THE ANALYSIS OF SOIL RESISTANCE DURING SCREW DISPLACEMENT PILE INSTALLATION

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1 Sudia Geoechnica e Mechanica, Vol. XXXVI, No. 3, 014 DOI: /sgem THE ANALYSIS OF SOIL RESISTANCE DURING SCREW DISPLACEMENT PILE INSTALLATION ADAM KRASIŃSKI Gdańsk Universiy of Technology, Deparmen of Geoechnics, Geology and Mariime Engineering, ul. Naruowicza 11/1, Gdańsk, Poland, akra@pg.gda.pl Absrac: The applicaion of screw displacemen piles (SDP) is sill increasing due o heir high efficiency and many advanages. However, one echnological problem is a serious disadvanage of hose piles. I relaes o he generaion of very high soil resisance during screw auger peneraion, especially when piles are insalled in non-cohesive soils. In many siuaions his problem causes difficulies in creaing piles of designed lengh and diameer. I is necessary o find a proper mehod for predicion of soil resisance during screw pile insallaion. The analysis of screw resisances based on model and field ess is presened in he paper. The invesigaions were carried ou as par of research projec, financed by he Polish Minisry of Science and Higher Educaion. As a resul of ess and analyses he empirical mehod for predicion of roaion resisance (orque) during screw auger peneraion in non-cohesive subsoil based on CPT is proposed. Key words: screw displacemen pile, pile insallaion, pile auger, orque, soil resisance 1. INTRODUCTION During he insallaion of SDP pile, when he auger peneraes he non-cohesive subsoil, high soil resisance is observed. This screw resisance is one of he bigges problems of SDP pile echnology and is a subjec of ongoing scienific invesigaions and analyses. I was also he reason ha many ypes of screw auger have already been inroduced ino pracice. I is believed ha soil resisance could be reduced by appropriae auger consrucion. Due o he difficulies conneced wih he insallaion of screw displacemen piles in various ypes of soil condiions, hese piles are ofen omied in geoechnical projecs. High soil resisance during screw auger peneraion means he high value of orque (M T ), which should be applied o roae he auger. Therefore, very massive drilling machines wih high orque (a leas 00 knm) need o be used for screw displacemen pile insallaion. The analysis of insallaion effor conneced wih SDP piles was carried ou, among ohers, by NeSmih [10]. The problem is addiionally complicaed due o he fac ha high soil resisances during he insallaion no always mean he high bearing capaciy of screw pile. The analogy o driven piles, where he high blowing resisances indicae he high pile bearing capaciy, canno be aken in he case of SDP piles. This resuls from he fac ha he load bearing capaciy of screw displacemen piles is primarily based on he shaf resisance Q s, [4 6]. Learning from pracice shows ha we raher will no be able o reduce significanly he soil resisances by changing he shape or he consrucion of screw auger. The only way o solve he problem seems o idenify he phenomena occurring in he soil during he screwing process and o propose a mehod for predicing he soil resisances. This paper presens he resuls of he auhor s own model and field ess as well as heoreical analyses concerning soil resisances during screw auger peneraion in non-cohesive soil. Model and field ess were correlaed wih CPT ess. Thanks o hese resuls, he empirical mehod of screw orque M T value predicion based on CPT ess has been developed and presened.. DESCRIPTION OF SCREW AUGER PENETRATION PROCESS The main ask of screw pile auger is o creae a hole in he ground by radial soil displacemen and o fill he hole wih he concree. Boom par of he

2 50 A. KRASIŃSKI auger is helical bu in he upper par he diameer of auger core gradually increases. Thanks o is shape he auger peneraes he soil during roaion and displaces he soil radially. Someimes, a verical compressive force is added from drilling machine o iniiae he auger peneraion. In oher cases a ension force is used o decrease he peneraion velociy and o reduce he orque value. Screwing process iniiaes a significan increase of radial sresses in he surrounding soil. These sresses are he main reason of high soil fricion around he auger surface. There are also soil resisances under he auger base. The inserion of a drilling ool ino he ground is a complex process, difficul o heoreical describe. Therefore, a simplified scheme of he auger was aken for analysis. In his scheme, presened in Fig. 1, he real auger is represened by he equivalen cylinder. I is assumed ha he oal value of orque M T consiss of wo componens: M Ts and M M = M + M. (1) T Ts M Ts is conneced wih soil fricion Ts around he auger shaf and is generaed by he auger roaion. M is conneced wih soil uni fricion under he auger base and is generaed by he roaion and auger verical movemen. Componens M Ts and M are defined by he formulas π Ds = Ts; i hs i, () M Ts ; M 3 π Ds = 1. (3) Fig. 1. Simplified scheme of SDP auger aken for analysis Screw resisance is also defined by he number of auger roaions n T per uni deph of peneraion. In he auhor s opinion M T and n T are he mos imporan parameers of SDP pile insallaion. In pracice, in he repors of screw displacemen pile insallaion here is given he informaion abou: oil pressure in he hydraulic roary ool, roaion velociy RPM (roaion per minue) and peneraion velociy (meers per hour). M T and n T values are obained from recalculaing he Fig.. Tes and measuremen resuls concerning example of SDP pile, [6]

