土木工程类专业英文文献及翻译

PA VEMENT PROBLEMS CAUSEDBY COLLAPSIBLE SUBGRADESBy Sandra L. Houston,1 Associate Member, ASCE(Reviewed by the Highway Division)ABSTRACT: Problem subgrade materials consisting of collapsible soils are com- mon in arid environments, which have climatic conditions and depositional and weathering processes favorable to their formation. Included herein is a discussion of predictive techniques that use commonly available laboratory equipment and testing methods for obtaining reliable estimates of the volume change for these problem soils. A method for predicting relevant stresses and corresponding collapse strains for typical pavement subgrades is presented. Relatively simple methods of evaluating potential volume change, based on results of familiar laboratory tests, are used.INTRODUCTIONWhen a soil is given free access to water, it may decrease in volume,increase in volume, or do nothing. A soil that increases in volume is calleda swelling or expansive soil, and a soil that decreases in volume is called a collapsible soil. The amount of volume change that occurs depends on thesoil type and structure, the initial soil density, the imposed stress state, andthe degree and extent of wetting. Subgrade materials comprised of soils that change volume upon wetting have caused distress to highways since the be- ginning of the professional practice and have cost many millions of dollarsin roadway repairs. The prediction of the volume changes that may occur inthe field is the first step in making an economic decision for dealing withthese problem subgrade materials.Each project will have different design considerations, economic con-straints, and risk factors that will have to be taken into account. However,with a reliable method for making volume change predictions, the best design relative to the subgrade soils becomes a matter of economic comparison, anda much more rational design approach may be made. For example, typical techniques for dealing with expansive clays include: (1) In situ treatmentswith substances such as lime, cement, or fly-ash; (2) seepage barriers and/or drainage systems; or (3) a computing of the serviceability loss and a mod- ification of the design to "accept" the anticipated expansion. In order to makethe most economical decision, the amount of volume change (especially non- uniform volume change) must be accurately estimated, and the degree of road roughness evaluated from these data. Similarly, alternative design techniquesare available for any roadway problem.The emphasis here will be placed on presenting economical and simplemethods for: (1) Determining whether the subgrade materials are collapsible;and (2) estimating the amount of volume change that is likely to occur in the'Asst. Prof., Ctr. for Advanced Res. in Transp., Arizona State Univ., Tempe, AZ 85287.Note. Discussion open until April 1, 1989. To extend the closing date one month,a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on February 3, 1988. This paper is part of the Journal of Transportation.Engineering, V ol. 114, No. 6, November, 1988. ASCE, ISSN 0733-947X/88/0006-0673/$1.00 + $.15 per page. Paper No. 22902.673field for the collapsible soils. Then this information will place the engineerin a position to make a rational design decision. Collapsible soils are fre-quently encountered in an arid climate. The depositional process and for-mation of these soils, and methods for identification and evaluation of theamount of volume change that may occur, will be discussed in the following sections.COLLAPSIBLE SOILSFormation of Collapsible SoilsCollapsible soils have high void ratios and low densities and are typically cohesionless or only slightly cohesive. In an arid climate, evaporation greatly exceeds rainfall. Consequently, only the near-surface soils become wettedfrom normal rainfall. It is the combination of the depositional process andthe climate conditions that leads to the formation of the collapsible soil.Although collapsible soils exist in nondesert regions, the dry environment inwhich evaporation exceeds precipitation is very favorable for the formationof the collapsible structure.As the soil dries by evaporation, capillary tension causes the remainingwater to withdraw into the soil grain interfaces, bringing with it soluble salts,clay, and silt particles. As the soil continues to dry, these salts, clays, andsilts come out of solution, and "tack-weld" the larger grains together. Thisleads to a soil structure that has high apparent strength at its low, naturalwater content. However, collapse of the "cemented" structure may occurupon wetting because the bonding material weakens and softens, and the soilis unstable at any stress level that exceeds that at which the soil had been previously wetted. Thus, if the amount of water made available to the soilis increased above that which naturally exists, collapse can occur at fairlylow levels of stress, equivalent only to overburden soil pressure. Additionalloads, such as traffic loading or the presence of a bridge structure, add