Rosenberg 1976

NOTE,

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Friction and end bearing tests on bedrock for high capacity socket design PETERROSENBERG A N D NOELL. JOURNEAUX Geotechniccrl Depcrtment, Professionul Services Division, Warnock Her-sey Professional Services Ltd., 128 Elmslie Street, LaSnlle, P.Q., Crrnrrda H8R I V8 Received October 22, 1975 Accepted May 4, 1976 Rock sockets are used to transfer axial loads to bedrock by a combination of periferal contact bond and end bearing. The load distribution with embedded depth in relation to rock properties actually measured is discussed. Case histories are presented. The results of full scale and bearing and friction tests carried out on various rock types are given. A relation between the unconfined compression strength of rock and the developed concrete to rock bond value is reported. Les emboitures dans la roche servent a transmettre les charges verticales a la roche par le frottement lateral et la resistance a la pointe combines. Des cas types de la ripartition de la charge selon les proprietes de la roche en place sont decrits. On prtsente les resultats d'essais en vraie grandeur de la capacite portante a la pointe et en frottement lateral effectuis sur differents types de roche. Une relation est proposee entre la resistance a la compression simple de la roche et la valeur ultime du frottement entre le biton de I'emboiture et la roche. [Traduit par la revue] Can. Geotech. J . . 13,324(1976)

Introduction A rock socket consists of a hole churned or drilled into bedrock and filled with concrete. The function of the socket is to transfer structural loads through upper non-competent strata to the bedrock. The load is transmitted to the bedrock along the contact surface between the concrete and the rock and by end bearing on the rock surface. Such a method of support is frequently used, especially for heavy highrise structures and large bridges. In spite of the increasing use of rock sockets, very little information is available regarding their design. The meager amount of work reported is probably due to the high cost involved in obtaining full scale field test results. The diameter of the sockets vary from 12 up to 120 in. (0.3 up to 3.0 m). The diameter associated with most building construction is of the order of 24 to 60 in. (0.6 to 1.5 m). The sockets supporting the Pont Bisson Bridge which crosses the Rivikre des Prairies in Montreal are 120 in. (3.0 m) in diameter (Heavy Construction News 1974).

Moore (1974), Thorburn (1966), Matich and Kozicki (1967), and Seychuck (1974) have reported the results of field test on sockets embedded in bedrock. Freeman et al. (1972) reviewed design criteria, and gave range of bond values presently used in shale bedrock. Few field evaluations of actual measured bond and developed end bearing stresses in instrumented sockets are available with the exception of the results given by Jackson et al. (1974). Osterberg and Gill ( 1973) have carried out theoretical studies concerning stress transfer of axially loaded concrete sockets embedded in rock of varying stiffness. This note briefly reviews the various methods used to design rock sockets. The results of end bearing and bond values determined by field tests are examined, as are data reported in the literature. On the basis of limited data a tentative correlation between bond and rock properties is presented. A design method based on predicted end bearing and bond mobilized on a rock socket is given.

NOTE

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Present Design Methods Freeman et al. (1972) reviewed methods of socket design and gave the various assumptions made. Four basic socket design methods may be considered. ( 1) Design For End Bearing Only

This method requires that the socket base be extended to a depth sufficient so that the stress does not exceed the allowable end bearing capacity of the rock. The effect of bond between the concrete and the surrounding rock is ignored. In this method the assumption made that all the axial stress is transmitted to the socket base. Since the load transferred to the rock by bond is ignored, the result is that actual stresses on the rock at the socket tip may be significantly lower than assumed, since actual field tests indicate that even in fractured rock the concrete to rock bond is significant. ( 2 ) Design on Concrete-Rock Bond Only This technique assigns an average concrete to rock bond value for the socket periphery and any contribution to the capacity derived from end bearing is ignored. Designs based on average concrete to rock bond values are given by Moore (1964) and Matich and Kozicki (1967); socket lengths of the order of 15 to 30 ft (4.57 to 9.1 m) were required. Under extremely poor rock conditions or if the bottom of the socket cannot be cleaned out properly, this technique may be advantageous. Under most conditions it will result in extremely long sockets. ( 3 ) Design on Allowable End Bearing and Carry Remaining Load in Bond This procedure assigns an allowable end bearing value for the socket base area and subtracts this value from the axial load. The socket length is made long enough to carry the remaining load by bond. This method does not consider the actual stress develo~ed at the socket end and as shown by ~ a c k s o net al. (1974) on an instrumented rock socket, the stresses actually developed may be at variance with the assumptions made, in this case the socket base unit stress were lower and the bond stress higher than anticipated.

