Prepared By: Curtis Berthelot Assistant Professor of Civil Engineering University of Saskatchewan

Transcription

1 COLD IN PLACE ROCK CRUSHING AND STABILIZATION OF NORTHERN LOW VOLUME ROADS: A CASE STUDY APPLICATION OF GROUND PENETRATING RADAR FOR COLD-IN-PLACE RECYCLED ROAD SYSTEMS Prepared By: Curtis Berthelot Assistant Professor of Civil Engineering University of Saskatchewan Ron Gerbrandt Preservation Engineer Saskatchewan Highways and Transportation Larry Safronetz Area Manager Saskatchewan Highways and Transportation Gordon Sparks Professor of Civil Engineering University of Saskatchewan Presented at 80 th Annual Meeting, Transportation Research Board Washington D.C. CDROM Proceedings Paper # January, 2001

2 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 1 ABSTRACT Saskatchewan Department of Highways and Transportation (SDHT) is responsible for maintaining approximately 6500 kilometers of northern gravel surfaced roads. Many of these northern gravel roads are built on poorly graded sand subgrades and may contain protruding bedrock and/or large boulders. Because of this, washboarding, protruding rocks, rutting and potholes are common performance problems of many northern gravel roads. Routine blading of these roads is often ineffective because unstable sand does not maintain its shape and compaction, protruding bedrock and boulders damage motor grader blades, boulders may become dislodged leaving holes in the road, and dislodged boulders are a safety hazard when windrowed along road side-slopes. Clay capping and base stabilization have been used to provide a stable wearing surface, cover protruding bedrock and large boulders, and reduce traffic dust. However, the long-term performance of clay capping and base stabilization can be highly variable and the associated costs can make these conventional solutions untenable. As a result, SDHT investigated the use of in-place rock crushing and stabilization/modification for northern gravel roads with significant proportion of boulders in the grade using a rotomixer/stabilizer. Based on the findings of this study, inplace rock crushing and stabilization/modification is a technically feasible solution for eliminating protruding bedrock and boulders contained near the surface. However, in-place crushing of boulders with unconfined compressive strengths over 50,000 psi resulted in significant damage to the rotomixer mandrel resulting in an approximate cost of just under $13,684 CDN per kilometer. in situ rock crushing with subgrade stabilization and double seal was found to cost approximately $52,017 CDN per kilometer.

3 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 2 1 INTRODUCTION Saskatchewan Department of Highways and Transportation (SDHT) is responsible for maintaining approximately 6500 kilometers of northern gravel roads. Many of these gravel roads are built on poorly graded sand subgrades and may contain protruding bedrock and/or large boulders. Because of this, these roads may exhibit wash boarding, protruding rocks, rutting and potholes which results in decreased service level and potential driving hazards. Routine blading is often ineffective because unstable sand does not maintain its shape and compaction, protruding bedrock and boulders damage motor grader blades, boulders may become dislodged leaving holes in the road, and dislodged boulders are a safety hazard when windrowed along road side-slopes. In the past, clay capping and base stabilization have been used to provide a stable wearing surface, cover protruding bedrock and large boulders, and reduce traffic dust (1). However, the long-term performance of clay capping and base stabilization can be highly variable and the associated costs can make them untenable. As a result, significant benefits could result from a more effective maintenance and upgrade treatment for northern gravel roads characterized by protruding boulders and bedrock and/or comprised of unstable sand subgrades. In the search for more appropriate methods for maintaining northern gravel roads, an in-place rock crushing and stabilization pilot project was undertaken on Highway in 1999 between kilometers and Historic performance problems with Highway include rough road surface due to severe potholes, rutting, protruding boulders and washboarding. Routine blading of Highway has been relatively ineffective due to unstable subgrade conditions and protruding boulders. Commercial traffic on Highway is limited to supplies to the Patunak First Nations Reservation, which is comprised of approximately 1500 residents. There is no residential or commercial development along Highway PRELIMINARY SITE INVESTIGATION A preliminary site investigation and visual site survey was performed along Highway in early summer Figure 1 and Figure 2 show protruding boulders and unstable sand subgrade conditions common to Highway