3 The analysis of soil resisance during screw displacemen pile insallaion 51 hree parameers menioned. In mos drilling machines, he value of orque M T depends on hydraulic oil pressure and on RPM (he smaller he RPM, he higher he M T for a given oil pressure). Figure presens he example of measuremen resuls concerning SDP pile insallaion in correlaion wih CPT es of subsoil. CPT was carried ou before and afer pile insallaion. The pile was insalled by drilling machine of maximal orque M T = 00 knm and by he auger of shape presened in he figure. In he CPT diagram one may noe ha afer pile insallaion here is an increase of q c resisances in he region around he pile shaf, bu under he pile base ha increase is no very high. Similar phenomenon was observed wih regard o he oher SDP piles esed. This allowed us o conclude ha screw displacemen piles affec he soil condiions (densiy and sress level) mainly around heir shafs. In he M T and n T diagrams one may noice ha during auger peneraion ino he bearing sraum an inensive increase of orque is observed wih consan value of he roaion number n T (abou 4 roaions per meer). When he orque reached he maximal value of 00 knm, a he deph of 7.5 m under he ground level, he peneraion velociy was reduced. As a resul, he n T value increased whereas he M T value decreased. Thanks o such acions i was possible o creae a pile of lengh L = 8.85 m. This example shows ha he orque value can be influenced by regulaion of auger peneraion velociy, hus by roaion number n T. 3. MODEL TESTS Model ess were conduced using SDP auger models prepared in 1:7.5 scale in wo diameers (Fig. 3). Auger model SDP1 represens he original auger of 400 mm in diameer, while model SDP represens he original auger of 500 mm in diameer. The experimens were carried ou a he Geoechnical Laboraory in Gdańsk Universiy of Technology. The research device consiss of seel conainer (.0 m diameer and.0 m high) for soil. Mois fine sand was prepared wih appropriae densiy achieved by vibraion and conrolled by CPT ess. The augers were screwed ino he soil using hand operaed device and verical load of consan value of 0.5 kn. Soil resisances during screwing were measured in erms of orque M T and number of roaions n T per each 0.1 m of auger peneraion. Several series of ess were carried ou. Apar from he measuremens of soil resisances during auger peneraion also he SDP and Alas pile models were insalled and esed in respec of heir bearing capaciy. A deailed descripion of pile model ess is presened also in oher auhor s work [3]. Fig. 3. Models of SDP augers (model scale 1:7.5) During screw auger peneraion usually only he oal value of orque M T is measured, and i is difficul o divide is value ino M Ts and M componens. Therefore, in one of he ess he measuring of screwing resisances was performed in a special way. I was assumed ha if he auger verical peneraion was blocked, hen during roaion only he M Ts componen was measured. Before blocking he oal M T value was measured. M componen was hen calculaed as a difference beween M T and M Ts. During he es, which is shown in Fig. 4, also he value of verical force Q Tv was measured. The auhor named his ension force as auger hrus. The measuremens of M T, M Ts and Q Tv were carried ou every 0.1 m of peneraion from 0.5 m o 1.0 m of deph. The resuls of measuremens, which were done for boh augers SDP1 and SDP, are presened as he diagrams in Fig. 5, ogeher wih diagrams of cone resisances q c and roaion numbers n T. Nex, using he auger scheme presened in Fig. 1, he values of uni soil resisances Ts and were calculaed and presened in Fig. 6 as diagrams ogeher wih diagrams of raios Ts /q cs and /q cb. The q cs value was aken as an average q c from he auger lengh h s, whereas q cb was aken as an average q c of D s o +D s region around he auger base level.

