Soil disturbance during continuous flight auger piling in sand

Transcription

1 Soil disturbance during continuous flight auger piling in sand Perturbation du sol au cours de forage à la tarière en milieu sableux M.J.Kenny & K. Z. Andrawes - Department of Civil Engineering, University of Strathclyde, UK H.Canakci -Department of Civil Engineering, University of Gaziantep, Turkey ABSTRACT : Continuous flight auger piling in sand can cause disturbance of the surrounding soil if the auger is allowed to rotate without vertical penetration. The results of an experimental programme are presented which show that the degree of disturbance depends on the initial density of the sand and the auger geometry as well as the number of over-rotations. An analysis of the auger transportation action in sand is presented which considers the sand particle motions within the flights. R E SU M E : Forage à la tarière pour la construction des pieux CFA en milieu sableux peut perturber le sol environnant si la pénétration de la tarière tou rn an t n est pas verticale. Les résultats d un program m e experimental sont présentés ici et m ontrent que le degné de perturbation depend de la d en sité initiale du sable et de la géom etrie de la tarière, assi bien que du nom bre de sur-rotations. U ne analyse de l action de transport forage de la tarière dans le sable est présentée, et considère les mouvem ents des particules de sable. 1 INTRODUCTION The continuous flight auger (CFA ) piling technique has now becom e well established as a fast, low cost and low vibration m ethod o f pile installation. H ow ever, along w ith these advantages, a num ber o f potential construction problem s have been identified which may adversely affect the perform ance o f the piles o r the integrity o f adjacent structures. C hief am ong these is the potential for overaugenng during boring, resulting in soil transportation by the auger and loosening o f the surrounding soil (U nsw orth and Fleming, 1989). The risk is considered to be greatest w hen piling in loose cohesionless soils. A num ber o f case histories have been described by Thorbum et al. (1993) w here C FA piling operations in sandy soils resulted in subsidence problem s in adjacent buildings. In som e o f these cases, the m ost likely cause w as attributed to over-rotation o f the auger whilst penetrating a hard stratum or otherw ise difficult ground, resulting in the partial removal o f supporting soil from beneath the adjacent foundations. England and H arding (1993) presented statistical evidence w hich show s a large variation in the boring rate for similar rigs but w ith different operators, with boring rates frequently in excess o f 16 revolutions per metre. To date, consideration o f this problem has been confined mainly to qualitative descriptions o f the auger action in sand w hich are based mainly on field observations, and som e analytical w ork. There has as yet been no com prehensive experim ental or field study o f this problem which could be used to support a model for continuous flight augering. The results o f recent experimental testing at the University o f Strathclyde are presented which provide insight into the conditions which produce over-augering and soil loosening during augering. In addition, recom m endations are given as to how the problem can be minimised. 2 BACKGROUND The function o f the piling auger is to bore to the required depth, during which time disturbed material is taken onto the auger flights. The auger is then withdraw n, rem oving the disturbed soil from the ground, while concrete is pum ped through the hollow central stem at a suitable rate to form the pile. The auger m ust maintain the integrity of the bore and the surrounding ground throughout this process. The auger consists o f a continuous helical flight, as show n in Figure 1, with a cutting blade arrangem ent at the boring tip. 2.1 Augers usedfor materials processing A ugers have long been used for transporting m aterials such as grain. A s such they consist o f a helical flight o f the type show n in Figure 1 w hich revolves at high speed in a stationary tubular casing with the flight projecting beyond the low er end o f the casing to provide the intake. They can operate at any angle o f elevation up to the vertical. The principle originates from the Archim edean screw, which differs from the grain auger in that the casing is fixed to and rotates w ith the flight. Tests on m odel augers by R oberts and Willis (1962) show ed that the A rchim edean screw will not transport grain when the elevation angle exceeds 20. H ow ever w hen the casing is not attached to the auger and remains stationary, grain can be transported even w hen the auger is vertical, although in this position it operates at