325

( 4 ) Design on Estimated Developed End Bearing and Bond This method assumes that the applied load is transferred by bond and by end bearing actually developed at the base. A prediction of the load reaching the base of the socket is required. Based on this prediction an allowable end bearing value is assigned and it is assumed that the rest of the stress is carried by bond. The socket length is adjusted so that the allowable end bearing and bond values are not exceeded. The main difference between this and the other methods is that the relation between applied axial stress and actual developed end bearing stress for various socket embedments and rock properties is required.

Load Transfer Finite element studies by Osterberg and Gill (1973) indicate that the percentage of load transferred to the base for a given socket geometry is dependent on the relative stiffness of the concrete socket with respect to the surrounding rock. Figures 1 and 2, taken from Osterberg and Gill (1973) give theoretical load transfer curves for sockets embedded 1.5 to 2 diameters into rock of varying moduli ratios. The results show that as the relative stiffness of rock to concrete increases the proportion of load carried by the socket base decreases. For most practical purposes concerning sockets in sound hard rock such as massive granites and limestones the modulus ratio (Er/ Ec) may be considered to be close to 1, for medium sound rock the ratio would be expected to be of the order of 1/5 and for highly fractured and weathered rock the ratio would be expected to be in excess of 1/50. With a reasonable assessment of the rock modulus a prediction of the proportion of the load transferred to the base may be made. Allowable End Bearing Most work carried out to determine the bearing capacity of rock relates the ultimate bearing capacity to the unconfined compressive strength. The unconfined compressive strength is based on laboratory tests of intact rock cores

CAN. GEOTECH. J. VOL. 13, 1976

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PERCENT

OF APPLIED AXIAL LOAD

POISSON'S RATIO -0.26

FIG. 1. b a d transfer curves for varying moduli ratio computed for embedment ratio of 3.0 (adapted from Osterberg and Gill 1973).

and does not reflect the zones of weakness in the rock mass; such as clay filled joints, soft layers, or the pattern of jointing. Any relation between the ultimate bearing capacity based on measured laboratory strengths can be used only after the laboratory strength test results have been reduced to represent the average in situ strength of the rock mass within the zone stressed by the socket. Such reductions may be based on experience with the local rock types and with the degree and nature of jointing known to exist. Teng (1962) reports that the ultimate bearing capacity of rock varies from five to eight times the unconfined compressive strength of intact cores. Coates (1967) considers that the ultimate bearing capacity is about three times the unconfined compressive strength; this theory is based on the Griffith Fracture Theory which concerns the initiation of cracking. Research by Rehman and Broms (1971) determined that the ultimate surface bearing capacity of solid rock varies from about four to

six times the unconfined compressive strength of the rock material. It was also determined that as the depth of embedment increased up to 1.0 the ultimate bearing capacity also increased from 25 to 70% depending on the rock tYPeBased on the bearing capacity theories reviewed, it would appear that the ultimate bearing capacity of rock as obtained from the above is in excess of any allowable bearing values presently recommended in building codes. For example, the National Building Code of Canada (1970) permits for footing design an allowable pressure of 10 ton/ft2 (9.7 kg/cm2) on the surface of limestone. This value is intended to allow for the existence of voids, open joints, and soil filled seams below the selected bearing level. The Chicago Building Code as quoted by Baker and Khan (1971 ) allows end bearing pressures of 100 ton/ft"97.6 kg/cm2) on 'sound' limestone with a 20% increase for each foot of confinement up to a maximum of 200 ton/ft2

NOTE

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PERCENT

OF APPLIED

A X I A L LOAD

FIG. 2. Load transfer curves for varying moduli ratio computed for embedment ratio of 4.0 (adapted from Osterberg and Gill 1973).