4 5 A. KRASIŃSKI Fig. 4. Measuremen of orque componen M Ts and auger hrus Q Tv in model es Fig. 5. The resuls of auger SDP screwing ess wih measuremens of orques M T, M Ts, M and force Q Tv Fig. 6. Uni fricion resisances Ts and and heir relaions o cone resisances q c

5 The analysis of soil resisance during screw displacemen pile insallaion 53 I can be noed ha raio Ts /q cs is consan along he whole deph, which means ha uni resisance Ts is proporional o cone resisance q c. On his basis he following relaion is creaed = (4) Ts q cs The raio /q cb is no consan bu is increasing wih deph. I was assumed ha his increase is caused by auger hrus Q Tv. The mechanism of auger hrus iniiaion can be explained by analogy o ordinary screw. In such a screw, when he fricion on he hread is negleced, a relaion beween M Ts componen of orque and hrus Q Tv resuls from he principle of energy conservaion. In he case of auger which is peneraed ino he soil he following relaion is assumed π MTs QTv = a (5) nt s where s pich of he auger helix, a reducion facor, aking ino accoun he losses caused by he soil fricion and by he fac ha verical velociy of auger inserion is less han i would resul from pich of helix s (a < 1.0, he exac value of a is no known). The number of auger roaion per 1.0 m (n T ) should be aken ino accoun as a value wihou unis. The hrus Q Tv causes he auger base impac on he soil by verical pressure q which can be expressed by he formula q 4Q MTs = = a. (6) π D n s D Tv 8 s The verical pressure q influences direcly he soil fricion under he auger base. The following relaion is assumed q T s = μ (7) where μ fricion coefficien of he soil under he auger base. On he basis of model es resuls i was concluded ha he value μ is no consan bu increases wih deph approximaely proporionally o he increase of cone resisance q c. The following relaion is assumed qcb μ = b, (8) p ref where p ref = 1.0 MPa reference sress, b conversion facor (no known). and Equaion (7) akes he form M Ts cb = 8 a b. (9) nt s Ds pref Afer inroducing wo new coefficiens m Ts Ts s ref q = M s D p (10) m we receive a new form of equaion (9) T = 8 a b (11) = 1 mts m qcb. (1) n The coefficien m was deermined experimenally from es resuls by using back analysis mehod. I was no necessary o deermine values of a and b coefficiens separaely. The m value was found o be abou 4.40 and was approximaely consan in relaion o deph and similar in he case of boh augers SDP1 and SDP. Finally, from he analysis one can obain empirical equaions for he calculaion of orque componens M Ts and M and oal orque M T of screw displacemen augers on he basis of cone resisances q c. Moreover, he analysis has shown ha M Ts componen depends mainly on he cone resisance q c, whereas M componen depends on M Ts value, cone resisance and number of auger roaions per uni deph n T. 4. FIELD TESTS A mehod for predicion of soil resisance during screw auger peneraion was prepared also for piles in naural scale. This mehod concerns SDP and CMC (Conrolled Modulus Column) augers. There were applied he same assumpions and very similar equaions as in he model es analyses, bu several addiional facors and effecs had o be aken ino accoun. These facors concerned he auger shape, nonhomogeneiy of soil profile and sress level in he subsoil. The same scheme of auger as presened in Fig. 1 and he same equaions (1) (3) of M T, M Ts and M calculaions were applied. Afer many rial replicaions of orque values measured in he field ess, he following formulas for Ts and resisances were deermined

6 54 A. KRASIŃSKI Fig. 7. Diagrams for deerminaion of η1, η and η4 coefficiens Fig. 8. Torque MT predicion for he SDP pile insallaion on he field sie Fig. 9. Torque MT predicion for he CMC pile insallaion on he field sie

