UNUSUAL MOVEMENTS OF ABUTMENTS SUPPORTED BY ENDBEARING PILES: DISCUSSION
2
366 CANADIAN GEOTECHNICAL JOURNAL UNUSUAL MOVEMENTS OF ABUTMENTS SUPPORTED BY END- BEARING PILES, by A. G. Stermac, M. Devata, and K. G. Selby (Vol. V, no. 2, pp. 69-79). LAURITS BJERRUM Nofwegian Geotechnical Institute, Oslo, Norway. When reading Stermac, Devata, and Selby's paper on the movements of abutments on end-bearing piles it struck me that these movements appeared to be very similar to those observed on abutments resting on a compressible foundation. Now recent measurements on piles driven to rock through thick deposits of soft clay have demonstrated that even small settlements of the clay will result in negative skin friction which is so large that the lower part of the pile will yield, resulting in considerable settlements of the piles (Johannessen and Bjermm 1965; Bjerrum et al. 1969). The order of magnitude of these settlements was from 1 to 4 inches (2.5-10 cm). Taking these findings into consideration there may be good reasons to try to review available experience from abutments resting on a compressible foundation in order to investigate whether they may help to explain the behavior of the cases presented. The feature of an abutment most essential for its non-uniform deformation is obviously the abrupt change in load on the ground surface on its two sides. Provided the vertical reaction from the bridge structure is relatively small compared with the weight of adjacent fill, the settlements of the ground behind the abutment where the load is largest will always be greater than the settle- ments of the front of the abutment. The movement of the abutment will there- fore, in addition to a vertical settlement, be a backward tilt resulting from the simple fact that the back of the abutment will settle more than the front. This movement will appear as an increase in the distance between the abutments and may therefore cause problems to the bearings of the bridge. This type of non-uniform movements has been described by several authors (see e.g., Casagrande 1939). A well documented set of observations illustrating the corre- lation between differential settlements and horizontal movements of an abut- ment was published by Bjerrum et al. in 1957. Most likely the same explanation is valid for the bridges described by Ster- mac et al. As mentioned above, there is probably no doubt that the piles carry- ing the abutments of the bridges described in the paper are subjected to considerable negative friction due to the settlement of the fill, which is of the order of 4 ft to several feet. There are also reasons to believe that the piles below the back of the abutments, where the height of fill is large, have under- gone settlements which may very well be of the same order as observed on Norwegian piles under similar conditions, as mentioned above. In contrary, the batter pile, below the front part of the abutments, where the height of fill is small, obviously has settled less than the piles below the back of the abut- ments. The effect of the negative skin friction on the piles is, therefore, likely to be a differential vertical settlement of the piles leading to a backward tilt of the same type as observed on abutments resting on the compressible foundation. If the above explanation is correct, the observed behavior is a result of negative skin friction on the piles. To avoid similar occurrences in the future, it would, for instance, only be necessary to reduce the skin friction Can. Geotech. J. Downloaded from www.nrcresearchpress.com by WA STATE UNIV LIBRARIES on 11/09/14 For personal use only.
Transcript of UNUSUAL MOVEMENTS OF ABUTMENTS SUPPORTED BY ENDBEARING PILES: DISCUSSION
366 CANADIAN GEOTECHNICAL JOURNAL
UNUSUAL MOVEMENTS OF ABUTMENTS SUPPORTED BY END- BEARING PILES, by A. G. Stermac, M. Devata, and K. G. Selby (Vol. V, no. 2, pp. 69-79).
LAURITS BJERRUM Nofwegian Geotechnical Institute, Oslo, Norway.
When reading Stermac, Devata, and Selby's paper on the movements of abutments on end-bearing piles it struck me that these movements appeared to be very similar to those observed on abutments resting on a compressible foundation. Now recent measurements on piles driven to rock through thick deposits of soft clay have demonstrated that even small settlements of the clay will result in negative skin friction which is so large that the lower part of the pile will yield, resulting in considerable settlements of the piles (Johannessen and Bjermm 1965; Bjerrum et al. 1969). The order of magnitude of these settlements was from 1 to 4 inches (2.5-10 cm). Taking these findings into consideration there may be good reasons to try to review available experience from abutments resting on a compressible foundation in order to investigate whether they may help to explain the behavior of the cases presented.
