Design lessons from full-scale foundation load tests

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Edwin Arnold Rogers
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1 Design lessons from fll-scale fondation load tests B.A. McCabe Department of Civil Engineering, National University of Ireland, Galway, Ireland D.T. Phillips Department of Materials Science and Technology, University of Limerick, Ireland ABSTRACT: A soft estarine silt site located near Belfast, Northern Ireland, has been sed in recent years for load tests on varios fondation types. In this paper, reslts from axial static load tests on a shallow pad footing, a single pile and a pile grop are smmarised. The reslts offer some sefl insight into the measrement and selection of sitable design parameters for fondations in soft silt. INTRODUCTION A series of fll-scale instrmented field tests on shallow and deep fondations has been carried ot in the soft estarine silt deposits that nderlie mch of the greater Belfast area. Reslts from the individal tests (shown schematically in Figre ) have been reported in fll detail in Lehane (23), Phillips and Lehane (24) and McCabe and Lehane (26). Some of these findings have been combined into this paper. In addition, the fondation test reslts are analysed in the context of varios laboratory and in sit test data. The paper concldes with a discssion on the selection of strength parameters for the geotechnical design of fondations in soft clays/silts..6m P Granlar fill AL G S GL 6m The test site is located at Kinnegar, approximately km north east of Belfast city centre. The geotechnical profile comprises approximately 7.5m of lightly overconsolidated estarine organic silt (known locally as sleech) exhibiting the typical properties shown in Table. The sleech is overlain by.-.5m of granlar made grond and nderlain by medim dense sand (SPT N vales of 5) at 8.5m depth. The water table varies both seasonally and tidally between.m and.5m. Detailed characterisation of the made grond and sleech is reported in McCabe (22) and Phillips (22). Table : Typical properties of the sleech Clay Fraction (%) 2 ± Fines Content (%) 9 ± 5 Water Content (%) 6 ± Plasticity Index (%) 35 ± 5 Organic Content (%) ± Peak Vane Strength (kpa) 22 ± 2 Yield Stress Ratio (YSR). to 2. Friction Angle ( o ) 33 ± 7m Figre : Schematic of Belfast fondation tests 2 GROUND PROFILE soft estarine silt (sleech).5m medim dense sand 3 SUMMARY OF TEST PROGRAMME AND KEY FINDINGS 3. Shallow footing test Lehane (23) reported load test reslts on a 2.m 2.m reinforced concrete pad footing at the Kinnegar site. The.7m thick pad was fonded in the sleech at a depth of.6m below grond level (P in Figre ). The footing was loaded via kentledge over a period of 5 hors in ten increments of approximately 9kPa each. The applied stress and mean settlement measred in the test is shown in Figre 2. The pad nderwent a deep-seated rotational failre when the applied fondation stress reached 96.5kPa.

2 Pile head settlement (mm) Axial load (kn) Vernier card readings DG Lateral load direction PLAN DG2 Inclinometer tbe Vernier sighting card Figre 3: Load-displacement behavior for AL Figre 2: Load-settlement behavior of pad fondation (after Lehane 23) 3.2 End bearing single pile test Phillips and Lehane (24) reported load tests on individal piles sed to measre the soil s response to lateral loading and in particlar the effect of an axial pile load on the soil s lateral response. A pair of 35mm sqare m long precast concrete piles (designated L and AL) were driven 2m apart and penetrated the medim dense sand by approximately.5m (Figre ). Kentledge spported symmetrically overhead was sed to load pile AL in advance of jacking it against pile L while an array of electrical resistance and vibrating wire strain gages recorded the load transfer to the soil over the entire pile length. Predictions of axial load capacity presented in Phillips (22) were sed in the design of the axial load test. These estimates show the pile has an average total resistance of kn; a load test was hence designed to apply half of this load to AL (i.e. FOS=2) in a series of eqal increments. However, a falty load cell readot nit, replaced prior to commencing the test, revealed on sbseqent calibration that only 7kN was applied to AL. The pile head movement nder this load is charted in Figre 3 and shows the pile to have settled by 2.5mm (.4mm of which can be acconted for by elastic shortening of the pile). This is consistent with the load shedding distribtion plotted in Figre 4 which indicates that 35% the load is transferred throgh skin friction in the sleech and the remaining 65% being transferred throgh end bearing to the medim dense sand. A noteworthy observation was made dring the seven weeks between pile installation and commencing the axial load test: the strain gage data indicated that a load of 37kN had developed in the pile de to negative skin friction. Depth (m) Axial load (kn) Figre 4: Load distribtion for AL 3.3 Friction pile tests Pit level (z=) Applied vertical load 3.3kN kn 35kN 6kN 64kN 68kN McCabe and Lehane (26) report load tests on a pile grop (G) comprising five 25mm (=B) sqare precast concrete piles, with for corner piles spaced at 2.8B (centre to centre) from a centre pile. Each pile was installed to a depth of 6.m and the pile heads were linked by a rigid pile cap. Parallel load tests were carried ot on the pile grop and on an identical single reference pile (S). Some of the piles were eqipped with electrical resistance and vibrating wire strain gages to measre the load distribtion. A pile cap linking the pile heads consisted of orthogonal grillages of H-sections sandwiched between steel plates. Each pile head was eqipped with a load cell and an electronic displacement transdcer. The compression tests were carried ot sing conventional kentledge (Figre 5) and a cstommade jacking arrangement was devised for the tension piles. For brevity, only the compression tests are presented in any detail.

