Robert Parsons University of Kansas Milad Jowkar Berkel and Company Construction, Inc Jie Han University of Kansas

Transcription

1 Robert Parsons University of Kansas Milad Jowkar Berkel and Company Construction, Inc Jie Han University of Kansas

2 First a little about KU Engineering 139,000 ft 2

3 Structural Testing and Student Projects Facility (~25,000 ft 2 )

4 Strong Wall/Floor

5 Student Project Facility

6 On with the Program: Outline Problem description Research goal Testing Results Comments

7 Problem Description Ballast degrades over time Particles break down and become rounded Ballast is fouled Poor drainage Reduced strength ARTC artc.com.au

8 Tensar International Current practice - undercutting Ballast is removed from beneath the track Fines and small particles are screened out Ballast is recycled back into service This is a convenient time to add geogrid reinforcement to the ballast layer

9 Research Goal Attempt to quantify the benefit of using triaxial geogrid reinforcement with recycled ballast Construct and test two full scale sections Unreinforced Reinforced with a geogrid located 7 inches below the ties ballast (with fines) to achieve 2:1 slope 2 screened ballast subgrade 7 Tie 10 geogrid (reinforced test) 24 ballast (with fines) to achieve 2:1 slope

10 Subgrade Properties Classification (Unified Soil Classification System, USCS) CH LL ( liquid limit) 52 PL (plastic limit) 21 PI (plasticity index) 31 % < 0.075mm (#200) 51 % % < 0.002mm 39 % Max dry unit wt lb/ft 3 Optimum Moisture 23 % Undrained shear moisture 630 psf Undrained shear moisture 1900 psf Undrained shear moisture 3300 psf

11 Stress(Psi) Subgrade strength with moisture Unconfined Compressive Strength S 10 u = 630 psf 5 0 0% 2% 4% 6% 8% 10% 12% Strain % 17% m.c 23% m.c 27% m.c

12 Percent Passing Gradation of Recycled Ballast % KU Ballast 90% BNSF Lower Bound 80% 70% BNSF Upper Bound Sieve Size(in) Ballast is heterogeneous igneous rock 60% 50% 40% 30% 20% 10% 0%

13 Test Frame

14 Subgrade compaction Unreinforced: Max dry density = 92%, w = 26% Reinforced: Max dry density = 93%, w = 27% LWD and CBR were similar (LWD ~ 12-13MN/m 2, CBR ~ 2.0)

15 Test Setup

16 Geogrid used for Reinforced Test TX 190L

17 Loaded Section

18 Cyclic Loading Plan Unreinforced Test Load Step Cycles Loading Rate (Sec/Cycle) Target Supply Pressure (psi) Actual Average Total Load (lb) Tie Bearing Pressure (psi) (dry) (soaked) Reinforced Test 1 inch ± of rainfall Load Step Cycles Loading Rate (Sec/Cycle) Target Supply Pressure (psi) Actual Average Total Load (lb) Tie Bearing Pressure (psi) (dry) (soaked)

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20 Excavation-Unreinforced Test

21 Excavation-Unreinforced Test

22 Excavation-Unreinforced Test

23 Excavation-Reinforced Test geogrid

24 Excavation-Reinforced Test

25 Deformation(in) Deformation (In) Track Settlement Pump Pressure (psi) Unreinforced Test Pump Pressure (psi) Reinforced Test East String Pot 7 String Pot East

26 Pressure Cell Pressure (psi) Average Tie Bearing Pressure The 4 surviving cells break over after soaking. Cell 1 starts to break over before soaking Pressure Cell 1 Pressure Cell 2 Pressure Cell 3 Pressure Cell 4 Pressure Cell 5 10 Top View Tie Telltales N E Rail Tie Bearing Pressure (psi)

27 Pressure Cell Pressure (psi) Average Tie Bearing Pressure Higher and more even pressures observed at maximum load. No breakover before soaking Pressure Cell 1 Pressure Cell 2 Pressure Cell 3 Pressure Cell 5 Top View Tie Telltales N E Rail Tie Bearing Pressure (psi)

28 Settlement (in) Number of cycles vs. track settlement Number of Cycles Load for reinforced section was 4.4% lower than for the unreinforced section Load for reinforced section was 5.2% higher than for the unreinforced section Unreinforced East String Pot Reinforced East String Pot

29 Track Settlement (in) Settlement vs. Tie Bearing Pressure Tie Bearing Pressure (psi) water applied prior to next step unreinforced reinforced

30 Section settlements Reinforced Unreinforced 5.0" 18 % Strain 3.8" 6.1" 1.2" 7" 2.0" 28 % Strain upper ballast 4.1" 14 % Strain 2.4" 1.4" lower 10" ballast 1.1" 3.0" 11% Strain 10% Strain 24" 13% Strain subgrade

31 Percent Passing Grain Size Distributions % 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Sieve Size(in) Original material after sieve Unreinforced under tie Reinforced under geogrid Reinforced under tie

32 General Observations Unevenness of the subgrade after both tests was observed, but, based on visual inspection, was greater after the unreinforced test. During excavation after both tests, the lower 8 inches of ballast was damp. A higher percentage of small diameter material was observed beneath the ties after the unreinforced test than after the reinforced test.