The feature of an abutment most essential for its non-uniform deformation is obviously the abrupt change in load on the ground surface on its two sides. Provided the vertical reaction from the bridge structure is relatively small compared with the weight of adjacent fill, the settlements of the ground behind the abutment where the load is largest will always be greater than the settle- ments of the front of the abutment. The movement of the abutment will there- fore, in addition to a vertical settlement, be a backward tilt resulting from the simple fact that the back of the abutment will settle more than the front. This movement will appear as an increase in the distance between the abutments and may therefore cause problems to the bearings of the bridge. This type of non-uniform movements has been described by several authors (see e.g., Casagrande 1939). A well documented set of observations illustrating the corre- lation between differential settlements and horizontal movements of an abut- ment was published by Bjerrum et al. in 1957.
Most likely the same explanation is valid for the bridges described by Ster- mac et al. As mentioned above, there is probably no doubt that the piles carry- ing the abutments of the bridges described in the paper are subjected to considerable negative friction due to the settlement of the fill, which is of the order of 4 ft to several feet. There are also reasons to believe that the piles below the back of the abutments, where the height of fill is large, have under- gone settlements which may very well be of the same order as observed on Norwegian piles under similar conditions, as mentioned above. In contrary, the batter pile, below the front part of the abutments, where the height of fill is small, obviously has settled less than the piles below the back of the abut- ments. The effect of the negative skin friction on the piles is, therefore, likely to be a differential vertical settlement of the piles leading to a backward tilt of the same type as observed on abutments resting on the compressible foundation. If the above explanation is correct, the observed behavior is a result of negative skin friction on the piles. To avoid similar occurrences in the future, it would, for instance, only be necessary to reduce the skin friction
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y W
A S
TA
TE
UN
IV L
IBR
AR
IES
on 1
1/09
/14
For
pers
onal
use
onl
y.
DISCUSSIONS 367
between piles and clay and, as described in the recent paper (Bjerrum et al. 1969) this can be done at a reasonable cost by coating the piles with bitumen.
CASAGRANDE, L. 1939. Setzungsbeobachtungen an Briickenbauten der Reichsautobahn. Bodenmechanik und neuzeitlicher Strassenbau. 2. Folge. Berlin, Volk und Reich Verlag. pp. 106-125.
BJERRUM, L., JONSON, W., and OSTENFELD, C. 1957. The settlement of a bridge abutn~ent on friction piles. Proc. 4th Intern. Conf. Soil Mech. Foundn. Eng., London, 2, pp. 14-18.
BJXRRUM, L., JOHANNESSEN, I., and EIDE, 0. 1969. Reduction of negative skin friction on steel ~ i l e s to rock. Paper to be presented at the 7th Intern. C o d . Soil Mech. Foundn. Eng., Mexico, 1969.
JOHANNESSEN, I., and BJERRUM, L. 1965. Measurement of the compression of a steel pile to rock due to settlement of the surrounding clay. Proc. 6th Intern. Conf. Soil Mech. Foundn. Eng., Montreal, 2, pp. 261-264. (Also published in Norwegian Geotechni- cal Institute Publ. 65.)
Manuscript receiued March 28, 1969.
ERRATA
1. The captions for Figs. 6 and 7 in the paper by DeLory and Salvas (May 1969, 11.104 and 105) are interchanged.
2. The caption for the upper photograph on p. 122 in the paper by Robinson and Taylor (May 1969) is "Fig. 3. Typical p~dl-out test during anchor design".
3. On line 18, p. 127, the numbers in parentheses "(.46 x 13.7 m)" should read " ( .46 x 1.37 m) ".
Can
. Geo
tech
. J. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y W
A S
TA
TE
UN
IV L
IBR
AR
IES
on 1
1/09
/14
For
pers
onal
use
onl
y.