its low est efficiency. This is analogous to the case o f the piling auger w hen rotating with no vertical penetration, with the bore walls acting as the casing. The grain auger testing clearly show s that it is the frictional force acting on the stationary casing which arrests the rotational m otion o f the grain and provides the upw ards m ovem ent. This force mainly results from the centrifugal force o f the rotating grain generated by the high rotational speed o f the auger (up to 2500 rpm). H ow ever for a piling auger, this frictional force is caused by the lateral earth pressures developed around the bore wall w hich act on the soil within the flights. A further distinction to be m ade is that the bore walls may not be stable so that sand may be fed into the auger from the sides as well as the bottom. 2.2 Augers usedfor CFA piling in sands A qualitative description o f the action o f the auger in sand w as given by T horbum et al. (1993). D uring boring, if the rate o f penetration o f the auger, v (m/min), is such that it equals the rate o f revolution o f the auger, n (revs/min), multiplied by the pitch length, p (m) (ie. v/np = 1.0), then the auger w ould penetrate the ground in the m anner o f a screw w ith the helical flight always following the same path. The soil w ould rem ain relatively undisturbed within the flights, assuming that the soil could be com pressed sufficiently to accom m odate the volum e o f the auger. H ow ever, in practice the torque required is seldom available from the auger rig and the rate o f penetration o f the auger is m uch low er (ie. v/np < 1.0). H ence the soil is cut by the blades at the tip o f the auger and taken onto the flights in a disturbed state. Typically the penetration rate v/np is equal to 0.5 or less. As a result, there is a tendency for the flights to push against the soil ribbon and hence for the soil either to rotate w ith the auger or to be transported upw ards relative to the auger, depending on the forces mobilised 1005

2 D ia m e te r, D K :------* Stem diameter, d F igure 1 T he C F A piling auger around the soil ribbon. Thus the tw o auger actions - cutting and transporting - need to be kept in balance so that the auger flights remain full during boring. I f the penetration rate o f the auger is too slow in relation to the rotational speed, an insufficient volum e o f sand may be taken onto the flights from the cutting edge to balance the sand transported up the auger causing an inflow from the sides o f the bore and consequent ground disturbance. Flem ing (1995) proposed a model for continuous flight boring in w hich the force equilibrium o f the soil within the flights is analysed. T he soil is considered to be a continuous solid with no relative particle motion. T he force driving the upw ards m ovem ent o f the soil is the shear stress acting at the bore wall, w hich is taken as the active normal pressure required to maintain stability o f the wall. This is determ ined using the m ethod proposed by Terzaghi (1943) for vertical shafts. T he forces resisting upw ards soil m ovem ent are self w eight and the dow ndrag acting against the upw ard flow at the bore wall. T he ratio o f the driving to the resisting forces is term ed the Flighting Ratio, and w hen its value exceeds 1, the auger can be expected to transport soil. B ased on this analysis, Fleming draw s the following conclusions: 1. T he occurrence o f excessive flighting is m ore likely with large than with small diam eter augers; 2. T he occurrence o f excessive flighting is m ore likely as the angle o f friction o f the soil external to the bore decreases ie. m ore likely for loose sand than m edium dense sand; 3. L ess soil will be transported as the auger flight angle is steepened. T he first tw o conclusions can be inferred from the calculated lateral pressures on the bore walls. Concerning the third conclusion, steepening the flights increases the rotational m otion o f the sand within the bore while reducing the upw ards motion. 3 T H E E X PE R IM E N T A L SET U P In order to investigate the validity o f the above ideas, a program m e o f laboratory tests using small augers w as carried out. The test procedure sim ulates the field condition o f the auger boring through sand from the ground surface, hitting an obstruction at depth and continuing to rotate with no further advance o f the auger. T o assess the level o f soil disturbance occurring during each test, the following w ere monitored: 1. T he volum e o f sand transported by the auger to the surface; 2. The surface settlement profile around the auger; 3. T he extent o f the zone o f disturbed sand around the auger; 4. T he change in density o f the sand around the auger. T he variables w hich influence the extent o f soil disturbance, and which w ere investigated during the test programme, are as follows: 1. T he initial density o f the sand; 2. T he auger geom etry ie. the pitch length/overall diam eter (p/d ) ratio and the stem/overall diameter (d/d ) ratio, 3. T he rate o f penetration during boring in relation to the rotational speed ie. the ratio v/np; 4. T he num ber o f over-rotations occurring after the auger advance is stopped. 