(195.2 kg/cm" at a 5 ft (1.5 m) depth. The higher values may be used provided the rock is core drilled and inspected to insure its quality. Freeman et al. (1972) based on in situ pressuremeter tests on shale with a lower bound unconfined compressive strength of about 1350 psi (94.9 kg/cm2) allowed a bearing value of 50 ton/ft"48.8 kg/cm2) for embedded caissons. Plate bearing tests carried to failure on the surface of bedrock are costly to perform and therefore only a few are published and readily available for review. There are, however, tests

available which although not carried to failure are of interest. Table 1 presents the results of such plate load tests, included are tests carried out by the authors. It would appear from the results of limited data presented that the range in allowable end bearing values for design of sockets would be from about 15 ton/ft2 (14.6 kg/cm2) in poor rock, 25 to 75 ton/ft2 (24.4 to 73.2 kg/cm2) in medium rock and 100 ton/ft2 (97.6 kg/cm2) in sound rock. These values are realistic provided adequate drilling below the fourdation is done to insure that no voids exist below the socket tip.

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TABLE 1. Results of plate and pile load tests on bedrock Reference

Rock type

Seychuck (1970)

(1) Ottawa Shale, core lengths. 3 to 10 in. (76.2 to 254 mm), qu = 5000 psi (351 kg/cm2) (2) Cherty limestone; q, = 8000 psi (562 kg/cm2) (a) sound limestone

Northern Quebec (WHI Report 1967) Tavenas (1971) Labrador (WHI Report 1968b)

260 (254)

Remarks No failure, linear elastic to maximum load 0 b

204 (199) 204 (199)

No failure, linear to maximum load; settlement 0.08 in. (2.03 mm) No failure, linear to maximum load; settlement 0.08 in. (2.03 mm)

(c) slightly weathered limestone Steeply dipping weathered slates; core recovery BX size 65% Sound granite; core recovery AXT size 95% Quebec City sandstone RQD < 10%

204 (199) 32 (31) 230 (225) 250 (244)

No failure, linear to maximum load; settlement 0.10 in. (2.54 mm) No failure, linear to maximum load; settlement 0.025 in. (0.6 mm)

Friable iron formations similar to weathered and friable sandstone

50 (49)

(b) slightly weathered limestone

Halifax (WHI* Report 1968a)

Maximum load or failure in tons/ftz (kg/cm2)

*Wornock Hersey International Ltd.

2:

8 0 -I

0 m 3:

C 0

r

-

W

No failure, linear to maximum load; settlement 0.02 in. (0.5 mm) Practically linear to 250 ton/ft2 (244 kg/cm2) then failure defined Linear to 45 ton/ft2 (44 kg/cm2) then failed; settlement 0.5 in. (12.7 mm)

Aw .

0\

NOTE

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LOAD

'I

IN P S I (kg/crn2)

I T

F

F

!

I EARS

'

I

I

surfaces;

FIG.3.. Load test on concrete socket.

Bond Value Sockets in bedrock are designed to transfer the total load by bond or adhesion along the cylindrical concrete to bedrock contact and by end bearing. Bond is that portion of the total resistance developed on the surface between the concrete and rock contact. The bond value, required in the design of sockets, is difficult as well as expensive to evaluate. Common practice used to arrive at an allowable concrete to rock bond is to use the results of pullout or isolated plug load tests. The results of two such tests carried out by the authors are given in Figs. 3 and 4, the pertinent

rock properties are also summarized on the figures. The test shown on Fig. 3 was performed on an 18 in. (0.45 m) diameter by 22 in. (0.56 m) long concrete socket. In order to eliminate the effect of end bearing the concrete socket was poured on a 4 in. (0.1 m ) styrofoam isolating pad. A closed end 12 in. (0.3 m) pipe pile was set on the concrete surface. The test was carried at a depth of 40 ft (12.2 m) below ground level. Loading was applied to the pile tops by a hydraulic jack reacting against a timber frame filled with steel bars. The test socket was

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CAN. GEOTECH. J. VOL. 13, 1976

ROCK TYPE Hot~zofltolly bedded shale core lengths 3 t o 5 inches ( 7 8 to 130MM ) qu: 3000 psi I211 kg /cm2)

FIG.4. Pullout test on concrete socket.

