quires plug systems to prevent migration through boreholes and shafts of harmful amounts of radionuclides to the biosphere. Plugs formed by expanding clay, which migrates through the perforation of metal pipes in which it is confined at the insertion in the boreholes, get a very low permeability and maintain a sufficient swelling potential to create a per­fect contact with the rock.

Tests indicate that the water uptake, swelling, and redistribution of water and minerals is a very complex process. Theoretical prediction of the rate of maturation can be made for the simple case of unlimited ac­cess to water at the peripheral boundary of the clay. Thus, it can be described as a diffusion process where differences in particle concentra­tion forms the gradient (2). For practical purpose it may be sufficient to know that the dominant part of the water uptake and swelling of clay plugs in boreholes takes place in about a mon th if the holes are water-filled from the start.

Experience from laboratory and field tests as well as from recent full-scale plugging operations from drill rigs at sea, shows that the technique should now be applicable in practice even to very long boreholes. It will be necessary to stabilize crushed rock zones crossed by the holes so that the insertion of the pipes can take place without hindrance.

APPENDIX.—REFERENCES

1. Pusch, R., and Bergstrom, A., "Highly Compacted Bentonite for Borehole and Shaft Plugging." Workshop on Borehole and Shaft Plugging, Organization for Economic Cooperation and Development, Nuclear Energy Agency, Colum­bus, Ohio, May 7-9, 1980.

2. Pusch, R., 1981, "Borehole Sealing with Highly Compacted Na Bentonite," Technical Report 81-09, Swedish Nuclear Fuel Supply Co., Dec, 1981.

A SIMPLE QUESTION ABOUT CONSOLIDATION

By John H. Schmertmann,1 F. ASCE

INTRODUCTION

This technical note asks the reader to consider a question about the mechanics of the behavior of soil. It appears that the profession has not produced an answer to this question and perhaps has not adequately

'Principal, Schmertmann and Crapps, Inc., Gainesville, Fla. 32601. Note.—Discussion open until June 1, 1983. To extend the closing date one

month, a written request must be filed with the ASCE Manager of Technical and Professional Publications. The manuscript for this paper was submitted for re­view and possible publication on April 3, 1981. This paper is part of the Journal of Geotechnical Engineering, Vol. 109, No. 1, January, 1983. ©ASCE, ISSN 0733-9496/83/0001-0119/$01.00. Proc. No. 17601.

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considered it. Yet, the answer involves predicting correctly a funda­mental aspect of soil engineering behavior. This note does not answer the question. Rather, it calls attention to it and to the wide divergence of professional opinion on the answer and to what this says about our understanding of soil consolidation behavior.

THE QUESTION

Consider this apparently simple question: Does the effective lateral stress in one-dimensional compression of a normally consolidated cohe­sive soil, such as in the oedometer test, increase, remain the same, or decrease during secondary compression aging? Fig. 1 illustrates the question using the effective stress path (ESP) format suggest by Lambe (1). Imagine that after a time interval of primary normal consolidation along ESP path 1-2 on the appropriate K0 line the investigator then leaves the soil at constant a[ during its subsequent secondary compres­sion aging. Herein K0 = <J'J<J[. During such aging will a'3 (and therefore also K0) increase, causing shear stresses to decrease and produce a con­tinuing 2-3 ESP along an A = - 1 path? Or, will <r'3 and K0 decrease and thus increase shear stresses and produce a continuing ESP along an A = +1 path? Or will they remain the same and Points 2 and 3 coincide on the Lambe ESP but not on the consolidation ESP?

• t

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SSI

0 IN l-D COMPRE

ID RATI

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> CC

o

VOLUME

J V c t ^ 1+Ko ^ J S T 5 ^ * ? ? ^ ^

*^7yz N* \ ^ ^ / ^ l \ \ \

?-<— ——?

03'

\ A,

VERTICAL CONSOLIDAT

T' 1

«" constant

\

\

X secondary

t ime (

ION STRESS

1

3

o r '

—>.