600 mm Figure 2 T he auger rig (not to scale) The experim ental set up, show n in Figure 2, consisted o f a test tank o f plan dimensions 0.6m x 0.6m and variable height into which Leighton B uzzard sand w as placed at a uniform density using the sand raining technique described by R ad and Tum ay (1987). The properties o f Leighton B uzzard sand are given in Table 1. Three different relative densities w ere used ; loose (Dr = 0.29), medium dense (D r = 0.58), and dense (D r = 0.81). An auger was then penetrated into the sand at a rate o f v/np = 0.9 to the required depth, normally 540 mm (H /D = 8.3), and then rotated with no further penetration. This relatively fast penetration rate w as chosen to minimise disturbance at the end o f auger penetration. T he auger was then rotated w ith no further penetration (ie. v/np = 0) for up to 200 rotations, during w hich the various m easurem ents w ere taken. A num ber o f small augers o f mm diam eter, but with varying pitch length/overall diam eter (p/d ) ratios and varying stem/overall diam eter (d/d ) ratios w ere used, as detailed in Table 2. The various m easurem ents w ere m ade as follows. The volume of sand transported by the auger to the surface w as measured by collecting the sand and weighing it at intervals o f 1 m inute (every 12 rotations). T he volum e w as then calculated by assum ing that the sand Table 1 The properties o f Leighton Buzzard sand Property M ineral com position Specific gravity Particle size range (mm) Uniform ity coefficient, (W d io M axim um unit weight (kn /m 3) M inim um unit w eight (kn /m 3) Friction angle, <(>' (peak) Friction angle, < >' (constant volume) Values Mainly Q uartz

3 60 A uger N o. 3 p/d = 0.93 m. dense dense N um ber o f rotations (a) Effect of initial density on sand transportation Number o f rotations (b) Effect of initial density on sand transportation Sand density; loose Sand density, m edium dense N um ber o f rotations (c) Effect of p/d ratio on sand transportation C Number o f rotations (d) Effect of p/d ratio on sand transportation i 35 'S ^ 30 r 25 V S o P 10 Auger No.2 p/d = 0.75 Sand density; loose O d/d = O - d /D = A d/d =0.58 d «C 8. I Auger No.4 p/d = = 24 Sand density. rough loose 8 = Number o f rotations Number of rotations (e) Effect of stem diameter d/d ratio Figure 3 Relationships betw een the volume o f sand transported per rotation and the number o f rotations (f) Effect of auger flight surface roughness Table 2 The auger param eters Auger no a I 2b 3 4 4a 5 Pitch, p (mm) Diameter, D (mm) Stem diameter, d(mm) Length, 1(mm) Ratio, p/d Ratio, d/d Flight surface/sand friction angle, is transported within the flights at its critical density. T he surface settlement profile around the auger w as m easured using tw o row s o f five linear displacement transducers w ith a resolution o f 0.05m m, with the rows placed at right angles to each other. T he volum e o f transported sand and the surface settlem ent profile w ere m easured in separate tests since it w as difficult to collect the sand w ithout disturbing the settlement readings. Changes in the density o f the sand around the auger were m onitored using therm al conductivity probes (Singh et al., 1980). T he technique has the advantage o f allowing the density to be m easured at the sam e location at intervals during the test. The main limitation is that relative densities below about 0.3 cannot be m easured accurately, although if the density changes from the m edium dense to the loose state, this can readily be identified. The probes consist o f a steel tube o f 102 mm length and 1.2 mm diam eter containing a heater element and a therm ocouple. The m ethod is based on the principle that the rate o f tem perature rise o f an em bedded heated body depends on the thermal conductivity o f the surrounding medium. Since there is a direct relationship betw een the therm al conductivity o f sand and its density, the probes can be calibrated to m easure the density. This w as done by fixing the probes to the bottom o f a density pot and recording the thermal conductivity for a range o f sand densities obtained in the pots using the sand raining technique. T he probes can m easure the unit weight to within ±0.2 kn /m 3o fth a t determ ined using the density pot method. 4 R E SU LT S O F T H E E X PE R IM E N T A L P R O G R A M M E T he test results for the volum e o f sand transported by the auger during over-rotation are shown in Figures 3a-f In each figure, the 1087

