The Laboratory Test for Estimate Freezing Effect of Backfill Material on a Buried Pipe

Transcription

1 , pp The Laboratory Test for Estimate Freezing Effect of Backfill Material on a Buried Pipe Jaemo Kang 1,1, Du-Hee Park 2,2, Jangguen Lee 1,3,Pill-Jae Kwak 1,4, Sang-Hyuk Park 1,5, Hyun-Dong Lee 1,6, 1 Korea Institute of Civil Engineering and Building Technology, 283, Goyandae-Ro Ilsanseo-Gu, Goyang-Si, Gyeonggi-Do, , Korea 2 Hanyang University, 222, Wangsimni-ro, Seongdong-gu, Seoul, , Korea Abstract. This paper is intended to identify, through a full scale model test, the ground temperature variation around the buried pipe depending on variation of ambient temperature as well as the displacement of the buried pipe and the stress behavior, and consequently, stress concentration and frost heaving were observed around buried pipe when ground was frozen. And despite of using sandy soil which is considered non-frost susceptible material for the test, frost heaving was occurred depending on water content and drainage condition. Viewing such result, even in case of displacing bedding and backfill material with non-frost heaving material, the countermeasure to protect from frost heaving is necessary depending on surrounding drainage condition, groundwater level and water contents. Keywords: Frost heaving, Frost susceptibility, Buried pipe, Backfill, Drainage 1 Introduction The techniques for predicting ground expansion caused by freezing were proposed in many studies (Kim et al., 2010; Kang et al., 2013) but the study to identify the stress behavior of buried pipe through a full-scale test has been far behind. US Army Corps of Engineers proposed the classification method to determine the frost susceptibility of soil in a bid to prevent the damage by frost heaving and even in case of using sandy 1 The Ph.D. student, Dept. of Civil and Environmental Eng. Hanyang University / Researcher, Korea Institute of Civil Engineering and Building Technology ( , jmkang@kict.re.kr) 2 Associate Prof., Dept. of Civil and Environmental Eng. Hanyang University ( , dpark@hanyang.ac.kr, Corresponding author) 3 Senior Researcher, Korea Institute of Civil Engineering and Building Technology 4 Senior Researcher, Korea Institute of Civil Engineering and Building Technology ( , pjkwak@kict.re.kr) 5 Senior Researcher, Korea Institute of Civil Engineering and Building Technology ( , karasin811@kict.re.kr) Corresponding author 6 Senior Researcher Fellow, Korea Institute of Civil Engineering and Building Technology ( , hdlee@kict.re.kr) ISSN: ASTL Copyright 2015 SERSC

2 soil or gravel with larger particle size, it requires identifying the frost susceptibility of the ground through lab test (US Army TM , 1985). To identify the occurrence of frost heaving depending on ground saturation and drainage condition despite of installation in compliance with current standard, effect on buried pipe was monitored under undrained condition at below zero temperature after saturating the specimen. Model test was conducted at large chamber and to maintain the temperature for test purpose, chamber was installed in freezing chamber separately. 2 A Full-Scale Model Chamber Test A full-scale model test was conducted to identify the actual behavior of buried pipe in undrained and saturated non-frost susceptible ground. (a) Cross-sectional diagram of test chamber (b) Plane diagram of test chamber Fig. 1. Schematic diagrams of chamber test (c) Thermocouple attached bar The chamber was 2 m 2 m 1.5 m and was designed to withstand maximum 10 ton at 40 or below. To simulate the freezing temperature, the chamber was placed in freezing chamber (see Fig. 1). Copyright 2015 SERSC 19

3 3 Analysis of the Result To monitor the ground temperature variation around buried pipe depending on freezing chamber, temperature sensors were placed on column at 15cm interval as seen in Fig. 1(c) so as to check the temperature variation by depth and temperature distribution depending on distance from buried pipe with the layout in Fig. 1(a) and Fig. 1(b). As a result of monitoring temperature variation around the chamber, no variation depending on distance was monitored but the depth. Temperature distribution by depth depending on freezing time at points A and D, the nearest arrays from the buried pipe (see Fig. 1(a)) is indicated in Fig 2. Fig. 2. Temperature distribution in the specimen Fig. 3 shows the variation in temperature and displacement at the pipe center. Temperature variation was based on average value measured by temperature sensor(d-1, E-1) placed at similar depth as buried pipe (Fig 1). While surrounding temperature dropped rapidly and maintained at a certain level due to phase change in pore water, displacement of buried pipe was monitored insignificant. As specimen temperature falls below zero when pore water phase change was finished, swelling occurred at the bottom of buried pipe leading to frost heaving. 20 Copyright 2015 SERSC

4 Fig. 3. Temperature and displacement at the middle of the buried pipe In conclusion, strain of buried pipe occurred during freezing in winter but also in spring when air temperature begins rising because of movement of pore water while deep ground was still frozen. In general, rupture to buried pipe was considered to be caused by strain by settlement resulting from ground thawing. Given the rupture in spring is attributable to potential heaving force at the bottom of buried pipe, site investigation seems to be necessary to verify the reliability of the result. Conclusion This study is intended to identify the effect of the stress by frost heaving on rupture or deformation of buried pipe. Frost heaving test also indicated the occurrence of frost heaving in saturated sandy soil and a full-scale model test was conducted to monitor the effect on buried pipe and the result is as follows. 1. In general, rupture to buried pipe was considered to be caused by strain by settlement resulting from ground thawing. Given the pipe rupture occurred in spring was attributable to movement of melted pore water due to freezing at the bottom which causes potential heaving force, site investigation is necessary to verify the reliability of the result. 2. As a result of a full-scale model test, maximum displacement of buried pipe at freezing index 300 day was 20.2mm which corresponds to 40.4% of pipe diameter 50mm which far exceeds allowable deflection 5% as defined in water pipe design or buried pipe design criteria and is interpreted as performance deterioration or problem with stability. 3. According to the test result, even in case of using sandy soil which is considered the nonfrost susceptible, frost heaving may occur depending on water content and ground drainage condition and to secure the stability of buried pipe, it s necessary to verify the frost susceptibility of backfill material and drainage condition in surrounding ground. Copyright 2015 SERSC 21

5 Acknowledgements. This research was supported by the research project Development of Technology for Life Detection and Rapid Rescue in Disaster Area of Urban Underground Collapse" funded by the Korea Institute of Civil Engineering and Building Technology. References 1. Kang, J.M., Kim, Y.S. and Lee, J.: Evaluation Method of Frost Heave for Unsaturated Soils, Journal of the Korea Geosynthetics Society, Vol.12, No. 1, pp (2013) (in Korean). 2. Kim, Y.S., Kang, J.M., Hong, S.S. and Kim, K.J.: Heat Transfer Equation and Finite Element Analysis Considering Frozen Ground Condition the Cyclic Loading, Journal of the Korea Geosynthetics Society, Vol.9, No.3, pp (2010) (in Korean). 3. U. S. Army Corps of Engineers : Pavement Design for Seasonal Frost Conditions, TM , pp.2-5 (1985) 22 Copyright 2015 SERSC