33 Conclusions The reinforced section settled less than the unreinforced section. Settlement of the ballast between the ties and geogrid was substantially less than the settlement of the equivalent portion of the unreinforced test section. The observed reduction in settlement was more significant for larger deformations, which is consistent with normal geogrid mobilization behavior.

34 Conclusions For settlements of more than 1 inch, more loading cycles (traffic) were required to cause the same amount of settlement. More ballast breakdown resulting in the generation of small particles and dust was observed for the ballast beneath the ties in the unreinforced test than in the reinforced test. The reinforced test section subgrade supported more load prior to subgrade failure than the unreinforced test section.

35 Acknowledgments Mid-America Transportation Center BNSF Tensar International KU Laboratory Staff

36 Questions?

37 Robert Parsons University of Kansas AJ Rahman PSI Jie Han University of Kansas Thomas E. Glavinich University of Kansas

38 Background Fouling can lead to excessive track settlement and/or substructure failure. As fouling increases, permeability decreases. As permeability decreases, more water is retained in the ballast for longer periods and drainage under dynamic loading from trains is restricted. Increasing pore pressures results in lower effective stress and lower strength. Resistivity will be lower when more water is retained in the ballast.

39 Hypothesis A relationship should exist between fouling and resistivity where resistivity is lower for higher levels of fouling. Therefore it should be possible to estimate the level of fouling by measuring the resistivity of the ballast.

40 Key Question Can relationships between resistivity, permeability, and fouling be characterized through laboratory tests?

41 Wenner probe arrangement voltage measured between probes a a a Ground surface a approximate depth of measurement

42 Source Materials The ballast used to host the fouling materials was heterogeneous igneous ballast provided by BNSF. It was recovered during an undercutting operation in Gardner, Kansas. The crushed ballast was from the same source. The coal was from a BNSF line in Wyoming. The clay was a local clay from Lawrence, Kansas.

43 Gradation curve of the ballast aggregate

44 Percent Finer (%) Gradation curves for fouling materials Coal dust Crushed ballast Clay Grain-Size (mm)

45 Permeability/resistivity test setup

46 Actual test setup

47 Measured resistivity range for all fouling materials Resistivity Range (ohms-cm), in thousands Fouling ratio Percentage of Fouling Crushed ballast clay soil coal dust 10% % % % %

48 Measured permeability (log scale) versus fouling ratio for crushed ballast fines, clay and coal dust

49 Permeability (log scale) versus fouling index of fouled ballast

50 Resistivity (ohms-cm) Resistivity of fouled ballast (crushed ballast fines) versus time Time(min) 20% fouled 30% fouled 40% Fouled 50% fouled

51 Resistivity (ohms-cm) Resistivity of fouled ballast (clay) versus time Time (min) 20% fouled 30% fouled 40% fouled 50% fouled

52 Resistivity (ohms-cm) Resistivity of fouled ballast (coal dust) versus time % fouled 20% Time fouled (min) 30% fouled 40% fouled 50% fouled

53 Comparison between permeability Hydraulic Conductivity, k (cm.s) Resistivtiy (ohms-cm) and resistivity at 18th hour versus fouling ratio Crushed Ballast Fines (Hydraulic Conductivity) Clay (Hydraulic Conductivity) Coal dust (Hydraulic Conductivity) Crushed Ballast Fines (Resistivity) Clay (Resistivity) Coal dust (Resistivity) Fouling Ratio (%) 0

54 Large scale resistivity test setup

55 Large scale resistivity four point test Water was added just before the test, which should result in a higher water content and lower resistivity near the surface. Test Reading Soil Resistivity Depth (in) Test R (Ω) ρ (Ω-cm)

56 Conclusions A series of laboratory tests were conducted at the University of Kansas on fouled ballast obtained from Gardner, Kansas, and coal dust from Wyoming. The tests measured the permeability and resistivity of ballast fouled with three different fouling materials.

57 Conclusions Relationships between resistivity, fouling, and permeability were identified for crushed ballast fines, clay, and coal dust. This suggests there is significant potential for using resistivity measurements as a proxy for fouling and permeability.

58 Conclusions A large scale resistivity test on heavily fouled ballast was performed on a sample prepared outdoors with a near-surface wet condition (not shown here). Differences in resistivity were observed between the upper, wet layer and the drier, lower layer. These results show the potential exists for characterizing fouling and permeability in the field using resistivity.

59 Future Research Research is ongoing to evaluate the use of resistivity and other technologies for field estimation of fouling quickly and without excavation. DCP LWD Resistivity (multiple tools)

60 Use of resistivity in the field

61 Acknowledgments The authors thank the Mid-America Transportation Center (MATC) for providing financial support for the research described in this report and Mr. Hank Lees and Mike Wnek of BNSF for providing material and technical support for this research. Their participation is greatly appreciated.