3 capacity efficiency of compression and tension pile grops were only slightly less than nity. Pile head load (kn) at.b displacement Figre 5: Compression pile grop load test 2 Centre pile of G The measred load displacement crves for the single pile and corner and centre piles of the grop are illstrated in Figre 6. As the degree of pile interaction increases, it can be seen that there is a marked decline in pile stiffness, even thogh the grop comprises relatively few piles. In addition, the ltimate capacity of the centre pile (conventionally defined as the load corresponding to a pile head displacement of.b) is seen to fall considerably below that of the single pile. Depth (m) Single pile S Corner pile of G Pile head load (kn) single pile S increasing load interaction Pile head displacement (mm) corner pile of G centre pile of G Figre 6: Load-displacement behavior for S and corner and centre piles of G Comparative ltimate load distribtions for the single and centre grop pile at a pile head displacement of.b are presented in Figre 7. It can be seen that the centre pile carries a greater proportion of its applied load at its base compared to an eqivalent single pile, consistent with theoretical predictions presented by Polos and Davis (98). In addition, McCabe and Lehane (26) have shown that the stiffness of single and grop piles in the sleech does not depend on loading direction (i.e. compression and tension stiffnesses were the same, once the inflence of pile cap flexibility was ncopled from the data). Single pile average ltimate shear stresses (.5 ±.5 kpa) were also eqivalent in compression and tension, once corrections were made for the weights of the piles and pile cap. The 6 Figre 7: Load distribtions for S and corner and centre piles of G 4 DISCUSSION OF TEST RESULTS A more comprehensive interpretation of the fondation tests presented in Section 3 has been provided elsewhere (Lehane 23, Phillips and Lehane 24, McCabe and Lehane 26). In this section, attention is focssed on presenting some simple lessons learned from these tests which wold be of interest to practitioners designing fondations in soft soil. 4. Negative Skin Friction Negative skin friction (NSF) is normally estimated by relating it to the vertical effective stress (σ vo ), according to: NSF = βσ ' vo [] where β is an empirical constant. Brland and Starke (994) measred NSF on pile shafts on nmeros sites, for periods of a few months p to seventeen years, and fond that average vales for β in soft compressible sediment lay within relatively narrow limits of.25<β<.35. The downdrag load of 37kN in Section 3.2 has been interpreted by Phillips

