Time-dependent displacement behaviour of model ...

Time-dependent displacement behaviour of model adfreeze aAdzgroukd piles in saline frozen soils %

K.W. BIGGAR Department of Civil E~lgineering,Royal Military College of Cnncrcln, Kitzgston, ON K7K5L0, Carlacla

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AND

D.C. SEGO Depcirt~rzetztof Civil Engineerirzg, The University of Alberta, Erlrizorlto~z,AB T6G2G7, Carzncln Received June 14, 1993 Accepted November 26, 1993 The findings of a laboratory study on the time-dependent displacenlent of model piles in saline frozen soil are reported. The short-term time-dependent pile deformation in ice-poor saline silty sand was best described using a simple power law of time, whereas the long-term time-dependent deformations were best described using a flow law formulation similar to that used to describe the long-term time-dependent deformation of ice or ice-rich permafrost. The use of cementitious grout as a backfill resulted in doubling of the pile load carrying capacity for a given displacement rate. The laboratory test results compare well with other laboratory and field studies. Key words: frozen soil, saline, model pile, time-dependent deformation, sand backfill, grout. Les rCsultats d'une Ctude en laboratoire sur le fluage de pieux modides dans des sols salins gel& sont prCsentCs. Le fluage B court terme reprksentant le tassement du pie^^ dans un sable silteux ii faible teneur en glace est bien dCcrit par une loi exponentielle en fonction du temps alors que le fluage 5 long terme est mieux dCcrit avec une loi d'Ccoulement similaire B celle utilisCe pour reprCsenter le fluage i long terme de la glace ou du $ergClisol riche en glace. L'utilisation d'un coulis ii base de ciment comme matkriau de remplissage permet de doubler la capacitC portante du pieu pour un taux de tassement donnC. Les rCsultats des essais en laboratoire se compaient bien avec ceux d'autres essais en laboratoire et au chanties. Mots clds : sols gel& salin, pieu modkle, fluage, remplissage en sable, coulis. [Tsaduit par la ridaction] Can. Geotech. J . 31, 395-406 (1994)

Introduction The performance of a number of different model pile backfill configurations in saline frozen silty sand under constant displacement rate loading conditions was examined in Biggar and Sego (1993b). Sandblasted pipe piles with clean sand or grout backfill were observed to undergo similar stress-displacement behaviour, although the capacity of the clean sand backfilled piles was limited by the strength of the adfreeze bond, whereas the grout-backfilled piles behaved in a strain-stre~~gthening manner. Untreated pipe piles with a clean sand backfill and the use of silty sand c ~ ~ t t i n gass a backfill material were observed to provide inferior loadcarrying capacity. The use of a grout as a backfill material is more expensive and requires greater construction control than use of a sand backfill, so a study was undertaken to determine whether the enhanced load-carrying capacity of grout-backfilled piles observed in the short-term constant displacement rate tests was also applicable under long-term, constant-load conditions. The study examined the performance of model sandblasted pipe piles with both grout and sand backfill in frozen silty sand at salinities of 0, 10, and 30 ppt and a constant temperature of -5°C. The objective was to examine pile performance in soil at salinities in which either adfreeze bond strength or time-dependent deformation govern the pile capacity, thereby providing guidance on which pile configuration provides optimum performance in the vario~ts native soil conditions. Experimental procedure General The test cell, pile material, and sample preparation for the constant-load tests were identical to that used for the P:intcil cn Cnnada 1 IinprimC :lu Cttondn

