Road Embankment Design Alternatives Over Permafrost

ROAD EMBANKMENT DESIGN ALTERNATIVES OVER PERMAFROST

' By David C. Esch,' , ABSTRACT The side slopes of roadway embankments in warm permafrost areas often cause particularly severe problems from long term thaw-related settlements. This situation results primarily from the snow plowed from the roadway, which thickens and increases the insulating cover on the side slopes. Soils underlying the slopes therefore often do not totally refreeze each winter, as normally occurs beneath the cleared portion - of the roadway. These annually deepening thaw zones beneath the slopes result in progressive settlements of the outer edges of the roadway and often cause longitudinal cracking of the roadway surface. Experimental installations of air convection ducts, in conjunction with insulation layers and embankment toe berms, were made during 1973 and 1974 on a newly constructed roadway embankment approximately 25 miles west of Fairbanks. Performance has been monitored since that time. Results through the thawing season of 1977 are presented, and demonstrate that satisfactory annual refreezing can be assured by this method, preventing long term thaw-settlements of snow covered embank- men t slopes.

INTRODUCTION: Previous studies by the Alaska Department of Highways of roadways constructed over permafrost have shown that progressively deeper thawing beneath the roadway side-slopes often still occurs, even in areas where the embankment thickness beneath the roadway centerline is ade- quate to prevent thawing into the underlying permafrost. Because the roadway side sl'ope areas are well drained and generally only sparsely vegetated, their mean surface temperature during a given thawing season will generally be higher than that of a well vegetated surface overlying permafrost. This will result in deeper thawing beneath the side slopes, an increase in the thickness of the annual thaw layer, and consolidation and settlements if the underlying permafrost has a moisture content higher than it can retain in a thawed state. Since a considerable portion of a roadway embankment may be supported struc- turally by the soils beneath the side slopes, any settlement in these areas will be reflected by embankment slumping and lateral spreading and cracking (Fig. 1). Because the snow cover on the side slopes is increased by the snow plowed from the roadway surface, the side slopes will also not cool and freeze back as efficiently as either the adjacent undisturbed ground or the travelled roadway surface areas. There- fore, annually deeper thawing of underlying permafrost often occurs beneath the roadway side slopes.

'Engineer of Tests, Alaska Department of Transportation, Fairbanks

1 *!

J i

Literature reviews indicate that very little study has been given to side slope effects of roadways over permafrost. The problems which result from side slope thaw are intensified by increased fill heights and probably by the steeper side slopes comonly used on high fills for economic reasons. To study the benefits of several alternate embankment slope designs in controlling thaw of permafrost beneath roadway side slopes, a research project was instituted in 1973 under funding of the Highway Planning and Research program of the Federal Highway Administration. Six different combinations of insulation layers, toe berms, and air ducting systems were installed on a newly constructed roadway and have been monitored since the completion of construition in 1974.

Thaw and Consolidation Zones

Fig. 1 .--Typical Roadway Distress from Thaw beneath Embankment Side Slopes.

PROJECT LOCATION AND DESCRIPTION

The site selected for the experimental embankment construction is located at Bonanza Creek, approximately 25 miles west of Fairbanks, on the "Parks Highway"; the primary trucking route between Fairbanks and Anchorage. In this area, a new roadway segment was to be routed across a generally undisturbed muskeg, underlain by ice-rich silt permafrost soils, with segregated ice. The variations in frozen water contents of the silt are shown by Figure 2. Overlaying the permafrost was a thin (1' to 2') peat layer, covered with a surface layer of sphagnum moss, with some areas of sedge covered tussocks also present. Vegetation consisted of scattered black spruce, tamarack, willow, and alder. In the interior of Alaska, such muskeg environments are ideal for the protection of permafrost at shallow depths. This site was selected because the new routing required a relatively high and uniform embankment height, ranging from 22 to 25 feet, over very poor soils. The critical nature of these soils was evidenced by the old roadway embankment crossing this muskeg approximately 500 feet to the north, which had exhibited distress similar to that depicted in Figure 1. On-site soil temperatures were monitored for one year prior to construction, and indicated that the permafrost in this area ranged from 28O to 30°F in mean annual temperature.