7 The analysis of soil resisance during screw displacemen pile insallaion 55 Ts = η η3 q cs η [kpa], (13) q cb = 1. η 3 η4 mts [kpa], (14) nt where η 1, η 4 coefficiens dependen on he deph of auger peneraion in bearing soil layer, η coefficien of auger shape influence, η 3 coefficien of sress level in he subsoil. Coefficiens η 1, η and η 4 are read from diagrams presened in Fig. 7, whereas η 3 coefficien is calculaed from he formula σ v 0 η3 = (15) 100 kpa where σ v 0 is a verical effecive sress in he subsoil. The value of m Ts coefficien is calculaed using formula (10). The number of auger roaions per uni deph n T which has o be applied in equaion (14) is unknown and we mus assume is value. I was menioned earlier ha he n T value can be conrolled during screwing process by he piling machine operaor. In he calculaion we should ake he planned or maximal acceped value of n T (usually no more han 0 o 40 roaions per meer) COMPARISON OF PREDICTED AND MEASURED SCREW RESISTANCES Figures 8 and 9 presen he resuls of calculaed replicaions of real screw resisances measured during he insallaion of wo seleced piles in he field. Paricular emphasis was placed on M T value predicion in he boom par of subsoil when he auger was peneraed in he bearing sandy layer. The organic soils in he upper pars of subsoil were less imporan, hen he screw resisances in hese layers were aken approximaely as: Ts = f s and = 5 f s, where f s is he sleeve fricion measured in CPT es. The values of η 1, η, η 3 and η 4 coefficiens were aken as equal o 1.0 in organic soil. The values of n T were aken as equal o hose read from he pile insallaion repors. On he basis of diagrams presened in Figs. 8 and 9 i can be concluded ha he proposed mehod for predicion of soil resisance during SDP and CMC auger peneraion in non-cohesive soils provides good resuls. 5. CONCLUSIONS The ess and analyses presened in he paper showed ha roaion resisances during screw displacemen auger peneraion in non-cohesive soils depend on soil srengh properies (expressed by CPT cone resisances q c ), auger geomery and screw echnique (he velociy of roaion and peneraion). The oal value of orque M T generaed by screwing process can be divided ino wo componens: M Ts momen resuling from soil fricion around he auger shaf and M momen resuling from soil resisances under he auger base. The calculaion mehod developed as a resul of ess and analyses allows us o predic he roaion resisances (a value of orque M T ) during SDP auger peneraion ino non-cohesive subsoil on he basis of CPT resuls. The mehod was verified posiively wih several pracical examples of SDP and CMC pile insallaion in he field. On he base of i, we will be able o know wha energy of piling machine is required o insall he designed screw displacemen pile. This predicion mehod can be applied o design screw displacemen piles ogeher wih he auhor s own mehod for pile bearing capaciy calculaion [7] or wih oher calculaion mehods, proposed, for example, by Busamane and Gianeselli [1], [], Maerens and Huybrechs [8] or by NeSmih [9]. I should be underlined ha many elemens of he calculaion mehod proposed were assumed in a simplified way. Due o his fac i may no work well in every case. This mehod should raher be considered as a preliminary proposal, which require many ess and verificaions and will cerainly be subjeced o modificaions. REFERENCES [1] BUSTAMANTE M., GIANESELLI L., Design of auger displacemen piles from in siu ess, Proceedings of Inernaional Geoechnical Seminar on Deep Foundaions on Bored and Auger Piles, BAP II, Balkema, Roerdam 1993, pp [] BUSTAMANTE M., GIANESELLI L., Insallaion parameers and capaciy of screwed piles, Proceedings of Inernaional Geoechnical Seminar on Deep Foundaions on Bored and Auger Piles, BAP III, Balkema, Roerdam 1998, pp [3] KRASIŃSKI A., Model ess of screwed piles, Proc. of he XIV Danube-European Conf. on Geoechnical Engineering, Braislava, Slovak Republic, June 4, 010, 37+CD. [4] KRASIŃSKI A., Field ess of screw displacemen piles and columns SDP and SDC, Drogi i Mosy, 011a, No. 1, pp (in Polish).

8 56 A. KRASIŃSKI [5] KRASIŃSKI A., Resuls of field ess of screw displacemen pile and columns, Inżynieria Morska i Geoechnika, 011b, No. 6, pp (in Polish). [6] KRASIŃSKI A., Bearing capaciy and ineracion wih soil of screw displacemen piles, Final repor of research projec No. N N for he Polish Minisry of Science and Higher Educaion, Gdansk (in Polish, no published), 011c. [7] KRASIŃSKI A., Proposal for calculaing he bearing capaciy of screw displacemen piles in non-cohesive soils based on CPT resuls, Sudia Geoechnica e Mechanica, 01, Vol. XXXIV, No. 4, pp [8] MAERTENS J., HUYBRECHTS N., Belgian screw pile echnology. Design and recen developmens, Swes & Zeilinger B.V., Lisse, The Neherlands, 003, p. 37. [9] NESMITH W.M., Saic capaciy analysis of augered, pressure-injeced displacemen piles, Proc. of he In. Deep Foundaion Congress 00, Geoechnical Special Publicaion, 00, No. 116, Vol., ASCE, pp [10] NESMITH W.M., Insallaion effor as an indicaor of displacemen screw pile capaciy, Deep Foundaion on Bored and Auger Piles, BAP IV, Van Impe (ed.), Millpress, Roerdam 003, pp