4 (22) to be compatible with β =.3, falling at the centre of the range recommended by Brland and Starke (994). This enables NSF to be estimated with confidence in the sleech or similar low YSR cohesive materials. 4.2 Skin Friction Mobilisation The instrmentation in end bearing pile AL indicated that a positive skin friction of 9kPa was mobilized along the top 3.75m of pile embedment. The shear stresses along the shaft then drop significantly confirming that the pile load was being transferred in end bearing to the stiffer sand at the base of the pile. The reslts from AL indicate that the bearing pressres at the pile toe are in the region of 98kPa, which is well below the bearing capacity of the sand, which is shown in Phillips (22) to be over 2 times stiffer than the sleech. The average ltimate shear stress on the single friction pile S (dedced from Figre 7) is kpa. This wold sggest (within site variability) that, even thogh pile AL was only loaded to 3% of its overall capacity, its skin friction was flly mobilized over the initial 3.75m of pile. The fll mobilization of skin friction at relatively small pile settlement is consistent with the findings of Brland and Cooke (974). They fond that skin friction develops rapidly and linearly as the pile settles and that fll skin friction is mobilised at small pile head displacements of magnitde.5b or 2mm in the case of AL. 4.3 Undrained Strength The ndrained shear strength (c ) is a commonly sed parameter in the design of fondations in soft clays/silts. However, choosing the most appropriate means of establishing c for se in total stress analysis often cases confsion among practitioners. The profile of c with depth at Kinnegar is plotted in Figre 8 and was determined sing the varios test and empirical methods listed below: (i) Consolidated ndrained triaxial compression tests on 54mm/mm diameter piston samples, shown by means of the shaded section in Figre 8(a) (ii) In sit (Geonor) vane testing. The vales qoted in Figre 8(a) incorporate Bjerrm s correction for plasticity index (I p ) (iii) A combination of two empirical relations (Ladd et al, 977 and Skempton and Henkel, 952). The eqations shown in Table 2 are plotted Figre 8(a). (iv) Cone pressremeter testing (interpreted sing the method of Holsby and Withers, 988); see Figre 8(b) (v) Standard correlations between the piezocone corrected cone end resistance q t and ndrained strength (Table 2 and Figre 8(b)). Note that the vale of N kt = is derived from Lnne et al (997) as being appropriate for the sleech. c c ' = v ' σ σ v c ' σ v c NC Table 2: Correlations for ndrained strength c Correlation NC YSR.8 =. +.37I t vo =, kt = N kt Depth (m) q σ p [2] [3] Reference Ladd et al (977) Skempton and Henkel (953) N [4] Lnne et al (997) Undrained Strength c (kpa) centre pile of G single pile S Empirical trend (see Table 2; Ladd et al 977, Skempton & Henkel 952) footing in sit vane range of triaxial test data Figre 8a: Measred pile shear stress distribtions (at displacement of.b) and varios ndrained shear strength profiles

5 Depth (m) Undrained Strength c (kpa) centre pile of G Cone pressremeter single pile S footing Piezocone test Figre 8b: Measred pile shear stress distribtions (at displacement of.b) and varios ndrained shear strength profiles The maximm shear stress (q s ) on the shaft of a pile is sally related to c according to: q = α [5] s c where α is an empirical constant. Semple and Ridgen (985) sggested that α= when c <.35σ v, i.e. in normally and lightly overconsolidated clays. However, α vales backfigred from S in conjnction with mch of the data in Figre 8(a) lie in the range.5-.7, sggesting that the typical se of α= wold have lead to nconservative design. Moreover, the shear stresses in the centre pile of G (also shown in Figres 8(a) and 8(b)) wold have been significantly overpredicted had this vale of α been sed. The foregoing wold sggest that the driving distrbance cased in the vicinity of this single pile is not captred in rotine laboratory strength tests. The ltimate end bearing load of 8.7kN measred in S eqates to an end bearing resistance of 4kPa. End bearing resistance is sally related to ndrained strength according to: q = N c [6] b c where an N c vale of 9 is sally deemed appropriate for piles whose base is embedded by more than 3 eqivalent diameters into the bearing stratm. Using N c =9, best predictions of the actal behavior are obtained from c vales (at pile base level, 6.m) determined from empirical or piezocone methods. Again the laboratory and in sit vane determinations of c wold lead to an nconservative design in this instance. Lehane (23) sed the data presented in Figre 2 to backfigre an operational strength for the sleech beneath the footing. A stress of 2.5kPa was added to the applied stress from the Kentledge (96.5kPa) to represent the difference between the effective stress at the level of the base of the footing and that adjacent to the footing. Based pon shape, depth and footing inclination factors of.2,.28 and.98 respectively, the average operational strength was fond to be 4kPa. This average strength is higher than the skin friction on the shaft of S and clearly reflects the lesser degree of distrbance to the grond cased by constrction and loading of the footing. The sleech beneath the footing has been sbjected to minimal distrbance (i.e. in sit effective stress and the footing weight still fall short of the preconsolidation pressre of 55 kpa). The best estimate of c for the footing lies above the piezocone data and below the empirical data; once again the triaxial and in sit vane strengths were fond to overpredict the operational strength by 5%. 5 CONCLUDING COMMENTS This paper has provided some sefl practical design tips for the design of fondations in soft clay/silt, and indicates that cation is needed in the choice of parameters. The following smmarises the main findings: Negative skin friction can be mobilized qite soon after driving and its magnitde appears to be predicted very well by Brland and Starke (994). Comparison of load distribtions in friction and end bearing piles illstrates the relative rates at which skin friction and end bearing are mobilized. Skin friction is flly mobilised at relatively small displacements, i.e..5% of pile width. When choosing an appropriate test method to determine the ndrained strength for fondation design, the stress history of the soil needs to be considered. For driven piles, it has been shown that the piezocone and cone pressremeter provide more appropriate strengths as they model the intense shearing associated with installing driven piles. For the footing experiment, a higher operational strength was obtained as distrbance to the grond was less. For both pile and footing tests, se of the triaxial and vane tests cold potentially lead to nsafe design.