2 ' .\

(Frozen) Moisture Content Yo

.. . . 0. .*. . 0 . e .. . .. e . m . . . . -0 *-.. . . 0.

Fig. 2--Frozen Moisture Contents of Silt FounZation Soils

DESIGN OF EMBANKMENTS:

Thermal analyses of the proposed embanknent, using the "Modified Berggren" calculations approach (l), indicated that thawing beneath the cleared roadway portion of the embanhent would reach to a depth of 12 to 15 feet, but that thaw beneath the lower portions of the side slopes would penetrate into the underlying permafrost. Three different basic types of modifications were considered for field evaluations to retard this side slope thawing. These included the use of rigid foam insulation layers, the construction of toe berms, and the installation of air convection ducts. Insulation layers have been utilized beneath roadways and airfields, to prevent thawing of underlying pernafrost, (2), (3), in addition to xuch more common usage beneath roadways in warmer regions for frost heave prevention. Both experience and thermal calculations have demonstrated that insulation layers are most beneficial when placed at or near the ground surface, and installed with a layer of thaw-stable soil between the insulation and the soil to be protected from thawing. An additional consideration made in ezbankiaent designs for this project xas that the insulation layers be nearly horizontal, both to simplify construction operations, and to preveat the insulation from creating a slippage plane, as might occur if it were installed parallel to the side slopes. Insulation layer widths were selected from one-dimensional thaw depth analyses and estimations of thaw depths within the embank- Zen t . Toe berms were designed with a thickness of six feet, the calculated thickness required to prevent the first year's thaw from penetrating beneath the berm. The berm width of twenty feet was selected to provide a major contrast to the no-berm condition, with a 100 foot transition length to permit studies of the effect of berm width. Berms ;:ere designed for use of excess silt caste materials from an adjacent section. The air convection ducts were included in this study to determine their benefits in removing heat and thereby refreezing the soils be-

3 neath the insulative snow cover. Because of the many variables which affect the efficiency of convective flow air ducts, the ducting system was not fully analyzed in the design stage, but was selected, installed, and experimentally evaluated. A detailed discussion of air ducts as related to building foundations is presented in Reference 1. The ducts function in convection only in winter, as the adjacent soil warms the air in the duct and causesit to rise up the exhaust stack, at the same time drawing in cool air at the inlet. Two different layouts for the ducts were utilized on the north and south sides of the embankment, as shown by Figure 3. The ducts were constructed from 8 inch galvanized corrugated metal pipe, installed with the inlet ends above the depth of maximum snow cover, and with 10 foot high exhaust stacks. The 50 to 100 foot long buried portionsslopedslightly upward toward the exhaust stacks. Inlet end dampers were provided to permit positive summer air flow prevention, and rain and snow hoods were added.

Fig. 3--Overall View of Embankment Details

CONSTRUCTION OPERATIONS:

Prior to construction, the embankment area was hand cleared, and all small trees and brush were placed in an even layer beneath the embankment. To provide a working pad for the experimental installations, and to assure a well frozen embankment foundation, the initial three feet of the embankment was constructed in September of 1973, and periodically cleared of snow during the subsequent winter. This resulted in lowering the soil temperatures to 25' to 26'F at a depth of 15 feet. In early April of 1974, prior to the start of seasonal thawing, the berms were constructed, and insulation layers and convection ducts were installed. The remainder of the embankment fill was conpleted by early June, 1974, followed by pavement pla e.ent in July of 1975. Insulation Was expanded polystyrene foam, HI-35CRy by DOW Chemical co.

