Design Parameters of Micropile in Permafrost Sandy Ground

Transcription

1 IWM 2014 Krakow, Poland June 12, 2014 Design Parameters of Micropile in Permafrost Sandy Ground Changho Choi, PhD., PE Joonyong Lee, Tae-Hyung Kim, Sung-Gyu Ko Korea Institute of Construction Technology

2 Contents 1. Research overview 2. Research objectives 3. Experimental test 4. Test results 5. Conclusion and future work 6. Why micropiles?

3 1. Research Overview 1) Engineering Characteristics of Frozen Ground - Classification of frozen ground a. Seasonal frozen ground Freezing during winter season at the surface b. Permafrost Frozen ground regardless of season - Ground temperature of permafrost Active layer Permafrost layer Unfrozen layer Ground Temp. Distribution of Permafrost

4 1. Research Overview 2) Definition of Adfreeze Bond Strength - Adfreeze bond (adfreezing) : Bonding of frozen soil and other materials - Adfreeze bond strength : Maximum stress measured during tangential failure between frozen soil and other materials Soil Pile Ice Bonding of Ice bar and tongue is a kind of ADFREEZING Design Parameter for Pile Foundation in Frozen Ground 4

5 1. Research Overview 3) Pile Design in Frozen Ground Design Step Bearing Capacity Estimation Uplift Force Check Creep Loading condition Weight+ Down drag force due to thawing < Adfreeze Strength Uplift adfreeze bonding force due to freezing < Weight + Adfreeze Strength Creep settlement Check Winter PWeight Summer Uplift adfreeze force due to ground freezing Adfreeze bond strength Q ad Qad P I L E Q df Q ad Down drag force due to ground thawing Adfreeze bond strength

6 2. Research Objectives Material Type τ a = r τ 1) Weaver and Morgenstern(1981) s f Steel 0.6 Concrete 0.6 Uncreosoted Timber 0.7 Corrugated Steel 1.0 Soil types and freezing temperature were not considered 2) In this study Suggest a reasonable method to characterize the shear strength of the frozen soils and the adfreeze bond strength with different normal stress and freezing temperature conditions. Obtain r s with different soil types and freezing temperature condition and compare it past research Provide a key parameters for design of pile in cold regions r s 6

7 3. Experimental Test 1) Freezing Chamber - Temp. Control upto -20 degree Celsius with ±1 degree precision - Size of chamber : 2000mm(W) 3000mm(L) 2500mm(H) 10/34

8 3. Experimental Test 2) Direct Shear Device for Frozen Soils - Working Temperature for the all parts : Max Horizontal Displacement Speed : 0.5mm/min - Shear Box size : 100mm 100mm 40mm (Height) - Working Temperature for the all parts : Max Horizontal Displacment Speed : 0.01mm/min - Shear Box size : 100mm 100mm 40mm (Height) 10/34

9 3. Experimental Test 3) Soil Properties - Use two type of soils; 1) standard sand (Joomoonjin Sand), 2) weathered granite soil which is classified as SW with 10% of #200 passing particles J.M. W.G G s Joomoonjin Sand e max e min D C u C c USCS SP SW #200 0 ~10 Passing Percentage (%) Weathered Granite Dr(%) S(%) w(%) Particle Diameter (mm)

10 3. Experimental Test 4) Shear Strength Test Procedure - Fill the shear box with three layers (controlled with weight of soil for each layer) and compact with three different compacting devices - Saturate the specimen in water tank for 24hrs, place on the shear device inside chamber and then freeze it to target temperature for 24hrs. Compactor Compactor Compactor Three Layer Compactors 10

11 3. Experimental Test 5) Adfreeze Bond Strength Test Procedure - Specimen is prepared by inserting the aluminum pedestal at the bottom of shear box and fill the half of shear box with soils - Again, saturate the specimen in water tank for 24hrs, place on the shear device inside chamber and then freeze it to target temperature for 24hrs. i) Inserting the aluminum ii) Filling with sand iii) Compaction 11

12 3. Experimental Test 6) Failure shape of shear strength and adfreeze bond strength test Top Bottom After failure 12

13 4. Test Results Freezing Temperature ( ) Vertical Stress (kpa) Joomoonjin Sand Weathered granite soil Shear Strength Adfreeze Bond Shear Strength (kpa) Strength(kPa) (kpa) Adfreeze Bond Strength(kPa) Both shear -2 strength 100 and adfreeze 1624 bond strength 239 of two soils 543 increases 123 as the temperature went down and normal stress increased Both shear -5 strength 200 and adfreeze 3160 bond strength 816 of Joomoonjin 1690 sand 389 were smaller than those of weathered granite soil at 0, whereas those of Joomoomjin sand were larger than those of weathered granite s o il be lo w Th is was due to different ic e binding capacity with s o il pa rtic le s, and was strongly -15 affected by 100 a highly - c o mple x inte- ra c tio n between 2916 the s 961 o lid s o il skeleton -15 and the pore 200 ma trix, composed - of ic e - and unfrozen 3050 water