6 The piezocone is sggested to provide the most accrate representation of the ndrained strength of the grond, despite the empiricism reqired in deriving this vale from the cone end resistance. Even with small-scale pile grops, the stiffness and capacity of the grop may be considerably redced and this shold be reflected in the ndrained strengths sed for pile grop design. 6 REFERENCES Brland, J. B. and Cooke, R. W. 974, The design of bored piles in stiff clays, Grond Engineering Vol. 7, No. 4, Brland, J. B., and Starke, W. 994, Review of measred negative pile friction in terms of effective stress, Proceedings of the 3 th International Conference in Soil Mechanics and Fondation Engineering, New Delhi, India, Holsby, G.T. and Withers, N.J. 988, Analysis of the Piezocone in clay, Geotechniqe, Vol. 38, No. 4, Ladd, C.C., Foott, R., Ishihara, K., Schlosser, F. and Polos, H.G. 977, Stress-Deformation and Strength Characteristics, Proceedings of the 9 th International Conference on Soil Mechanics and Fondation Engineering, Tokyo, Vol. 2, Lehane, B.M. 23, Vertically loaded shallow fondation on soft clayey silt, Proceedings of ICE Geotechnical Engineering, Vol 56, No., 7-26 Lnne, T., Robertson, P.K. and Powell, J.J.M. 997, Cone Penetration Testing in Geotechnical Practice, Blackie Academic and Professional, London McCabe, B.A. 22, Experimental Investigation of Driven Pile Grop Behavior in Belfast Soft Clay, PhD Thesis, University of Dblin, Trinity College McCabe, B.A. and Lehane, B.M. 26 Behavior of Axially Loaded Pile Grops Driven in Clayey Silt, ASCE Jornal of Geotechnical and Geoenvironmental Engineering, Vol. 32, No. 3, 4-4 Phillips, D.T. 22, Field tests on single piles sbjected to lateral and combined axial and lateral loads, PhD Thesis, University of Dblin, Trinity College Phillips, D.T. and Lehane, B.M. 24, The Response of Driven Single Piles Sbject to Combined Loads, Proceedings of the 5 th International Conference on Case Histories in Geotechnical Engineering, New York, USA, CD Paper.7 Polos, H.G. and Davis, E.H. 98 Pile Fondation Analysis and Design, Wiley Semple RM and Ridgen WJ 986, Shaft capacity of driven pipe piles in clay. Grond Engineering Vol. 9, No., - 7 Skempton, A.W. and Henkel, D.J. 953 The post-glacial clays of the Thames estary at Tilbry and Shellhaven, Proceeddings of the 3 rd International Conference in Soil Mechanics and Fondation Engineering, Vol., 32-38

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