Study on the Classification and Evaluation Method of the Frost Susceptibility of Rock Mass

Transcription

1 The International Symposium on Systematic Approaches to Environmental Sustainability in Transportation (ISSAEST), August 2-5, 2015, Fairbanks. Study on the Classification and Evaluation Method of the Frost Susceptibility of Rock Mass Caichu Xia Jihui Huang Department of Geotechnical Engineering Tongji University,Shanghai, China 4rd, August, 2015

2 Layout BACKGROUND FROST HEAVE RATIO OF ROCK AND FRACTURES FROST HEAVE RATIO OF SATURATED ROCK MASS EVALUATION OF THE FROST SUSCEPTIBILITY OF ROCK MASS CONCLUSION 2

3 1.BACKGROUND The severe frost damage phenomenon in tunnels The frost heave of rock mass is considered a main reason for frost damage in coldregion constructions. Little attention has been paid to the frost heave of rock mass, because the expansion of rock upon freezing is less noticeable due to the constrain of rock itself. The classification and evaluation of the frost susceptibility of rock mass is significant to the design of constructions in cold regions. 3

4 2. FROST HEAVE RATIO OF ROCK AND FRACTURES 2.1 Frost Heave Ratio of Saturated Rock Frost heave ratio ηr closed (%) Matsuoka(1990) Tao (1990) Kang et al.(2012) 9% volume expansion of porous water The frost heave ratio of rock is caused by 9% volumetric expansion of porous water upon freezing. Creep of ice 2.17%n closed r Porous ratio n 0 (%) For all kinds of rock: Closed r N is porous ratio. 2.17%n In opened system, the frost heave ratio of the rock increases because of the effect of water migration from the external water resource. It can be evaluated using the following equation: 2.17% n Open r 0 Fig.1 The experimental relation of frost heave ratio and porosity of saturated rock under closed system Table 1. Frost heave ratio of rocks Frost susceptibility Frost heave ratio(%) Porous Rock Closed Opened average r ratio n type 0 system system average r (%) Closed Closed r r Tuff Frost susceptible Tuff % Tuff Non-frost susceptible Andesite % Open Closed 4

5 2. FROST HEAVE RATIO OF ROCK AND FRACTURES 2.2 Frost Heave Ratio of Saturated Fractures Extrusion of water Water migration Freezing direction Rock Rock Rock No dilation Ice (a) (c) (e) Dilation Ice Dilation Ice Water migration Freezing direction Ice Ice Ice Extrusion of water Rock Extrusion of water (b) (d) (f) No dilation Rock Dilation Rock Dilation Fig.2. Typical Freezing dilation of saturated fractures upon freezing In open fractures: When rock or the filler is frost susceptible: In closed fractures: Water is extruded with the progression of the freezing process. No dilation f 0 Water migrates toward the freezing zone. Water is trapped. Dilation f 3% 5

6 3. FROST HEAVE RATIO OF SATURATED ROCK MASS 3.1 Frost Heave Ratio of Various Lithological Rock Masses The distribution of fractures in rock masses can be simplified as parallel and characterized with an interval L, width e, trend Ɵ, and plunge ɸ, as illustrated in Fig. 3 3 The frost heave strain of rock mass can be described as a tensor in following equation. z sincos e e r f 0 sin sin L L 0 0 cos (1) E 2 e L f Fig. 3 Rock mass with a set of parallel fractures 1 N The frost heave ratio of rock mass is described equation 2. n e e n r f (sini cosi sini sini cos i) L L i1 The maximum frost heave ratio of rock mass with n set fracture is described equation 3. arctan1 arctan 2 1 n 3n max f r f f nf ne / L is the volume ratio of the fracture. The value of n f is calculated for different grades of rock mass using the parameter provided by the code for design of road tunnel, the national standard of China. (2) (3) 6

7 3. FROST HEAVE RATIO OF SATURATED ROCK MASS 3.2 Calculation Method of the Frost Heave Ratio of Rock Mass The frost heave ratios of typical lithological rock masses under open system are computed using Eq. 3 with parameters in Tab. 2 and Tab. 3 The volume ratio of the fracture of Grade V of rock mass is largest, equals to 9.0%. 7