4 INSTRUNENTATION DETAILS \ The thermal performance of the various design features is evaluated from temperatures measured with a system of 352 thermocouples and 20 thermistors installed both in horizontal strings and in 18 vertical borings made through the embankment. TherEocouples are of the copper- constantan type, and are connected through double pole rotary switches located on roadside posts. Readings of both thermocouples and thermistors are made monthly with a Hewlett-Packard >lode1 3465B Digital Volt- Ohm meter, using an ice-bath reference junction for the thermocouples. The combination of these two sensors was selected to take advantage of the accuracy of better than +. O5OF from calibrated thermistors, while avoiding the high cost of a Full thermistor installation. Accuracy of the thermocouples used has generally been better than +0.3'F, as judged by comparisons with adjacent thermistors and agreement between temperature indicated and probe measured thaw depths. Sit& air temperatures and wind velocities are obtained by battery operated recorders housed in a Weather Bureau type weather station. Temperatures 1" beneath the roadway pavement and at 1" depths on the north and south embankment side slopes also are recorded continuously, as is the exhaust stack air temperature ou one of the four air ducts. To provide a basis for repeated measurements of side slope settlements and lateral spreading, cross-section reference hubs and nails were placed at approximately 10' intervals from the centerline to the toe of the embankment slopes on all study sections. Surveys are made in late September to monitor annual slope mveraents. Thaw depths in the-.tae of slope and berm areas are also measured at this time by means of hand probing with 4 inch diam@&rsteel rods. Because of the very soft nature of the thawed foundation soils, this method provides an excellent annual check on the position of the permafrost table. Thaw depths beneath the central portions of the ezbankment are determined from the position of the 32'F isotherm, as indicated from thermocouple temperatures at 1' to 2' intervals of depth.

RESULTS OF OBSERVATIONS

Sire air temperature data during the period of this study are summar- ized by Table I, expressed as cumulative freezing and thawing indices in Farenheit Degree-Days above or below 32OF. Dates given for the start of the freezing and thawing seasons ere those on which the maxi- ' mum cumulative thawing or freezing indices were reached. TJinter freezing indices are listed for the year in which the winter began.

Start Thawing Start Freezing To tal liean of Index of Index Snowfall Temp. OF Year Thaw OF-days Freeze OF-2~:is -inches Annual

1974 4/13 3340 9/30 -6130 90.2 -- 1975 4/23 3550 10/12 -6820 51.0 23.8 1976 4/16 2870 10/8 -43SO 59.6 25.2 1977 4/20 3250 10/11 -- -- 25.2

Table I - Climatological Data - BOR~ZZCreek Test Site

5 Thaw Depths for the south-facing s le slopes of all sections at the . end of the 1974 and 1977 thawing season are shown by Figures 4 and 5, covering embankments without and with insulation layers. Thaw depths at the end of the 1975 and 1976 thawing seasons nearly always fell between the 1974 and 1977 data, indicating that progressively deeper annual thawing has occurred beneath all study slopes, with the notable exception of the area between the air convection ducts on the insulated berm section. Table 2 demonstrates the progression of annual thawing for seven different embankment designs as indicated by thermocouple strings located beneath the critical side slope area, at eight feet inside of the normal toe of a 2:l embankment slope. At this point, all designs are indicating progressively deeper annual thawing. Only the two insulated berm designs have successfully prevented thawing from reaching down to the original ground surface. These data demonstrate that the more extensive design treatments were progressively more successful -in retarding thaw in this critical area. However, all sections are still on a downward thawing trend and equilibrium may be several years away. Settlement observations in 1976 and 1977 for movement reference points on embankments without and with 20' wide berms are presented by Figures 6 and 7. The initial elevation surveys, made Tn September of 1975, serve as the zero reference for these plots. Note that some settlements, on the order of 1%of total height, have occurred through- out the central portion of all embankments. This is probably a result of the annual freeze-thaw cycling within the embankment so.i'Fs, since temperatures indicate that only about 15 feet of annual thawing occurs beneath the central portion of the roadway; considerably less than the 23 foot embankment height. Lateral movement measurements of the elevation reference hubs have indicated that outward movements are . generally similar in magnitude to the downward movements, resulting in toe of slope movements occurring roughly on a 1:l outward slope. The actual settlement magnitudes at the various points are to some extent dependent on the initial moisture contents of the underlying soils. Figure 2 indicates, from the widely differing moisture contents, that considerably different settlements may be expected from point to point, even with similar thaw penetrations.