constant displacement rate tests discussed in Biggar and Sego (199312), s o will not be discussed further. Only one soil temperature was examined (-5"C), and all piles had their surface sandblasted prior to installation. Piles of three different diameters were used in the test specimens. Pile diameters were two-thirds of the hole diameter, thus piles 33, 63, and 102 nlm in diameter were placed in prebored holes 52, 102, and 152 mm, respectively. To test piles of three different diameters with the same test specimen, the smaller piles were overcored with a larger core barrel. The variables examined in this program were limited to the native soil salinity (the soil in the test cell prior to pile installation) and the backfill material. Pile performance under constant load using a s a l i n e backfill material was reported in Hutchinson (1989), thus only clean nonsaline sand slurry and grout backfill materials were examined during this study. After the piles were placed and backfilled in the native soil test specimen, it was left for a minimum of 24 h to allow the backfill to freeze (or cure, for grouts). The test cells were then moved into the test frame and connected to a constant-temperature bath, and the soil temperature was allowed to stabilize for a further 24 h period. Loadirzg The loading frame, shown schematically in Fig. 1, consisted of reinforced 150 mm wide channels on the top and bottom separated by four 25 mm all-thread rods. A second type of frame was also used which had 150 m n channels for the vertical members instead of threaded rods. Load was applied to the cell via a Bellofram or a jack (further referred to simply as jacks) using compressed air controlled by a pressure regulator. Loads less than 25 kN were applied using Belloframs and the laboratory-supplied compressed air (up to

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396

CAN. GEOTECH. J . VOL. 31, 1994

The expressions creep and t h e - d e b e n d e n t deformation or displacement are often used interchangeably without regard to the mechanisms involved. Strictly speaking, creep refers to time-dependent deformation of a material under constant stress with no change in volume. In frozen saline soils the time-dependent deformation behaviour may be affected by consolidation due to higher unfrozen water contents (Domaschuk et al. 1983; Nixon and Lem 1984). To avoid such ambiguity in this paper, the expression timedependent deformation will be used to refer to soil deformation behaviour under constant stress, and time-dependent displacement will refer to the displacement behaviour of piles under constant load. Time-dependent deformation in frozen soils may b e described in four stages: an instantaneous strain; a period of decelerating strain rate, often called primary creep; a period of constant strain rate, often called secondary creep; and a period of accelerating strain rate, often called tertiary creep. To confirm that a constant strain rate has been achieved it is most useful to plot the data with the logarithm of strain rate as the ordinate axis versus the logarithm of time as the abscissa axis (Morgenstern et al.. P980). The four stages of time-dependent deformation aqd the relationship between strain rate and time are s h o w n ~ ~ c h e m a t i c ain l l ~Fig. 2.

FIG.1 . Constant load test apparatus.

875 kPa). Larger loads (up to 56 kN) were applied using bottled compressed extra dry air and jacks capable of sustaining air pressures up to 7000 kPa. Each loading system was capable of maintaining the load within 1% of the desired level. An initial small load ( 105 000 > 59 000 3 000 60 000 44 700 12 000 > 65 000

Grout Grout Grout Grout Grout Grout

0.1 0.5 3.4 0.2 1.6 3.4

6 700

0.3 Incremental 2.6

None None

Sand Sand

-5.5 -5.1

25 000 > 13 000

Incremental Incremental

2.4 None

Sand

-4.0

8 700

Incremental

4.9

-5.8 -4.8 -5.2 -3.2

> 67 000 > 30 000 > 50 000 800

0.6 Incremental Incremental 2.9

Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand Sand

Immediate failure -5.6 -4.6 -5.0 -3.0

Test temp. ("C)

?

11a

na 5.0 1.7 2.7 6.5 6.0 7.0 4.9 2.6 2.6 2.9 6.4 5.4 2.8

-5.2 -5.3 -5.5 -5.1 -5.1 -5.3 -5.2 -5.1 -5.0 -5.3 -5.3 -5.3 -5.3

gZ /i: 2

-5.1 -5.2 -5.5 -5.4 -5.4 RTD failure -5.5 RTD failure RTD failure -5.3 -5.4. -4.9-5.3 -3.2 -5.1 ' -5.0 -3.1 -5.2 -5.2 -5.0 -4.9 -5.1 -5.2 -5.0 -5.3

g