14 4. Test Results Joomoonjin Sand Weathered Granite Strength (kpa) Shear strength of frozen soil Adfreeze bond strength 100 kpa τ f (T)=-22.97T T R 2 = τ a (T)=-1.44T T R 2 =0.99 τ a (T)=0.21T T R 2 =0.97 increase rate 0 of those converged between and Temperature ( ) Temperature ( ) Strength (kpa) Shear strength of frozen soil Adfreeze bond strength 100 kpa τ f (T)=-5.06T T R 2 =0.99 Shear strength of two frozen soils increased dramatically until - 10, but Adfreeze bond Shear strength strength of frozen soil τ f (T)=-25.94T showed T a similar Shear strength pattern of frozen soil to shear strength 6000 Adfreeze bond strength R 2 = Adfreeze bond strength characteristics, even though increment of adfreeze bond strength due to temperature 300 and kpa normal stress conditions was 300 different kpa from that of shear τ f (T)=-10.75T T strength. R 2 =0.99 Strength (kpa) τ a (T)=-5.46T T R 2 =0.99 Strength (kpa) τ a (T)=0.0014T T R 2 = Temperature ( ) Temperature ( )

15 4. Test Results Joomoonjin Sand Weathered Granite kpa 200 kpa 300 kpa kpa 200 kpa 300 kpa r s (=τ a /τ f ) 0.4 r s (=τ a /τ f ) Temperature ( ) Temperature ( ) The coefficient r s decreases at temperature range between 0 and -5 degrees Celsius, and the ratio becomes constant as temperature decreases below -5 degrees Celsius. This indicates that the temperature condition has an influence on the proportional coefficient in initial freezing condition. Also, the normal stress condition has an influence on the proportional coefficient in initial freezing condition in case of weathered granite soil

16 4. Test Results Coefficient r s from this study and previous studies at Result of this study(joomoonjin sand) Result of this study(weathered granite soil) Weaver and Morgenstern(1981) Ladanyi and Theriault(1990) r s (=τ a /τ f ) Normal Stress (kpa) Both shear strength and adfreeze bond strength characteristics of frozen soils are governed by the intrinsic material properties such as grain size, ice and water content, air bubbles, and by externally imposed testing conditions such as temperature, freezing time, and strain rate. This means that field environmental conditions should be considered in determination of shear strength and adfreeze bond strength characteristics. 16

17 5. Conclusion and Future Works In this study, direct shear testing inside of a large-scale freezing chamber was conducted in order to analyze the shear strength and adfreeze bond strength characteristics with varying freezing temperatures and normal stress with two soil types, and the relationship between the adfreeze bond strength and shear strength of frozen soils was analyzed in terms of ratio rs. The test equipment described in this paper provides a promising approach to obtaining foundation design parameter, adfreeze bond strength, in frozen ground. According to our test results the previous proportional coefficients describing the relationship between shear strength and adfreeze bond strength should be used in care to obtain true adfreeze bond strength characteristics in all conditions. For pile design in frozen ground, it is recommended to compare the design parameters from several research results and obtain the most suitable one for the field conditions. 17

18 5. Conclusion and Future Works Df = 1 Df = 2 Fract al Dim. For Constr. Mat. (Brandt and Prokopski, 1993) m (Weaver and Morgenstern, 1981) Material Fractal Dimension Steel C o n c re te Steel Concrete cement paste Mortar τ = a r τ s f 18

19 6. Why Micropiles? This research work can be generally applied for foundation design in permafrost area such as Siberia, north Canada, etc. S. Korea has built a new Antarctic research station, JanBoGo research base in this Feb. and there is a need to small pile construction technology because the transportation of construction machine is key issue. That is why micropile method has got interest. 19

20 6. Why Micropiles? Korea- Paris(12hr)- Santiago(15hr)- Punta Arenas(4hr) Anta rc tic Chile Ba s e - King Se jong Ko r e a Station (1hr) 세계지도, Route 표시 20 20

21 6. Why Micropiles? - KSJ JangBoGo 21

22 6. Why Micropiles? - 22

23 6. Why Micropiles? - 23

24 6. Why Micropiles? - Elevated Building 24

25 6. Why Micropiles? 25

26 Gentoo Penguin 26

27 Who is Bad Guy? 27

28 Thank You!

29 2. Pile Design in Frozen Ground P u (=πdl f ) P+W p +P t +P s L f : depth of frozen layer in active zone L t : depth of thawed layer in active zone L e : depth of permafrost zone W p : weight of foundation P : downward load P t : downward skin friction force of thawed layer in active zone P s : downward skin friction force of permafrost zone P u : upward skin friction force of frozen layer in active zone τ u : adfreeze bond strength of frozen layer in active zone P+W p +P d P s (=πdl e τ a ) L a : depth of active layer L e : depth of permafrost zone W p : weight of foundation P : downward load P d : downward skin friction force of active layer P s : upward skin friction force of permafrost zone τ a : adfreeze bond strength of active layer