4 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 3 FIGURE 1 Highway Typical Boulders In Grade and Along Road Edge FIGURE 2 Highway Typical Unstable Sand Subgrade

5 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 4 In situ subgrade soil samples were retrieved from Highway The resulting grain size analysis from each sample is illustrated in Figure 3. As seen in Figure 3, Highway subgrade is comprised of uniform sand with approximately 15 to 20 percent fines. Atterburg limit characterization of the fines portion determined the fines portion to be non-plastic. AASHTO classification determined the subgrade to be non-plastic fine sand (A-3) and the Unified Soil Classification System determined the subgrade to be poorly graded sand with high non-plastic fines content (SM). Standard Proctor moisture density characterization was performed as per ASTM protocols and is shown in Figure 4. As can be seen in Figure 4, the optimum standard proctor density was found to be approximately 2070 kg/m 3 at 7.5 percent moisture content and the optimum modified proctor density was found to be approximately 2160 kg/m 3 at 6.5 percent moisture content % Passing Sieve Size 0.45 (mm) Kilometer 10 Kilometer 20 FIGURE 3 Highway in situ Subgrade Grain Size Distribution

6 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: Dry Density (kg/m 3 ) Moisture Content Percent 11 Modified Proctor Standard Proctor FIGURE 4 Standard and Modified Proctor Moisture-Density of SDHT Highway Subgrade In situ boulders were sampled from Highway , were identified as to their type, and characterized with respect to unconfined compressive strength. The rock type identification and unconfined compressive strength measurements are summarized in Table 1 and illustrated in Figure 5. As seen in Table 1 and Figure 5, the in situ boulders exhibited mean unconfined compressive strengths ranging from MPa (24.0 ksi) for weathered granitoid to MPa (51.9 ksi) for fine-grained metamorphic basalt.

7 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 6 TABLE 1 Highway in situ Rock Unconfined Compressive Strength Measurements Rock Type Sample Length (mm) Sample Width (mm) Sample L/D Ratio Load (KN) UCS (ksi) UCS (MPa) Fine Grained Metamorphic A Fine Grained Metamorphic B Granite A Granite B Pegamite A Pegamite B Pink Granitoid A Pink Granitoid B Weathered Granitoid A Weathered Granitoid B Weathered Granitoid A* Weathered Granitoid B Grey Granitoid A Grey Granitoid B * Sample contained internal fracture plane. Mean UCS (MPa) Mean UCS (MPa) Fine Grained Metamorphic Granite Pegamite Pink Granitoid Weathered Granitoid Grey Granitoid Weathered Granitoid Fractured FIGURE 5 SDHT Hwy in situ Rock Unconfined Compressive Strength Measurements

