DTSCUSSIONS

StabilitC des Culees de Ponts Ctablies sur des Pieux Traversant une Couche Mollel~': Discussion

GREGORY P. TSCHEBOTARIOFF Corlsitlting Erlgirccw., Lowrer~cevillc~, New .Ter.se.v

Received November 4. 1971 Accepted December 17, 1971

Fifteen cases of horizontal movements of bridge structures supported by piles driven through soft clay to sand, till, or rock are described and analyzed in the paper. Three of them, cases 12, 13, and 14, refer to arch type bridges where the superstructure is likely to have affected somewhat the move- ments of the abutments proper. In the remaining twelve cases no substantial hori- zontal forces could be transmitted by the steel bridge girders to their abutments and cer- tainly none at the ones providing rocker support. In all twelve cases the abutments, at rocker level, were found to have moved towards their backfill, "U(+)," indicating tilting, lateral translation, or a combination of both.

The authors have rendered an important service to the profession by their comparative analysis of these cases which clearly shows that the primary cause of the movements observed are the lateral plastic deformations of soft clays overstressed in shear by the weight of the embankment.

Attempts have been made, for instance by Stermac et ol. (1968) and by some of his discussers, to explain observed lateral move- ments of abutments towards their backfill by an imbalance of lateral forces against the abutment proper created by excessive settle- ments of the backfill due to the consolidation of the underlying soft clay. Supplementary piles of a reversed batter were suggested as a remedv.

This eiplanation does not go to the source of the problem and therefore can be danger- ously inadequate. Excessive settlements of

1Prepared discussion presented at the 24th Canadian Geotechnical Conference, Halifax, Nova Scotia, Sep- tember, 1971.

2Paper by R. Marche and Y. Lacroix, Can. Geotech.J.. 9, pp. 1-24.

the backfill are only a symptom but not the cause of the shearing deformations within the underlying clay - layer (Tschebotarioff 1970).

Measurements made by King and Gavaris, Consulting Engineers, under my direction at Allamuchy for the New Jersey State Depart- ment of Transportation (case 8) have shown, (see Nicu et 01. 1971) that the abutment movement of the "U(+)" type was essentially one of rotational tilting backwards around a point close to where the conventional batter piles were embedded in the footing of the abutment. Due to the formation of a settle- ment crater under the embankment, there actually is a decrease with time of lateral pressures of the granular backfill against the perched abutment proper, but this is accom- panied by an increase of lateral pressures of the underlying soft clay against the rear row of piles which support the heel of the abut- ment.

Bending stresses in these piles under the heel of the abutment, especially if they have a reversed batter which extends them below the fill, are then inevitable and failure may result unless they are designed to resist these stresses.

The importance of this point has been emphasized by case 15 in Quebec. Reversed batter piles under the heel of an abutment, apparently of the situ-cast concrete type which can be provided only with comparative- ly light flexural reinforcement, were found to have been fractured a few feet below the abutment footing which they supported.

Piles with adequate flexural resistance, battered in both directions, should be provid- ed. Conventionally battered piles resist the natural tendency of an overloaded embank- ment to slide towards its toe. In my opinion, it is the omission of such conventionally

Cn~lndinn Gcotccl~nicnl Journal, 9. 225 (1972)

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226 CANADIAN GEOTECHNIC.4L JOURNAL. VOL. 9. 19i2

battered piles which lead to what the authors term "U( -)" type abutment movements towards the center of the bridge.

The only actual case of such "U(-)" type movements quoted for girder bridge abutments is illustrated by Fig. 1 of the paper, as originally reported by Peck et 01. (1948). Tt will be noted from that Fig. 1 that only vertical piles were used and that they had only a very shallow penetration into the underlying sand layer. Also only vertical piles were used in the arch type bridges, cases 12 and 14. The latter also had a "U(-)" type of abutment movement, but the type of arch used in case 12 prevents lateral abut- ment movements or rotations, - however this could not prevent the piles from being fractured. The adequacy of the pile reinforce- ment under the arched bridge of case 13, also with a "U(-)" type of movement in- spite of battered piles, is not known.

T doubt that "U(-)" type of movement can actually take place, as shown for "type 2" on Fig. 7 of the paper and on Fig. 2 of the abstract thereof, once piles of an adequate flexural strength are battered in both direc- tions.

As I pointed out in my paper at the Lehigh Conference (1970), conventional design pro- cedures for abutments on piles driven through

soft clay should be drastically revised. The authors have contributed an important step in this direction.

Further discussion of this matter has been incorporated in the second completely revised edition of my 1951 book, (scheduled for publication early in 1973).

I should add that some of these findings may have direct application in the design of waterfront quay walls of the relieving platform type under similar soil and backfill conditions. I t is a topic where further experi- mental research promises to be very fruitful.

NICU, N. D., ANTES, D. R., and KESSLER, R. S. 1971. Field measurements on instrumented piles under an overpass abutment. Highw. Res. Board Rec. No. 354, Washington, D.C., Sept. 1971.

PECK, R .B., IRELAND, H. O., and TENG, C. Y. 1948. A study of retaining wall failures. Proc. 2nd Intern. Conf. Soil Mech. Found Eng., Rotterdam. 3, pp. 296299.

STERMAC, A. G., DEVATA, M., and SELBY, K. G. 1968. Unusual movements of abutments supported on end-bearing piles. Can. Geotech. J. 5, (2), pp. 69-79.

TSCHEBOTARIOFF, G. P. 1970. Bridge abutments on piles driven through plastic clay. Proc. Conf. on Design and Install. of Pile Found. and Cell. Struct; Lehigh University, April 1970, pp. 225-238.

---- -- 1973. Foundations, Retaining, and Earth Struct- ures, McGraw-Hill, New York. (In preparation for printing).

A Study of the Cementation Bonds of the Sensitive Clays of the Outardes River Region1 : Discussion

R. M. QUIGLEY The University of Wesfer~z Onforio, Lo~~rlon, 011fmio

Received December 14, 1971 Accepted December 21, 1971

The Authors (Loiselle, Massiera, and Sain- ani) have presented a most interesting and ambitious study of cementation bonds in a sensitive quick clay from central Quebec. They are to be complimented for undertaking and presenting a complex study which basically involved chemical pretreatment by EDTA so- lutions to destroy cementation bonds on the

lPaper by A. Loiselle, M. Massiera, and U. R. Sainani, Can. Geotech. J. 8, pp. 479-498.

one hand and re-creation of bonds by sodium chloride seepage on the other. Such a disser- tation is by its very nature always open to al- ternative interpretation and it is only through such discussion that a clearer view will be ob- tained of the complex interrelationships of physico-chemistry and the mechanical prop- erties of soils.

My comments are intended to be construc- tive and I hope will help launch the "further research on the behaviour of highly cemented

Canadian Gcotecllr~ical Journal. 9, 226 (1972)

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