SOIL SURVEY

-160-Table 64 . Guides for assessing soil limitations for local roads and streets 1This guide applies to soils evaluated for construction and maintenance of local roads and streets . These are improved roadsand streets having some kind of all-weather surfacing, commonly asphalt or concrete, and are expected to carry automobil etraffic all year . They consist of : (1) the underlying local soil material (either cut or fill) called the subgrade ; (2) the bas ematerial of gravel, crushed rock, or lime - or soil cement - stabilized soil called the subbase ; and (3) the actual roa dsurface or pavement, either flexible or rigid . They also are graded to shed water and have ordinary provisions for drainage .With the probable exception of the hardened surface layer, the roads and streets are built mainly from the soil at hand, an dcuts and fills are limited, usually less than 2 meters . Excluded from consideration in this guide ore highways designed fo rfast-moving, heavy trucks .Properties that affect design and construction of roads and streets are : (1) those that affect the load supporting capacity an dstability of the subgrade, and (2) those that affect the workability and amount of cut and fill . The AASHO and UnifiedClassification give an indication of the traffic supporting capacity . Wetness and flooding affect stability . Slope, depth ofhardrock, stoniness, rockiness, and wetness affect the ease of excavation and the amount of cut and fill to reach an evengrade . Soil limitation ratings do not substitute for basic soil data or for on-site investigations .ItemsDegree of Soil Limitatio nAffectingNone to Slight Moderate Severe Very SevereUseSoil Drainage Class2 Very rapidly, rapidly,Poorly and veryPermanently we twell, and moderately Imperfectly drained .(Wet)3poorly drained . soils .well drained .Flooding (Flood)Non eInfrequent Occasional Frequen t(once in 5 years) (once in 2 to 4 years) (every year )Slope (Slope) 0 to 9 (AD) 9 to 15 (E) 15 to 30 (F) > 30 (GH )Depth to Bedrock 4(Rock - D)>100cm 50- 100 cm < 50 cmSubgrade5 (Str . )a. AASHO groupindex 60 to 4 5 to 8 > 8b. Unified soil GW, GPy ,SW ;SP~SM CL (with p .1 . 8 < 15) CL(with p .1 . 8 of 15 o rclasses and GC and SC and ML more),CH,MH,OH,OL ,and P tSusceptibility toFrost Heave ? (Frost)Low (Fl, F2) Moderate (F3) High (F4)Stoniness10 Stones > 2 m apart . Stones 0 .5 to 2 m apart . Stones 0 .1 to 0 .5m apart .Stones < 0 .1 m apart .(Stony)(Class 0 to 2) (Class 3) (Class 4) (Class 5 )Rock exposures > 100 m Rock exposures 30 to 100 Rock exposures < 30 m Rock exposures tooRockiness 10 apart and cover < 2% m apart and cover 2 to apart and cover > 10% frequent to permi t(Rock) of the surface . 10% of the surface . of the surface . location of roads and(Class 0) (Class 1) (Class 2 to 4) streets .(Class 5)1. These guidelines, with some adjustment of slope and rockiness limits, will also be useful for assessin gsoils for use as parking lots .2. For an explanation of soil drainage classes, see the System of Soil Classification for Canada (Canada Soi lSurvey Committee 1974) pp . 220-221 .3. The abbreviations in brackets are used in Table 67 to indicate the nature of the limitation .4. If the bedrock is soft enough so that it can be dug with light power equipment and is rippable by machinery, reduc emoderate and severe limitations by one class .5. This item estimates the strength of a soil as it applies to roadbeds . When available, AASHO Group Index values fromlaboratory tests were used ; otherwide the estimated Unified classes were used . The limitations were estimatedassuming that the roads would be surfaced . On unsurfaced roads, rapidly drained, very sandy, poorly graded soilsmay cause washboard or rough roads .6. Group index values were estimated from information published by the Portland Cement Association (PCA,1962) pp .23-25 .7. Downgrade to moderate if content of fines (less than 200 mesh) is greater than about 30 percent .8. P .I . means plasticity index .9. Frost heave is important where frost penetrates below the paved or hardened surface layer and moisture transportabl eby capillary movement is sufficient to form ice lenses at the freezing point . The susceptibility classes are taken fro mthe United States Army Corps of Engineers (1962) pp . 5-8 . Table 66 is reproduced from the above article .10. See also definitions for rockiness and stoniness in the System of Soil Classification for Canada (Canada Soil Surve yCommittee, 1974) pp . 217-218 .