土木工程类专业英文文献及翻译.doc

1、PAVEMENT 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 andweathering processe

2、s favorable to their formation. Included herein is a discussionof predictive techniques that use commonly available laboratory equipment andtesting methods for obtaining reliable estimates of the volume change for theseproblem soils. A method for predicting relevant stresses and corresponding collap

3、sestrains for typical pavement subgrades is presented. Relatively simple methods ofevaluating 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 tha

4、t increases in volume is calleda swelling or expansive soil, and a soil that decreases in volume is called acollapsible 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. Subgra

5、de materials comprised of soils thatchange 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 e

6、conomic 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 designrelative to t

7、he subgrade soils becomes a matter of economic comparison, anda much more rational design approach may be made. For example, typicaltechniques for dealing with expansive clays include: (1) In situ treatmentswith substances such as lime, cement, or fly-ash; (2) seepage barriers and/or drainage system

8、s; 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 roadroughness evalu

9、ated 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 l

10、ikely to occur in theAsst. Prof., Ctr. for Advanced Res. in Transp., Arizona State Univ., Tempe, AZ85287.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 manuscriptfor this paper was submitted for r

11、eview and possible publication on February 3, 1988.This paper is part of the Journal of Transportation.Engineering, Vol. 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 enginee

12、rin 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 followings

13、ections.COLLAPSIBLE SOILSFormation of Collapsible SoilsCollapsible soils have high void ratios and low densities and are typicallycohesionless or only slightly cohesive. In an arid climate, evaporation greatlyexceeds rainfall. Consequently, only the near-surface soils become wettedfrom normal rainfa

14、ll. 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 collapsib

15、le 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 gr

16、ains 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 beenpreviously wetted. Thus,