method was also studied. The observed settlement, at the left shoulder 3.9 meters from the center line of road way closed to an abutment, was 1.5 cm. in 9.8 months after construction. The bearing unit settled 21.3 cm. at 50 meter from bridge abutment and the settlement of 14.00 cm. was measured at its middle. At present 86.96 to 97.10% of total settlement was observed if Asaoka's method of total settlement prediction was correct since was the settlement obtained by observed. The results of settlement prediction by Terzaghi's method indicated high settlement ratio about 1.18-1.82 to Asaoka's method. Tomlinson's method indicated a wide range of settlement ratio, so it couldn's be concluded. The best method was the modified theory of elasticity by Poulos's method which indicated settlement ratio about 1.02 and 1.67 when they were analysed by settlement of single pile foundation, included of interaction of other piles in the foundation and pile cap diameter effect was 7 times of single pile diameter or equal to 62% of the spacing, also using dead load of approach embankment. The Asaoka's graphical method using Terzaghi one dimensional theory show a good prediction of rate of settlement and indicates the field coefficient of consolidation about 59 to 104 times of laboratory coefficient of consolidation.
