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Road Construction in Palsa Fields 26 Transportation Research Record 978 Road Construction in Palsa Fields J. HODE KEYSER and M. A. LAFORTE ABSTRACT frost zone (.!_) , permafrost features are scarce in the highlands except at the tops of bare hills. In the lowlands, the m;tin permafrost feature is pals a Palsa is an important feat11re of the dis­ [for definition, see Stanek ll)J. As the development continuous permafrost regions of northwest­ of the area proceeds from south to north and from ern Quebec. Because of the development of west to east, most of the roads are or will be hydroelectric complexes along La Grande and located in the lowland area where muskeg and palsa Great Whale rivers, the road network will be fields are frequent. expanded by the addition of 2000 km of road The problem of design and maintenance of roads with many sections crossing palsa fields. crossing palsa fields is dealt with. Concl11sions are Problems related to the design, construc­ based on observations of the performance of an ex­ tion, and maintenance of roads in palsa perimental embankment constructed over a palsa, on fields are identified and described. The data gathered through subsurface investigations of a observations are mainly based on the perfor­ projected road 100 km long between LG-2 and GB-1, mance of a test embankment built 3 years ago and on the evaluation of five settlement sites along on a large palsa and the performance of 620 the 620-km James Bay access road. km of road, paved in 1976, that c11ts through several palsa fields. The topics discussed are topology, occurrence and distrib11tion of PALSA TOPOLOGY AND OCCURRENCE palsa fields in northern Quebec, dating of palsa ice, description of a typical palsa Palsa can be defined as a discontinuo11s permafrost field, description of the physical charac­ feature; it is a mound created by the formation and teristics of a typical palsa, temperature growth of an ice core under favorable microenviron­ regime in the palsa, performance of an in­ mental conditions (2). Two main types of palsas are strumented test embankment 3 years after identified in s11bar~tic Quebec: nonwooded palsas and construction, performance and maintenance wooded palsas (3). Both types occur in the lowland history of a 6-year-old paved road that areas at elevations not exceeding 200 m and can be crosses several pals a fields, and predicted found in clusters of 3 to 10 in both dry and wet versus observed rate of settlement of exist­ areas. ing embankments. Based on the results of Nonwooded palsas are mainly located north of the these investigations, recommendations are -3°C annual isotherm in the coastal zone of H11dson made for the design and maintenance of roads Bay. They are principally of organic origin (90 per­ that cross palsa fields. cent) and have either round or irregular shapes. They can be isolated or form important palsa fields 11p to 5 km 2 in area (palsa plateau north of Great Whale) • The origin of nonwooded palsas is thought to be the degradation of ancient permafrost. The development of a road network in subarctic Wooded palsas are found principally in the south­ Quebec is relatively recent. All the principal and ern part of the territory where the mean annual some local roads were built after 1972. With the temperature varies from -1°C to -4°C. They are nor­ development of the La Grande hydroelectric complex, mally covered with black spr11ce and tamarack, li­ approximately 2000 km of roads are being added. The chens and peat moss, forming a drunken forest at the principal access road runs from Matagami to LG-2; it edge of the palsa. Many palsas present signs of is 620 km long, and was paved between 1974 and 1976 degradation, with cracks and water ponding at the (Figure 1). surface. More than 400 sites of wooded palsas have The area under consideration is 40 000 km 2 , ex­ been identified by Dionne (3); the first access road tends from the 50th to the 56th north parallel, and cuts through at least five zones <!l. Each zone is divided into two main topographical regions: (a) could have 5 to 10 palsa fields. the lowlands, at an altitude under 200 m, that are Palsas are generally considered relic permafrost, composed of a silty clay plain south of the 52nd under tree cover, protected by microclimatic phenom­ degree parallel, and a glaciomarine plateau to the ena. However, a dating test by the 0-16/0-18 method north and (b) the highlands that are 150 to 250 km indicates that the palsas along the LG-2-GB-l access