DISCUSSION

The best indications of the long term performance of the experimental embankment sections come from examination of the changes in thaw depths with time, as indicated by Figures 4 and 5 and Table 2. On this basis, it appears that the insulated berms, with and without air ducts, will be the best long term performers. Toe insulation layers in the normal embankment show very little significant advantage. Increasing the width of a 6' high berm from 10 to 20 feet showed a significant reduction in thaw inside the toe of slope. It should be noted from Figure 6 that all three full bern sections have, for the four years since their construction, kept thaw at this point from reaching through the original two foot active layer, and into the underlying ice-rich permafrost. As demonstrated by air temperature data at this site, the winter of 1976-77 was much warmer than normal, and would be expected to ad- versely affect soil refreezing, particularly in the toe of slope areas.

6 I I 8 I I I I 1 1 1

I 20' Berm with Air Ducts

z3x, 8'' Air Ducts -,

I I t I 1 1 I 40' 48' 5 6' 64' 72' 80' 00' Distance to Rt. of Centerline

Figure 4 - Embankment without Insulation - Maximum Thaw Depths in Toe of Embankment Slope Areas for First and Fourth Years of Study

7 40' 48' 5 6' 6 4' 72' 8d 8 8' I I I I 1 I

Normal Embankment with Toe Insulation 12'

8' 2"x 16' Insulation Layer .c4 4' &P nQ) 0

-4'

12' 20' Insulated Berm

8' 2"%32' Insulation Layer

- 4'

20' Insulated Berm 8 Air Ducts

\ \ \ \\

2"x 32' Insulation Layer -4' I -8' I I I 1 I I 4 0' 48' 56' 64' 72' 80' 88' Distance to Rt. of Centerline

Figure 5 - Insulated Embankment Areas - Haximum Thaw Depths in Toe of Embankment Slope Areas for First and Fourth Years of Study

8 Section Type -1974 1975 1976 1977 Normal Embankment 4-0.2' -0.5' -4.0' -4.1t Toe Insulated Embankment -0.2 -0.8 -3.0 -3.6 10' Wide Berm -0.4 -0.9 -2.5 -2.8 20' Wide Berm 4-0.4 -0.4 -1.5 -1.8 20' Berm & Air Ducts 4-0.8 +0.1 -1.2 -1.7 Insulated 20' Berm 42.2 4-1.9 +1.3 +O. 7 Insulated Berm & Air Ducts 3-2.5 +2.1 +l. 7 +l. 3 Table 2 - f4aximum Thaw Depths (ft.) at Vertical Thermocouple Strings 8 feet inside of Normal Embankment Toe

8dU 40'U. E - 40'Rt. 80'Rt. ,

-.2' - - t' y. I *c -3'- - i!! Lc -5'- .. t 0, Em bankm ent v) -.8' - with Toe Insulation -

-.2' - -4 -

-6t Normal Embankment Legend

(0) 9/24

cAC Pavement 1 2 20' 6 IO' 0 80'U. 4dLf. 4dRt. 88Rt.

Figure 6 - Past-Construction Settlements, Embankments ifithout Berms

9 . 80'Lt. 40'Lt. 40'Rt. 8O'Rt. 120'Rt.

-.2# - +' y. I 1- + -.4 c 2E -.6' - 20' Berm 8 Air Ducts c a3 v) -.8' - I

20' Insulated Berm €3 Air Ducfs Y

20' Insulated Berm

Legend 9/24/76 (X) 9/30/77 30'1- cAC Pavement

Figure 7 - Post-Construction Settlernents, Embankments With Berms

10 Soil temperatures in March and April of 1977 were examined to determine whether residual thaw zones or "taliks" existed in areas beneath the side slopes. Thicknesses of thawed zones, which failed to refreeze during the '76-'77 winter, as determined from vertical thermocouple string temperature data, are presented in Table 3. The exact lateral extent of these taliks are difficult to determine because of the lack af thermocouples in many areas of the fill.

Section. @ 8' Inside Toe @ 16' Inside Toe

Normal Fill 3.5 5.0 Toe Insulated Fill 4.0 6.5 10' Wide Berm 4.0 6.0 20' Wide Berm 2.5 5.0 20' Berm & Air Ducts 2.0 5.0 20' Insulated Berm 1.5 1.0 Insulated Berm & Ducts 0. 0

Table 3 - Thicknesses of Residual Thaw Zones in April of 1977 - ft.