4 Distance from auger centre / auger diameter (L/D) =o O relative density after penetration - relative density after 200 rotations (a) After 36 rotations C.L I 0 Distance from auger centre / auger diameter (L/D) Figure 5 Changes in sand density after 200 rotations in loose sand for auger w ith p/d = 0.6 (b) After 144 rotations Figure 4 Surface settlem ent profiles for various augers over-rotating in loose sand volum e o f transported sand per rotation is plotted against the num ber o f rotations. T he figures show that irrespective o f the initial density o f the sand, the volum e o f transported sand is highest during the first few rotations after w hich it rapidly reduces and eventually reaches a reasonably constant value (steady state). Figure 3a show s that the volum e o f transported sand is higher for loose sand than for medium dense and dense sand w hen the auger flight is relatively shallow (p/d = 0.6). H ow ever, w hen the auger flight is steep (p/d = 0.93) the am ount o f transported sand is very low irrespective o f the initial density o f the sand, as show n in Figure 3b. This trend is confirmed in Figures 3c and 3d w here augers having different p/d ratios are com pared for loose sand and m edium dense sand respectively. The figures show that sand transportation by the auger during overrotation is only significant when the auger flight is shallow (p/d = 0.6) and is greatest in loose sand. These findings confirm the conclusions draw n by Fleming (1995) from his auger boring model. Figure 3e show s the effect o f increasing the stem diam eter on sand transportation. It can be observed that increasing the stem diam eter reduces the am ount o f sand transported by the auger. The cumulative transported volum e for d/d = 0.58 is about 40% less than for d/d = T he influence o f the flight roughness is show n in Figure 3 f T w o different flight surfaces w ere prepared using polished and roughened steel, and the flight surface/sand friction angle w as determ ined using direct shear tests. From the figure it is apparent that increasing the roughness o f the flight reduces the am ount o f sand transported by the auger. T he surface settlem ent profiles during over-rotation w ere found to be consistent w ith the results for the volum e o f transported sand. T hat is, the conditions which produce the greatest am ount o f transported sand also produce the greatest surface settlement. The ground surface settlem ent profile for over-rotation in loose sand are show n for various auger p/d ratios in Figure 4. F or augers used in the field, w hich have a boring rate o f up to 40 rpm, Figure 4a represents 1-2 minutes o f over-rotation while Figure 4b represents 4-8 minutes. T he figures show that the settlem ent is greatest close to the auger periphery and decreases away from the auger, becoming negligible at a distance o f 4.5D from the auger (approximately 50% o f the auger depth). T he settlem ent is m uch larger for the shallow pitch auger (p/d = 0.6) than for the steeper pitched augers (p/d = 0.75 & 0.93). It can also be seen that a longer period o f over-rotation produces greater surface settlement. The changes in sand density, m easured using the thermal probes, w hich occur during augering in initially loose sand are shown in Figure 5. It can be seen that som e densification o f the sand around the auger periphery occurs during the penetration stage, followed by loosening during the over-rotation stage. In general, a cone-shaped zone o f disturbed soil is produced, the surface extent o f which coincides w ith the surface settlem ent profile. The extent of the disturbed zone is similar to that found by T horbum et al. (1993), who carried out a field trial, although no details o f the auger parameters or am ount o f over-rotation w ere given. 5 A N A LY SIS O F T H E A U G E R T R A N SPO R T A TIO N ACTION A s previously m entioned, the transportation action o f a CFA piling auger is closely related to that o f augers used for materials processing. Consequently, the velocity diagram s produced by R oberts (1995) in his vertical auger conveyor analysis provide a useful m eans o f studying the m ovem ent o f soil particles on the auger flight. T he forces acting on the sand within the auger flights are show n in Figure 6. The force W is due to the w eight o f the soil within the flights. T he rotation o f the auger produces a force F as the flight pushes against the soil ribbon, resulting in sliding on the flight surface. This force is opposed by a drag force T which is caused by lateral earth pressures acting on the sand within the flights. Since