from the shores of James Bay and present a low rocky road were formed during the last 40 years (~). plateau strewn with lakes. The area bedrock is essentially Precambrian with many outcrops in the highlands. Unconsolidated de­ DESCRIPTION OF A TYPICAL PALSA FIELD posits are composed mainly of marine silty clay and beach deposits in the lowlands and glacial till and A palsa field can be defined as an assembly of indi­ fluvio-glacial sands and gravels in the highlands. vidual palsas in an environment that favors palsa Almost everywhere peat deposits can be found in formation. A typical palsa field in northern Quebec surface depressions. is shown in Figure 2. The climate is of a subarctic continental type Palsa fields have been characterized by in situ with maritime influence from the James Bay. The geotechnical and geophysical surveys; by borings, average freezing index is around 2500°C per day, and so11ndings, and sampling; and by testing. More than the thawing index is less than 1500°C per day; mean 10 palsa fields were studied in 1980-1982. Borings air temperature varies from O to -4°C. The in-place were made either with or witho11t B or N casing, and snow cover varies from 45 to 65 cm. samples were taken with thin-wall t11bes or split Although the area is in a discontinuous perma- spoons in the disturbed and 11ndisturbed state l!l . 27 Bale James \ • "- - 1•c. i '"~....... .. ~·~I , ' r- _~ , ' ' .... I I .MATAOA~!;J FIGURE 1 La Grande hydroelectric complex. A, I lJ "&4!1,600 'm ~~~ 11 :• • I ML~ ., i a ~ • FIGURE 2 Typical palsa field. 28 Transportation Research Record 978 The samples were examined in a cold room and in the and contains small depression zones that are at field to determine the density and water and ice times unfrozen (see Figure 3). The peat layer at the content. Classification tests were also made: peat surface of the palsa is often cracked around the was classified according to the Von Post index (6) boundaries and is accompanied by a slipping surface and the frozen structure was classified according to and, sometimes, exposed ice. Degradation is worst in ASTM standard procedure Ill . the open areas or in old burnt surfaces; in the deep There could be a palsa field in every low-lying wood-covered areas, the surface is hununocky but the area in a particular region; but more often palsa peat layer is not degraded. Around the palsa, under­ fields are distant from each other. A palsa field neath the pond, or in the natural peat cover, the generally contains 3 to 10 palsas; the distance soil is unfrozen and boundaries between frozen and between individual palsas is generally less than the unfrozen material are well defined. width of the palsas. All the wooded palsas investigated are composed The drainage pattern in a palsa field is not well of four typical layers: a layer of peat at the top, defined and is sometimes influenced by the underly­ a silty clay layer with no excess ice, ice inter­ ing rock. Usually the water table is close to the stratified with silt, and the unfrozen soil. Several surface, and, most of the time, a pond can be found typical borehole sections are shown in Figure 4 and in the low areas of a palsa field. the general characteristics of each layer are given in Table l. The thickness of the peat layer varies from 0.60 DESCRIPTION OF A TYPICAL PALSA to 2. 70 m. The peat material is generally black or brown with a low fiber content and high mineral Palsas are usually small and circular or ellipsoidal content. The Von Post index is always higher than 6. in shape; the length of a palsa generally varies Surface cover is normally lichen or moss with an from 10 to 100 m, and palsas have a thickness of l active layer varying from 20 to 50 cm. The frozen to 8 m with a maximum of 3 m above the surrounding peat is generally classified as Nbn because it is terrain. The surface of a palsa is often hummocked well bonded with a slight excess of ice, its density ' FIGURE 3 Typical unfrozen depression zones at the surface of a palsa. Keyser and Laforte 29 .;.,ii.. 11'1'6 ~ ~"pit.Tit.<, p~p.jroPM PM-o;,o. [llfl) 1IEf1lf '" llH1~ lu ""'ffllT ~~ri "'A'"- <>1411' "'....,TH ~,.;1:~ Nlon 19 f\.<.1 53~90 0 . 8-0. 9 100-3100 O. zt f 11onl : .os M Ito ~ .,,~,.,. cl>y 61-JS 1. 2-l. 5 59-110 "•l<;.i.i" ~V. 11u... ~n.. Q. 61 5. 11 ~ 5 .81 .. ~J ~"" ~ o. • 1 O. t6 <IUY JL·JJ l. 2-1.4 20-JO Mbn 0 . 15 29 l.44 t:Ut"l't.V tlb. 0 . 12 1.50 ~ ,. ' ~~· Sill)' .o.A'f 76-80 IG& t S.:i.. l. Z6 4. ZJ v. ~ 0.9-1.l '8·100 ~ · 11U.
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