tothe collapse, especially of shallow collapsible soil. The triggering mechanismfor collapse, however, is the addition of water.Highway Problems Resulting from Collapsible SoilsNonuniform collapse can result from either a nonhomogeneous subgradedeposit in which differing degrees of collapse potential exist and/or from nonuniform wetting of subgrade materials. When differential collapse ofsubgrade soils occurs, the result is a rough, wavy surface, and potentiallymany miles of extensively damaged highway. There have been several re-ported cases for which differential collapse has been cited as the cause of roadway or highway bridge distress. A few of these in the Arizona and New Mexico region include sections of 1-10 near Benson, Arizona, and sectionsof 1-25 in the vicinity of Algadonas, New Mexico (Lovelace et al. 1982; Russman 1987). In addition to the excessive waviness of the roadway sur- face, bridge foundations failures, such as the Steins Pass Highway bridge,1-10, in Arizona, have frequently been identified with collapse of foundation soils.Identification of Collapsible SoilsThere have been many techniques proposed for identifying a collapsiblesoil problem. These methods range from qualitative index tests conducted on674disturbed samples, to response to wetting tests conducted on relatively un- disturbed samples, to in situ meausrement techniques. In all cases, the en- gineer must first know if the soils may become wetted to a water content above their natural moisture state, and if so, what the extent of the potential wetted zone will be. Most methods for identifying collapsible soils are only qualitative in nature, providing no information on the magnitude of the col- lapse strain potential. These qualitative methods are based on various func- tions of dry density, moisture content, void ratio, specific gravity, and At- terberg limits.In situ measurement methods appear promising in some cases, in that many researchers feel that sample disturbance is greatly reduced, and that a more nearly quantitative measure of collapse potential is obtainable. However,in situ test methods for collapsible soils typically suffer from the deficien-cy of an unknown extent and degree of wetting during the field test. This makes a quantitative measurement difficult because the zone of material being influenced is not well-known, and, therefore, the actual strains, in- duced by the addition of stress and water, are not well-known. In addition, the degree of saturation achieved in the field test is variable and usually unknown.Based on recently conducted research, it appears that the most reliable method for identifying a collapsible soil problem is to obtain the best quality undisturbed sample possible and to subject this sample to a response to wet- ting test in the laboratory. The results of a simple oedometer test will indicate whether the soil is collapsible and, at the same time, give a direct measureof the amount of collapse strain potential that may occur in the field. Potential problems associated with the direct sampling method include sample distur- bance and the possibility that the degree of saturation achieved in the field will be less than that achieved in the laboratory test.The quality of an undisturbed sample is related most strongly to the arearatio of the tube that is used for sample collection. The area ratio is a measure of the ratio of the cross-sectional area of the sample collected to the cross- sectional area of the sample tube. A thin-walled tube sampler by definition has an area ratio of about 10-15%. Although undisturbed samples are best obtained through the use of thin-walled tube samplers, it frequently occurs that these stiff, cemented collapsible soils, especially those containing gravel, cannot be sampled unless a tube with a much thicker wall is used. Samplers having an area ratio as great as 56% are commonly used for Arizona col- lapsible soils. Further, it may take considerable hammering of the tube to drive the sample. The result is, of course, some degree of sample distur- bance, broken.bonds, densification, and a correspondingly reduced collapse measured upon laboratory testing. However, for collapsible soils, which are compressive by definition, the insertion of the sample tube leads to local shear failure at the base of the cutting edge, and, therefore, there is less sample disturbance than would be expected for soils that exhibit general shear failure (i.e., saturated clays or dilative soils). Results of an ongoing studyof sample disturbance for collapsible soils indicate that block samples some- times exhibit somewhat higher collapse strains compared to thick-walled tube samples. Block samples are usually assumed to be the very best obtainable undisturbed samples, although they are frequently difficult-to-impossible to obtain, especially at substantial depths. The overall effect of sample distur- bance is a slight underestimate of the collapse potential for the soil.675译文：湿陷性地基引起的路面问题作者：...摘要：在干旱环境中，湿陷性土壤组成的路基材料是很常见的，干旱环境中的气候条件、沉积以及风化作用都有利于湿陷性土的形成。