loaded to a maximum stress of 170 psi (1 1.9 kg/cm". As shown on Fig. 2 the socket movement at 160 psi (1 1.2 kg/cm" defined as failure, was 0.25 in. (6.35 mm); no drop in bond with further movement was observed. The test shown on Fig. 4 was performed on an 8 in. (0.20 m ) diameter, 36 in. (0.91 m) long concrete socket formed at the bottom of a test pile. The socket was grouted onto a steel anchor block through which steel cables were strung to the surface. Two tell-tales for deflection measurements were installed in the plug. Loading was by means of a hollow stem jack, reacting against the pile casing. The test was carried out at a depth of 55 ft (16.7 m) below ground level. Failure was taken at 245

psi (17.22 kg/cm" and further movement indicated no drop in strength. The tcst results show that even in poor rock the bond value is significant, that it is mobilized at small movements and that it remains fairly constant after failure. Examination of these figures as well as other published data show that, the ultimate bond strength is developed with very little movement usually less than 0.25 in. (6.35 mm) after the peak strength has been obtained the strength does not drop off but tends to remain constant even with further movement. This was also observed in an instrumented caisson by .Jackson et al. (1974). A list of observed values of bond between

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TABLE2. Observed values of bond resistance between concrete and rock

Reference Moore (1964)

Matich and Kozicki (1967) Thorburn (1966)

Seychuck (1970) Gibson and Deveny (1973) Jackson et al. (1974) This paper Fig. 3

This paper Fig. 4

Rock type

Bond resistance in psi (kg/cm2)

Weathered fractured interbedded sandstones and shales

140 (9.8)

Weathered shales,

45 (3.2)

q, = 70 psi (49 kg/cm2)

Weathered and fragmented shales; joints spaced 0.5 to 1 in. (12.7 to 25.4 mm) Shale, joints spaced 3 in. to 10 in. (76.2 to 254 mm), q, = 5000 psi (351 kg/cm2)

35 (2.5)

Sandstone

120 (8.4) 235 (16.5)

Cherty limestone RQD 50 Highly fractured hard, Andesite core rec. in BXL core 33 to 75%, q. = 1500 psi (105 kg/cm2) Shale, joints spaced 2 in. (50.8 mm), q, = 3000 psi (21 1 kg/cm2)

440 (31)

245 (17.2)

Remarks Concrete 5000 psi (351.5 kg/cm2) 20 in. (508 mm) diameter by 8.5 ft (2.59 m) long plug; n o failure established Concrete 5000 psi (351.5 kg/cm2). Test on 24 in. (609.6 mm) diameter by 13.5 ft (4.1 m) long plug; no failure Test on 3 in. (76.2 mm) diameter by 12 ft (3.65 m) long plug; ultimate bond obtained Test on 19 in. (482.6 mm) diameter by 36 in. (914.4 mm) long plug; no failure Failure defined Failure interpreted by authors Test on 18 in. (457.2 mm) diameter by 22 in. (558.8 mm) long plug. Concrete 3000 psi (21 1 kg/cm2) Test on 8 in. (203.2 mm) diameter by 36 in. (914.4 mm) long concrete plug. Concrete 5000 psi (351.5 kg/cm2)

2

1 0

rn

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CAN. GEOTECH. J. VOL. 13, 1976

L E G E N D REF. 0

0 A

V

+X

REMARKS

-

THORBURN(1966) qu eat. MATICH A N D KOZICKl(1967)No t a i l m . T H l S PAPER Fig.4 THlS PAPER F i g . 3 SEYCHUCK(1970) No faihrb GIBSON AND DEVENNY qu eat. ( 1973)

-

10

M E A S U R E D BOND VALUE ( psi)

FIG.5. Tentative relation between bond and unconfined compression strength of rock.

concrete and rock is given in Table 2. Although these observations reveal a wide range of values, and the precise data with respect to the rock properties are not available for each case, the data have nevertheless been plotted on Fig. 5 using an estimate of rock compressive strengths and give a tentative relation between compressive strength and the ultimate bond which may be used to estimate a preliminary bond value. The bond value actually developed will also

depend on such factors as, socket wall roughness, concrete strength, cleanliness of sides, and groundwater conditions. The effect of water in sockets was particularly pointed out by Baker and Khan (1971).