8 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 7 3 TEST SECTION DESIGN, LAYOUT, AND CONSTRUCTION The test sections constructed on SDHT Hwy were designed and constructed around the objective to evaluate the effectiveness of crushing protruding boulders in-place, and stabilizing the in situ gravel road sand subgrade. The stabilizers considered in this study included low plastic clay, calcium chloride, DL10 Special asphalt emulsion, and SS-1 asphalt emulsion. The in-place rock crushing and stabilization test sections constructed as part of the Highway test site include: a) In-place rock crushing, reshaping, and recompaction with no stabilization (control section). b) In-place rock crushing, reshaping, recompaction and surface modification with low plastic clay. c) In-place rock crushing, reshaping, recompaction and stabilization/modification with calcium chloride. d) In-place rock crushing, reshaping, recompaction and stabilization/modification with low plastic clay and calcium chloride. e) In-place rock crushing, reshaping, recompaction and stabilization with DL-10 asphalt emulsion. f) In-place rock crushing, reshaping, recompaction and stabilization with SS-1 asphalt emulsion. Stabilizer concentrations and layer thicknesses were based on the characterization results of the subgrade and the properties of the alternative stabilizers/modifiers considered in this study. The stabilized test sections are summarized in Table 2. As seen in Table 2, the asphalt emulsion stabilized test sections considered in this study included the addition of DL10 Special and SS-1 at concentrations of 2.5, 4.5 and 6.5 percent. Given a residual asphalt cement content of approximately 60 percent, the residual asphalt cement content in the field was 1.5, 2.7 and 3.9 percent by dry weight of soil respectively. Table 3 summarizes and Figure 6 illustrates the Marshall stability after a seven-day cure and the retained Marshall stability after a seven-day cure and a 48 hour soak of each SDHT Hwy stabilized material system. Marshall stability was chosen as the relative measure of stability because it is commonly understood and well referenced. As also seen in Table 3 and Figure 6, the Marshall stability of the Highway 918 subgrade soil was significantly improved with the addition of asphalt emulsion whereas the addition of the calcium chloride and clay only marginally increased Marshall stability. When soaked for 48 hours, the retained stability of the unmodified Highway 918 subgrade and that of the Highway 918 subgrade with clay and/or calcium chloride modification was negligible. However, the retained stability of the Highway 918 subgrade modified with asphalt emulsion was found to be only slightly less than that observed from the unsoaked samples, illustrating the moisture resistance of the asphalt emulsion stabilized soil.

9 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 8 Chainage Start (km) Chainage End (km) TABLE 2 Highway Test Sections Section Length (m) Stabilizer Type and Amount Surfacing Gravel (Tonnes) Surfacing Double Seal (m) none % clay (150 tonnes) % clay (150 tonnes) % clay (200 tonnes) % clay (100 tonnes) % clay (75 tonnes) % clay (25 tonnes) 1 l/m % clay (25 tonnes) 2 l/m % clay (25 tonnes) 3 l/m l/m 2 CaCl l/m 2 CaCl l/m 2 CaCl None None % DL10 Special % DL10 Special % DL10 Special % SS % SS % SS none Totals TABLE 3 Highway Subgrade Marshall Stability Across Stabilizer Types and Concentrations Stabilizer Type and Concentration 7-Day Cured Marshall Stability (KN) 7-Day Cured and 48- Hour Soak Marshall Stability Unmodified % Clay % Clay % Clay % Clay 11/m 2 CaCl % Clay 21/m 2 CaCl % Clay 31/m 2 CaCl /m 2 CaCl /m 2 CaCl /m 2 CaCl % DL10 Special % DL10 Special % DL10 Special % SS % SS % SS

10 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 9 Mean Marshall Stability (KN) Unmodified 10% Clay 15% Clay 20% Clay 10% Clay 11/m2 CaCl 10% Clay 21/m2 CaCl 10% Clay 31/m2 CaCl 1 1/m2 CaCl 2 1/m2 CaCl 3 1/m2 CaCl 2.5% DL10 Special 4.5% DL10 Special 6.5% DL10 Special 2.5% SS-1 4.5% SS-1 6.5% SS-1 7-Day Cure 7Day Cure-48 Hour Soak FIGURE 6 Marshall Stability of SDHT Highway Stabilized Systems Figure 7 through Figure 14 show the process used to construct the Highway test sections. The process includes primary in-place rock crushing with a rotomixer/stabilizer CMI650 with mandrel cutting down (Figure 7). Although the CMI R/S 650 did crush almost all in situ boulders, it provided on average a 200 mm minus crush, which required raking with a motor grader to remove the larger pieces of crushed rock out of the grade to achieve effective shaping and compaction (Figure 8). Following the in-place rock crushing process; primary shaping and compaction of subgrade (Figure 9); secondary in-place rock crushing and injection of stabilizers using the CMI R/S 650 with mandrel cutting up (Figure 10); final shaping and compaction (Figures 11 and 12); and application of surface coarse traffic gravel or double seal (Figures 13 and 14) was performed.