The presence of these talik zones indicates that long term progressive thawing will probably continue to occur with all treatments, except for the full combination of berms, air ducts, and insulation. However, acceptable performance of sone of the other sections may still result, since the present therm1 trends are to'some extent in- fluenced by the past very warm winter. It is possible that a cold winter combined with minimal snow cover ma.y reverse some of the present- thawing trends. Air ducting installations can be varied greatly from those used in these study embankments. No conclusions have yet been drawn on the comparative long term benefits of the two different air ducting layouts used. Temperatures along the length of the south side air ducts indicate that these ducts are probably too long for their diameter, because their refreezing ability is considerably less in the uninsu- lated berm area, where the ducts approach their exhaust stacks, The greatest cooling occurs near the inlet ends, the entry point for cold outside air. Other combinations of more, larger, or shorter ducts may be more efficient, and may eliminate the need for insulation laysrs above the air ducts.

CONCLUSIONS Performance and temperature observations on experimental embankmenc sections located in a warm permafrost area have shown favorable resulcs from a combination of embankment toe berms, insulation layers, and convective air duct cooling systems. Any of these design features can be expected to show some benefits in reducing thaw-related movements in the critical central portion of high embankments. However, the full combination of treatments section has been the only section exhibiting full annual refreezing in the critical toe of embankment slope areas. ACKNOWLEDGEMENTS

The writer acknowledges the cooperation and helpful recommendations of B. Balvin and H. Livingston in developments of the problem, selec- tions of the design alternatives, and preliminary field soil szudies. Funding for this project has been provided under the HPX program of the U.S. Department of Transportation, Federal Highway Addnistration. Opinions, findings, and conclusions expressed herein are those of the writer.

APPENDIX '1, - REFERENCES 1. Department of the Army, (1966), "Calculation Methods for Determination of Depths of Freeze and Thaw in Soils", TECHNICAL MANUAL TM 5-852-6. 2. Esch, D.C., (1973), "Control of Permafrost Degradacion Beneath a Roadway by Subgrade Insulation", - PROCEEDINGS, SECOND INTERNATION PER'IAFROST CONFERENCE, pp. 608-621. 3. Esch, D.C. and Rhode, J.J., (1977), "Kotzebue Airport Runway Insulation over Permafrost", PROCEEDISGS, SECOND INTERNATIONAL SYMPOSIUM ON COLD REGIOWS ENGINEERING, pp . 44-61. 4. Wellman, J.H., Clark, E.S., and Condo, A.C., "Design and Construction of Synthetically Insulated Gravel Pads in the Alaskan Arctic", PROCEEDISGS, SECOND ISTER- NATIONAL CONFERENCE ON COLD REGIOSS ENGINEERISG, pp. 62-85. 5. Williams, W.G.., (1968), DEVELOPMEXT ASD USE OF PLASTIC FOAM TO PREVENT FROST ACTION DAK4GE TO HIGHWAYS - A SUMMARY OF EXPERIENCE IN UIU'ITED STATES, Inter- national Conference on Highway Insulation, Wurcz- burg, Germany. 6. Esch, D.C., (1971), SUBGRADE INSULATION FOR FROST HEAVE CONTROL - SUIQfARY OF SECOND AND THIRD WINTERS PERFORNANCE, State of Alaska, Department of High- ways, Materials Division, Douglas, Alaska. 7. Gold, L.W., (1967), "Influence of Surface Conditions on Ground Temperature," CANADIAX JOURNAL OF EARTH SCIENCES, VO~.4, pp. 199-208. 8. Williams, G.P., (1965), "Heat Balance Ov2r Sphagnu-. Moss, I' PROCEEDINGS, FIRST CAXAD1A.X CONFEREXCE OS MICROMETEOROLOGY, pp. 173-193. (Reprinted as NRC 10017). 9. Brown, R.J., (1973), "Influence of Climatic and Terrain Factors on Ground Temperatures at Tnree Locations in the Permafrost Region of Canada", SECOND INTERNATIONAL PERMAFROST CONFERESCE PROCEEDISGS, pp. 27-34. 10. Kritz, M.A. and Wechsler, A.E., (1967), STTRFACE CHAXXTER- ISTICS, EFFECT ON THERMAL REGINE - PHASE 11, U.S. Army Cold Regions Research and Engineering Ldmratorj- Technical Report 189.