土木工程类专业英文文献及翻译

PAVEMENT PROBLEMS CAUSED

BY COLLAPSIBLE SUBGRADES

By 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.

INTRODUCTION

When 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 called

a 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 the

soil type and structure, the initial soil density, the imposed stress state, and

the 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 dollars

in roadway repairs. The prediction of the volume changes that may occur in

the field is the first step in making an economic decision for dealing with

these 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, and

a much more rational design approach may be made. For example, typical

techniques for dealing with expansive clays include: (1) In situ treatments

with 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 make

the 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 techniques

are available for any roadway problem.

The emphasis here will be placed on presenting economical and simple

methods 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,