8 3. FROST HEAVE RATIO OF SATURATED ROCK MASS 3.2 Calculation Method of the Frost Heave Ratio of Rock Mass 8

9 3. FROST HEAVE RATIO OF SATURATED ROCK MASS 3.2 Calculation Method of the Frost Heave Ratio of Rock Mass It can be seen from Tab. 3 that: First, the greater the porous volume of rock, the greater the frost heave ratio, It is less than 1.42% for rock masses composed of non-frost susceptible rock and less than 2.09% for rock masses composed of frost susceptible rock. Second, the frost heave ratio increases with the grade of rock mass. Third, the frost heave ratio increases with the grade of weathering. 9

10 4. EVALUATION OF THE FROST SUSCEPTIBILITY OF ROCK MASS 4.1 Water Supply Condition Water is indispensable for frost heave of rock mass, the water supply condition should be considered while the evaluation of frost susceptibility of rock mass. rock mass under open system the rock mass can be considered saturated for frost susceptibility evaluation. the rock mass under closed system no frost heave occurs 10

11 4. EVALUATION OF THE FROST SUSCEPTIBILITY OF ROCK MASS 4.2 Classification of The Frost Susceptibility for Rock Mass Three factors in the process of the classification are considered: Table 4. The classification of the frost susceptibility of soil and rock mass according to their frost heave ratio The degree of potential damage to the construction as a result of the frost heave In the code for engineering geological investigation of frozen ground, the national standard of China The elastic modulus of frozen rock is 7.5 times that of frozen soil 11

12 4. EVALUATION OF THE FROST SUSCEPTIBILITY OF ROCK MASS 4.3 Evaluation Method of Frost Susceptibility the frost susceptibility of the rock and fracture filler Table 5. Evaluation of the frost susceptibility of rock mass in force-based design the grade of the rock mass the porosity of the rock evaluation of the frost susceptibility of rock mass In table 5, the frost susceptibility of rock mass is classified into 5 degree of frost susceptibility,that is, non-frost, weakly, moderately, strongly, and very strongly susceptible. 12

13 4. EVALUATION OF THE FROST SUSCEPTIBILITY OF ROCK MASS 4.4 Application The No. 1 Baimang Snow Mountain Tunnel is located in the Yunnan Province, China. The altitude of the tunnel is m above sea level. Table 6. Frost susceptibility of surrounding rock of the No. 1 Baimang Snow Mountain Tunnel using the proposed method The surrounding rock in the No. 1 Baimang Snow Mountain Tunnel is non-frost susceptible to strongly frost susceptible. 13

14 EVALUATION OF THE FROST SUSCEPTIBILITY OF ROCK MASS 4.4 Application The Jianglulin Tunnel is located in the Qinghai Province of China. Table 7. Frost susceptibility of the surrounding rock of the Jianglulin tunnel Most of the surrounding rock of the Jianglulin Tunnel is non-frost susceptible, but the surrounding rock near the fracture zone and the tunnel entrance is nonfrost susceptible to moderately susceptible. This evaluation is based on the survey report and should be updated during excavation. These results provide references for anti-frost damage design of the tunnel. 14

15 5. Conclusion A calculation method for the frost heave ratio of rock is developed by fitting the experiment data. The frost susceptibility of rock mass is classified into 5 types according to its frost heave ratio and based on the frost heaving classification of the soil. The frost susceptibility of the surrounding rock is assessed in two cold-region tunnels. Frost heave ratio of rock and fractures Frost heave ratio of saturated rock mass Evaluation of the frost susceptibility of rock mass Classification and evaluation method of the frost susceptibility of rock mass 15

16 ACKNOWLEDGMENTS This work has been supported by the National Natural Science Foundation of China (Grant No ), Key Technologies Research and Development Program of China (No. 2014BAG05B05), and two China Ministry of Communications Western Construction Science & Technology Projects(project numbers: ; ). 16

17 Thank you! 17