11 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 10 FIGURE 7 Primary Rock Crushing FIGURE 8 Windrowing and Separating Crushed Rocks

12 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 11 FIGURE 9 Clay Modification FIGURE 10 Asphalt Emulsion Stabilization

13 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 12 FIGURE 11 Final Shaping and Compaction of Grade FIGURE 12 Typical Compacted Final Grade

14 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 13 FIGURE 13 Typical Gravel Wearing Coarse FIGURE 14 Typical Asphalt Emulsion Stabilized Wearing Coarse

15 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 14 4 CONSTRUCTION COST EVALUATION The total construction cost for each test section was equilibrated to a per kilometer basis segmented with respect to labor; equipment; gravel; stabilizer/modifier; miscellaneous costs including fuel, teeth, and housing for the crew; and double seal surfacing as summarized in Table 4 and illustrated in Figure 15. As seen in Table 4 and Figure 15, a significant portion of the cost to crush boulders in-place was equipment costs. Because the CMI R/S 650 employed in this project is designed to rotomix asphalt concrete with an unconfined compressive strength of one to five MPa at room temperature, boulder unconfined compressive strengths as 358 MPa inflicted severe mandrel damage when high concentrations of boulders were encountered. During the 7.25 kilometers of rotomixed road, $13,222 of mandrel teeth was consumed. However, this pilot was intended to be a worst case scenario and therefore, sections of Highway with high concentrations of boulders were selected in order to provide an extensive field evaluation of the in-place rock crushing process. It is believed that redesigned mandrel will improve efficiency, reduce teeth costs, and result in fewer equipment breakdowns. Figure 16 illustrates the mean total construction cost by test section type. As seen in Figure 16, the mean construction cost per kilometer ranged from $13,684 for in-place rock crushing and a gravel wearing coarse to $52,017 for in-place rock crushing, asphalt emulsion stabilization and a double seal wearing coarse. TABLE 4 Highway Test Section Construction Cost Per Kilometer by Cost Type Test Section (m) Gravel Cost ($/Km) Stab. Cost ($/Km) Seal Cost ($/Km) Misc. Cost ($/Km) Total Cost ($/Km) Labor Equip Test Section Cost Cost No Stabilization 640 3,127 11,931 1, ,220 20,122 20% Clay 500 3,104 11,407 2, ,022 21,296 20% Clay 500 3,353 12,320 2, ,264 21,296 15% Clay ,617 7,542 2, ,263 14,782 15% Clay 460 3,515 16,613 2, ,263 25,956 10%Clay-2/3l/m 2 CaCl 500 2,453 10,122 2,360 5, ,596 22,940 10%Clay-1l/m 2 CaCl 200 6,133 25,304 1,475 2, ,595 38,335 10%Clay-2l/m 2 CaCl 200 6,133 25,304 1,475 4, ,595 39,905 10%Clay-3l/m 2 CaCl 200 4,662 20,128 1,475 5, ,595 34,830 3l/m 2 CaCl 200 4,662 20,128 1,475 5, ,595 34,830 2l/m 2 CaCl 200 4,662 20,128 1,475 4, ,595 33,260 1l/m 2 CaCl & Seal 200 4,662 20,128 1,475 2,823 6,719 2,595 38,405 No Stabilization & Seal , ,959 2,593 18,760 No Stabilization ,286 1, ,769 9, % DL10 Special 150 6,410 21, , ,593 37, % DL10 Special 150 6,410 21, , ,593 50, % DL10 & Seal 80 7,125 22, ,106 6,463 2,600 51, % SS-1 & Seal 150 7,117 22, ,706 6,467 2,593 45, % SS-1 & Seal 150 7,117 22, ,766 6,467 2,593 55, % SS-1 & Seal 160 6,672 21, ,365 6,063 10,706 55,269 No Stabilization 660 1,457 4,971 1, ,595 10,954

