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,

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, Vol. 114, No. 6,

November, 1988. ASCE, ISSN 0733-947X/88/0006-0673/$1.00 + $.15 per page.

Paper No. 22902.

673

field for the collapsible soils. Then this information will place the engineer

in 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 the

amount of volume change that may occur, will be discussed in the following

sections.

COLLAPSIBLE SOILS

Formation of Collapsible Soils

Collapsible 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 wetted

from normal rainfall. It is the combination of the depositional process and

the climate conditions that leads to the formation of the collapsible soil.

Although collapsible soils exist in nondesert regions, the dry environment in

which evaporation exceeds precipitation is very favorable for the formation

of the collapsible structure.

As the soil dries by evaporation, capillary tension causes the remaining

water 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, and

silts come out of solution, and "tack-weld" the larger grains together. This

leads to a soil structure that has high apparent strength at its low, natural

water content. However, collapse of the "cemented" structure may occur

upon wetting because the bonding material weakens and softens, and the soil

is 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 soil

is increased above that which naturally exists, collapse can occur at fairly

low levels of stress, equivalent only to overburden soil pressure. Additional

loads, such as traffic loading or the presence of a bridge structure, add to

the collapse, especially of shallow collapsible soil. The triggering mechanism

for collapse, however, is the addition of water.

Highway Problems Resulting from Collapsible Soils

Nonuniform collapse can result from either a nonhomogeneous subgrade

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