NRC Publications Archive Archives des publications du CNRC Bearing capacity calculations for piles in permafrost Parameswaran, V. R. This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur. Publisher’s version / Version de l'éditeur: Proceedings of the 4th International Conference on Cold Regions Engineering, pp. 751-759, 1986 NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=da8a8773-2eab-41fd-979a-862c71c695b5 https://publications-cnrc.canada.ca/fra/voir/objet/?id=da8a8773-2eab-41fd-979a-862c71c695b5 Access and use of this website and the material on it are subject to the Terms and Conditions set forth at https://nrc-publications.canada.ca/eng/copyright READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site https://publications-cnrc.canada.ca/fra/droits LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB. Questions? Contact the NRC Publications Archive team at [email protected]. If you wish to email the authors directly, please see the first page of the publication for their contact information. Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à [email protected]. Ser THl National Research Consell nationel N21d Council Canada de rec- Canada no. 1363 institute for Imtitut de c. 2 Research in recherche en BLrn 1 Construction construction Bearing Capacity Calculations for Piles in Permafrost by V.R. Parameswaran ANALYZED Reprinted from Proceedin s of the 4th International Conference Cold 8egions Engineering, TCCqE, ASCE Anchorage, AK, February 24-26,1986 p. 751 -759 (IRC Paper No. 1363) Price $2.00 BLDG. RES. NRCC 25847 LIBRARY 86- 07- 1 5 ABSTRACT Small-scale model piles frozen in various soils were tested at constant rates of displacement and constant load. The derived values were used to calculate the naximurn allowable stress at the pile-soil interface. - - Des modsles reduits de pieux poses dans divers sols gel& ont 6t6 soumis B des essais 3 des vitesses de dgplacement et sous des charges constantes. On a utilisg les valeurs ainsi Obten,,P.- -- -- --I---%-- -1- - u-*--*a*- admissible 3 l'ir Reprinted from Proceedings of the 4th Int7. Conference Cold Regions Engineering, TCCRE, ASCE Anchorage, AK, February 24-26.1986 BEARING CAPACITY CALCULATIONS FOR PILES IN PERMAFROST V. R. Parameswaran* Small-scale model piles frozen in various soils were tested at constant rates of displacement and constant load. The derived values were used to calculate the maximum allowable stress at the pile-soil interface based on: 1) minimum or steady-state displacement rates observed in the constant-load creep tests, and 2) time to failure, which is equal to the time to attain peak load in tests carried out under constant dis9lacement rate and (for constant-load creep tests) time to the onset of tertiary OL accelerating creep. The latter method, which applies Vyalov's equation to constant-load creep tests, gives the lowest value for allowable stress, based on a failure time of 25 years, and can provide a better margin of safety during the life of a structure founded in permafrost. Introduction Pile foundations are probably the simplest and most commonly used support for structures built in permafrost areas. They bear the load of the structure by means of two mechanisms: adfreezing strength between pile and soil, and end bearing. The total bearing capacity of a pile embedded in frozen ground can be calculated by suitably combining the contributions of the two mechanisms. The effect of the active layer, which seasonally freezes and thaws, is usually ignored since the pile is embedded in the perennially frozen ground to a depth great enough to overcome the effects of heaving and slumping. In the early days of construction it was common practice to embed the pile at a depth below the permafrost table at least three times the thickness of the active layer. An empirical formula can be used to calculate the total bearing capacity of a pile (Vyalov and Porkhaev, 1976): where r:d is the adfreeze strength in the ith layer in ground consisting of n layers, Ai is the adfreezing area between pile and soil in that layer, o is the compressive strength of the soil under the tip of the pile, and Ab is the end-bearing area of the pile. In most soils the coefficients of homogeneity, kl and kp, can be assumed "Research Officer, Division of Building Research, National Research Council of Canada, Ottawa, Canada. KIA OR6 752 COLD REGIONS ENGINEERING to be equal to 1. For reticulate or layered ice-rich soils with a fractional ice content. i, k, = 1-i. The coefficients of performance, ml, and m , are equal to 1 for most soils for temperatures below -2OC. For most $ine-grained clayey soils m2 I 1 to 1.5. For coarse-grained soils, however, ma takes values of up to 2.5; and for piles resting on an ice layer or piles placed in steamed or slurried holes, m2 = 0. If end-bearing is neglected and it is assumed that the ground consists mostly of one type of soil, considered to be homogeneous, the equation reduces to: where Tad is average adfreeze strength between pile and frozen soil, and A is the total interfacial area. Thus the important parameter that governs bearing capacity is T, and the objective of any good design is to provide a value for allowable stress, that can be borne by the pile foundation without exceeding the total allowable settlement during the life of the structure. T~ will depend on temperature, grain size, and moisture content of the soil and the characteristics of the pile Itself. The soil at the pile-soil interface is under a constant load, and therefore the creep characteristics of the soil must be known. As sufficient data regarding adfreeze strength do not exist for all soils and terrains. the range of long-term adfreeze strength values for given soils has to be determined from short-term tests carried out in.the laboratory (using model piles), or the field (using full-scale piles) or from long-term tests of piles under constant load. As discussed in an earlier paper (Parameswaran 1985), there are essentially three methods of estimating the allowable stress, T~~~,at the pile-soil interface from such tests. 1) The peak adfreeze strength determined from constant-rate tests is plotted against the imposed rate of displacement (usually on a log-log or semilogarithmic scale) and the resulting curve extrapolated backwards to the allowable displacement rate. A total allowable settlement of 1 in. (25.4 mm) in 25 years, for example, will correspond to an allowable average rate of settlement of about 2 x mm/min. The value of T~~~ SO obtained, however, is an upperbound value, and large factors of safety may have to be used in design calculations. 2) The minimum displacement rates observed in the constant-load creep tests are plotted against corresponding stresses to give the value of allowable stress corresponding to allowable displacement rate. The assumption in this procedure is that the transient creep regime (consisting of instantaneous displacement as well as primary or attenuating creep) is negligibly small compared with the steady-state creep regime. This is not a valid assumption in most instances. The method does, however, give a smaller value for allowable stress than that obtained from the short-term tests carried out under constant displacement rates, PILES IN PERMAFROST 753 3) The third method makes use of an equation relating time, tf, for the onset of tertiary or accelerating creep and applied stress, T. Analogous to many viscoelastic materials, for frozen soils this relation is given by: where to and T~ are constants characteristic of the material and have dimensions of time and stress, respectively. The equation was first suggested by Vyalov (1959, 1962) and has been used extensively (Sayles, 1968, 1973; Sayles and Haines, 1974) for various frozen soils. The term tf is called failure time. Thus, by plotting In (tf) as a function of stress for various pile tests at different temperatures the values of the constants to and T~ can be determined. For a particular failure time, tf, the allowable stress can be obtained from the plot or by calculation. This method takes into account the transient creep regime and the secondary or steady-state region of creep, i.e., the total creep curve from the beginning of a test until the onset of tertiary or accelerating creep. This paper assesses the three methods by applying them to data obtained from pile tests carried out in the laboratory on frozen soils under controlled conditions. I Experimental Procedure I Small-scale model piles (diameter 3 in.; 76.2 mm) made from ! natural wood (B.C. fir or spruce), steel, and concrete were frozen I into three soils: sand of uniform grain size containing 14% moisture ' by weight and compacted to an optimum Proctor dry density, yd, of 1700 kg/m3; silty sandy soil from N.W.T. containing 20% moisture; and silty clay from N.W.T. containing 45% moisture by weight.