16 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 15 Construction Costs ($/Km) 60,000 50,000 40,000 30,000 20,000 10,000 0 No Stab (control) 20% Clay 20% Clay 15% Clay 15% Clay 10% Clay-2-3l CaCl 10% Clay-1l CaCl 10% Clay-2l CaCl 10% Clay-3l CaCl 3l CaCl 2l CaCl 1l CaCl Seal No Stab & Seal No Stab (control) 2.5% DL10S 6.5% DL10S 4.5% DL10S Seal Labor Equipment Gravel Stabilizer Misc. 2.5% SS1 Seal 6.5% SS1 Seal Seal Coat 4.5% SS1 Seal No Stab (control) FIGURE 15 Highway Test Section Construction Cost per Kilometer by Cost Type $60,000 Construction Cost ($/Km) $50,000 $40,000 $30,000 $20,000 $10,000 $13,684 $18,759 $24,448 $34,002 $35,497 $38,403 $43,790 $52,017 $0 Gravel Gravel w/ Seal Clay Stabilized Clay/CaCl Modified CaCl Modified CaCl Modified w/ Seal Asphalt Emulsion Stabilized Asphalt Emulsion Stabilized w/ Seal FIGURE 16 Mean Total Construction Cost per Kilometer Grouped by Test Section Type

17 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 16 5 POST CONSTRUCTION PERFORMANCE Inspection of the Highway test sections was performed immediately after construction and one year after construction. For comparative purposes, a control test section comprised of in-place rock crushing with a gravelwearing surface coarse and no stabilizers added to the subgrade was constructed. After one year of performance, the clay modified test sections were determined to exhibit improved stability of the subgrade, assisted in maintaining moisture after rainfalls, and assisted in retaining gravel into the wearing coarse. However, the clay modified test sections produced increased traffic dust in dry weather conditions and became slippery in wet weather conditions when compared to other test sections. Calcium chloride modified test sections were constructed at concentrations of one liter/m 2, two liter/m 2 and three liter/m 2 to a depth of 150 mm. To evaluate the relative influence that clay has on the retention and performance of calcium chloride modification, test sections with and without ten percent clay by dry weight of subgrade soil were constructed. It was determined that calcium chloride provided improved compaction properties of the sand subgrade, assisted in moisture retention within the grade, and assisted in embedding gravel into the wearing coarse. The calcium chloride modified test sections significantly reduced dust in dry weather conditions when compared to other test sections. Clay modification of the calcium chloride treated subgrade increased the amount and duration of moisture retention. It was concluded that with ten percent clay added to the subgrade, one liter/m 2 of calcium chloride provided the optimal balance of moisture retention and stability. Calcium chloride concentrations of two liters/m 2 and three liters/m 2 with ten percent clay resulted in surface ponding, potholes, rutting, and slippery driving surface after rainfalls. It was also determined that the calcium chloride without clay was less sensitive to rainfall and only exhibited minor surface ponding and potholes after rainfall up to concentrations of three liters/m 2. SS-1 and DL10 Special asphalt emulsion was used to stabilize test sections at respective concentrations of 2.5, 4.5 and 6.5 percent by dry weight of soil to a design depth of 150 mm. SS-1 is a slow set anionic asphalt emulsion that employs medium residual penetration asphalt designed to increase the stability and strength of granular materials. DL10 Special is an anionic soft pen residual asphalt emulsion designed to provide added stability, improved environmental durability and surface workability in hot weather. Based on the observations of the SS-1 and DL10 Special asphalt emulsion stabilized test sections immediately after construction, the test sections with increasing concentrations of asphalt emulsion increased the sand subgrade stability, provided a tighter surface wearing coarse and reduced dust as would be expected. It was found that the asphalt