Rock Socket Design Based on the information presented a proposal for the design of rock sockets is given for consideration. This is as follows: ( a ) On the basis of field and laboratory

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NOTE

investigation estimate an allowable end bearing capacity for the rock. ( b ) Estimate a bond value to be used. This selection may be based on the tentative relation given in Fig. 5 or on the results of field tests. (c) Estimate the ratio of the rock to concrete modulus. (d) Predict the developed end bearing for various socket geometries. Select the socket length in which the ultimate bond is mobilized and the allowable end bearing value assigned is not exceeded. The procedure given can be used only if close inspection is available during the construction. This should include drilling at the base of the sockets, .examination of the rock in the socket, ensuring proper cleaning.. and concreting of the socket. "7

Conclusions The following conclusions are drawn from the information ~resented. ( a ) Bond strength is significant, even in poor rock it is mobilized at small movements and remains reasonably constant after initial failure. (b) The bond strength can be tentatively correlated with the compressive strength of rock provided the lowest compression strength is used. (c) The ultimate end bearing capacity of rock is significantly higher than is usually allowed by the various building codes. (d) A reasonable estimate of the developed end bearing for various socket embedments can be obtained using Fig. 1 and 2. (e) Rock sockets can be designed using the ultimate bond strength and applying the

333

factor of safety against overall failure against the end bearing value chosen. BAKER,C. N. and KAHN,F. 1971. Caisson construction problems and correction in Chicago. ASCE, J. Soil Mech. Found. Div. 97 (SM2), pp. 417-440. COATES,D. F. 1967. Rock mechanics principles. Mines Branch Monogr. 874, Dep. Energy, Mines, and Res. Ottawa, Can. FREEMAN, C. F., KLAJNERMAN, D., and PRASAD, G. D. 1972. Design of deep socketed caissons into shale bedrock. Can. Geotech J. 9, pp. 105-1 14. GIBSON,G. L. and DEVENNY, D. W. 1973. Concrete to bedrock bond testing by jacking from bottom of a borehole. Can. Geotech. J. 10, pp. 304-306. HEAVYCONSTRUCTION NEWS. 1974. Big caissons are socketed 35 feet into rock. Heavy Construction News, September 16, 1974. JACKSON, W. T., PEREZ,J. Y., and LACROIX, Y. 1974. Foundation, construction and performance for a 34storey building in St. Louis, Geotechnique 24, pp. 63-90. MATICH, M. A. and KOZICKI, P. 1967. Some load tests on drilled cast-in-place concrete caissons. Can. Geotech. J. 4, pp. 357-375. MOORE,W. W. 1964. Foundation design. ASCE,Civ. Eng. 34(1), pp. 33-35. NATIONAL BUILDING CODEOF CANADA. 1970. Nat. Res. Counc. Can., Ottawa, Can. OSTERBERG. J. 0 . and GILL. S. A. 1973. Load transfer mechanism for piers socketed in hard soils or rock. Proc. 9th Can. Symp. on Rock Mechanics, Montreal, Que. REHMAN, S. and BROMS,B. B. 1971. Bearing capacity of piles driven into rock. Canadian Geotech. J. 8, pp. .. 151-162. SEYCHUCK. J. L. 1970. Load tests on bedrock. Can. ~ e o t e c h . ' J7, . pp. 464-470. TAVENAS, F. 1971. ContrBle du roc de fondation de pieux forks a haute capaciti. Can. Geotech. J. 8, pp. 400-416. TENG,W. C. 1962. Foundation design. Prentice-Hall Inc., Englewood Cliffs, N.J., pp. 260-261. THORBURN, S. 1966. Large diameter piles founded on bedrock. Proc. Large Bored Piles Conf. Inst. Civ. Eng., London, Engl. WARNOCK HERSEY INTERNATIONAL LTD. 1967. Report on pile load test, Grand'Mkre, Quebec. unpubl. rep. 1968a. Report on slate load test proposed development, Halifax, Nova Scotia. unpubl. rep. 1968b. Report on plate load test proposed building, Labrador City, Newfoundland. unpubl. rep.