FIG. 1.—Illustration of the Question

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Alternatively, one can also ask the question in this form: Will K0 = Vz/cr'x of a normally consolidated cohesive soil increase or decrease dur­ing secondary aging in 1-D compression?

IMPORTANCE OF THE QUESTION

The interested reader will most likely answer the question in accord with his or her preception of soil behavior. If one imagines cohesive soil to behave as an assemblage of particles separated by bound water layers with viscous behavior, then particle to particle contact shear stresses will diminish with time and the reader would presumably answer "in­crease." He or she would expect the soil to act in a plastic, time-soft­ening manner. The reader considering such soil to behave in an essen­tially elastic manner might logically answer "no change" because of a presumed constant Poisson's ratio. Still another reader might reflect on how in situ and laboratory-consolidated clays tend to increase their strength and modulus with time and answer "decrease" because a greater strength implies a reduced value for K0. He or she would expect the soil to act in a plastic, time-hardening manner.

The aforementioned give only some of the possible thought paths to different answers to the question. No doubt other though paths exist. But each should reflect something basic in the way the reader models the cohesive soil behavior in situ and in the laboratory.

THE UNCERTAINTY

One would think that somewhere in the nearly 60-yr history of Soil Mechanics since Terzaghi first formally introduced his effective stress consolidation theory that the question posed would have undergone in­vestigations and the answer would have become part of our basic un­derstanding of the mechanics of soils. However, a modest literature search by the writer has not, with one little known exception, (2), even produced the question much less the answer. During the recent prep­aration of part of a paper (3) considering a possible explanation for the quasi-preconsolidation effect, the writer just took for granted a "de­crease" answer to the question. A prominent reviewer promptly chal­lenged this assumption. This reviewer would have answered "increase." This led to the present more thoughtful consideration of the question and its implications. It also led the writer to attempt a written and verbal survey of 40 geotechnical engineers prominent for their research and work with soil consolidation or related soil behavior. The survey time interval ran from November, 1980 through January, 1981. Table 1 sum­marizes the results.

Table 1 breaks down the answers to permit comparisons with respect to: (1) Whether the writer received a written or verbal reply; (2) nation­ality; (3) the research or practice principal of orientation of the respond­ent; and (4) his estimated age bracket. Twenty three of thirty men polled in writing prepared written replies. The verbal poll usually involved only a conversation with the writer. Inspection of Table 1 will show gen­erally similar response patterns for each category in each comparison. Approximately one half said "increase," one quarter "remains the

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TABLf 1.—Summary of Response to Question

Categories d)

Written Verbal

USA Canada Europe

Research/teaching Consulting/practice

Estimated age > 50 Estimated age £ 50

Increase (2)

11 5

16

11 2 3

11 5

9 7

Number of Responses

Same (3)

8 1 9

3 4 2

6 3

6 3

Decrease (4)

2 2 4

4 0 0

2 2

2 2

Don't know (5)

2 1 3

2 0 1

1 2

1 2

Total (6)

23 9

32

20 6 6

20 12

18 14

same," and one eighth each for "decrease" and "don't know." Based on the scarcity of relevant data in the literature and the general diversity of opinion shown in Table 1, it seems fair to say that the geotechnical engineering profession does not have anywhere near a common under­standing of the soil behavior involved.

THE CHALLENGE

During these days of the development of stress path thinking, con­stitutive equations, Cam-clay models, etc., we perhaps need to step back and take a closer look at the state-of-the-art with respect to our under­standing of soil-structure behavior as exemplified by the proposed ag­ing-stress question.

APPENDIX.—REFERENCES

1. Lambe, T. W., "Methods of Estimating Settlement," ASCE, Journal of the Soil Mechanics & Foundations Div., Specialty Conf. on Design of Foundations for Control of Settlement, Sept. 1964, pp. 47-71.

2. Newlin, C. W., "A Laboratory Investigation of Lateral Stresses during One-Dimensional Consolidation," thesis presented to Northwestern University at Evanston, 111., in 1965, in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

3. Schmertmann, John H., "A General Time-Related Soil Friction Increase Phe­nomenon," Laboratory Shear Strength of Soil, ASTM STP 740, 1981, p. 481.

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