18 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 17 emulsion stabilized test sections required minor shaping for a period of approximately two weeks after placement until the emulsion fully cured. Minor wheel path abrasion was also observed two months after placement with no wearing surface coarse placed on the stabilized sand subgrade after construction. To minimize wheel path abrasion, it is recommended that a thin layer of crushed rock or traffic gravel be embedded into the wearing surface immediately after placement and prior to the asphalt emulsion curing. In addition, a residual pen asphalt emulsion and/or faster setting emulsion may help reduce wheel path rutting and abrasion and eliminate the need for blading during curing. Double seal wearing coarse was also placed on select portions of the asphalt emulsion stabilized test sections, one-year after construction the sealed test sections constructed on the asphalt emulsion stabilized subgrade was found to be performing well with only minor edge breaks present. 6 SUMMARY, CONCLUSIONS AND FUTURE RECOMMENDATIONS SDHT is responsible for maintaining approximately 6500 kilometers of northern gravel roads. Many of these gravel roads are built on poorly graded sand subgrades and may contain protruding bedrock and/or large boulders. Furthermore, many of these roads exhibit washboarding, protruding rocks, rutting and potholes, which result in severe road roughness and a potential driving hazard. Routine blading is often ineffective because unstable sand does not maintain shape and compaction, protruding bedrock and boulders damage motor grader blades, boulders may become dislodged leaving holes in the road, and dislodged boulders are a safety hazard when windrowed along road side-slopes. SDHT undertook an in-place rock crushing and subgrade stabilization pilot project on sections of Highway The objective of this project was to evaluate cold in-place rock crushing to reduce protruding boulders and to evaluate the feasibility of in-place modification/stabilization to stabilize sand subgrades. The construction of the Highway test sections determined that in-place rock crushing with the CMI R/S 650 is technically feasible and provides a workable surface free of large boulders. The addition of clay and calcium chloride and asphalt emulsion to the uniform sand subgrade resulted in increased stability and reduce traffic dust. The construction costs for this pilot project ranged from $13,684 for in-place rock crushing and a gravel wearing coarse to $52,017 for in-place rock crushing, asphalt emulsion stabilization and a double seal wearing coarse. Based on the results of this study, further investigation of in-place rock crushing and sand stabilization should consider: a) A roto mixer mandrel specifically designed for in-place rock crushing.

19 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 18 b) Stabilization designs should be based on fundamental subgrade soil behavior including physical and mechanical properties of alternative stabilization systems to quantify the performance related behavior of these systems. REFERENCES 1. Safronetz, J., Berthelot, C., Sparks, G., and Gerbrandt, R. Alternative Maintenance Treatments for Northern Saskatchewan Gravel Roads. Proc., Canadian Society of Civil Engineers, pp , Regina, June 1999.

20 Berthelot, Gerbrandt, Safronetz, and Sparks. Page: 19 LIST OF FIGURES FIGURE 1 Highway Typical Boulders In Grade and Along Road Edge FIGURE 2 Highway Typical Unstable Sand Subgrade FIGURE 3 Highway in situ Subgrade Grain Size Distribution FIGURE 4 Standard and Modified Proctor Moisture-Density of SDHT Highway Subgrade FIGURE 5 SDHT Hwy in situ Rock Unconfined Compressive Strength Measurements FIGURE 6 Marshall Stability of SDHT Highway Stabilized Systems FIGURE 7 Primary Rock Crushing FIGURE 8 Windrowing and Separating Crushed Rocks FIGURE 9 Clay Modification FIGURE 10 Asphalt Emulsion Stabilization FIGURE 11 Final Shaping and Compaction of Grade FIGURE 12 Typical Final Grade FIGURE 13 Typical Gravel Wearing Coarse FIGURE 14 Typical Asphalt Emulsion Stabilized Wearing Coarse FIGURE 15 Highway Test Section Construction Cost per Kilometer by Cost Type FIGURE 16 Mean Total Construction Cost per Kilometer Grouped by Test Section Type LIST OF TABLES TABLE 1 Highway in situ Rock Unconfined Compressive Strength Measurements TABLE 2 Highway Test Sections TABLE 3 Highway Subgrade Marshall Stability Across Stabilizer Types and Concentrations TABLE 4 Highway Test Section Construction Cost Per Kilometer by Cost Type