this force acts along the sam e line but in the opposite direction to the particle motion, the particle m otion is defined by the direction of the force T w hich acts at an angle X to the horizontal. The particle m otion is m ore clearly explained by the velocity diagram show n in Figure 7. The figure show s a soil particle on the auger flight and the related velocity diagram developed as a result of auger rotation. Vs is the tangential velocity o f the auger at the radius considered, Vr is the relative velocity o f the soil particle with respect to the auger flight surface, and VA is the absolute velocity o f the soil particle. T he direction o f VR is defined by the auger flight angle a. T he angle X defines the direction o f the absolute velocity and hence the helix angle o f the path follow ed by the soil particle. VA can be resolved into tw o com ponents, the lifting velocity VL and the rotational velocity VT. F or a flight with surface friction angle S. the maximum lifting velocity V u could only be approached if the frictional force T due to lateral earth pressures w as very high. This defines the m aximum possible value o f X, w hich is show n as /i If there w ere no lateral pressures, the angle X w ould be zero and the sand would rotate within the auger flights with no upwards motion. 1088

5 Direction of rotation C 7 > Bore wall Pitch, p F a+5 Sand m otioit 'V V " - - \ I W T Auger flight, angle a Figure 6 Forces acting on the sand within the auger flights As the lateral earth pressures increase, the angle X increases and a greater amount o f sand w ould be transported upw ards, to a maximum am ount equivalent to /L «. From Figure 7 it can be observed that as the flight angle a increases, the value o f /t, decreases. Similarly, as the flight surface roughness S increases, X^m decreases. Therefore in both cases the volume o f sand transported should decrease. This w as confirmed by the experimental results, as can be seen from Figures 3 c and 3 f The transporting capability o f an auger conveyor is usually defined in terms o f its volum etric efficiency t\ as follows: rj = actual volum e o f material delivered per rotation ( vi total volum e o f sand within one auger pitch - ( D 2 4 d 2 )p The volumetric efficiency can be readily calculated experimentally from Figure 3 for any stage o f the over-rotation tests. In Table 3, the experimentally obtained volum etric efficiencies calculated using Equation 1 are given for selected tests, for the initial stage o f overrotation and at the steady state tow ards the end o f the test. It can be observed that in each case the volum etric efficiency is much greater at the initial stage than at steady state. T he m ost likely reason for this is that high lateral earth pressures w ere produced during penetration o f the auger due to the high rate o f penetration, with sand being displaced laterally into the surrounding ground. However this process w as gradually reversed during the overrotation stage w ith sand being draw n tow ards the auger and transported to the ground surface. T he lateral pressures would gradually reduce to the point w here the stability depends on arching around the bore walls. At both stages, but particularly at the steady state, the volumetric efficiencies are very m uch less than previously determined. T horbum et al. (1993) calculates a typical volum etric efficiency o f about 70%, while Viggiani s (1993) analysis implies an efficiency o f 100%. Furtherm ore, for the auger w ith p/d = 0.6, the maximum possible efficiency (for X = X x) is about 73%, which Table 3 Experimental volum etric efficiencies Initial sand p/d d/d Auger Volumetric efficiency, r (%) density (Dr) ratio ratio friction, 5 Initial stage Steady state Loose (0.29) Medium (0.58) Dense (0.81) (1) w ould occur w hen the lateral earth pressures w ere very high. It should also be noted that for vertical conveyors running at optim um speed, the volum etric efficiency does not exceed about 40% (Roberts and Willis, 1962). This does not mean that the auger flights are not full o f sand, but rather that the sand within the flights is rotating w ith very little upw ards motion. 6 C O N C LU SIO N S From the experimental results and analysis o f the auger transporting action, the following conclusions can be drawn. 1. Prolonged rotation o fc F A piling augers at depth in the ground w ithout com m ensurate vertical penetration can cause ground disturbance and surface settlem ent up to a distance o f about 50% o f the auger depth. The am ount o f disturbance depends on the num ber o f over-rotations, so that for piling rigs with a high rotational speed, ground disturbance could com m ence rapidly. 2. G round disturbance is greatest in loose sand and w hen the auger pitch/diam eter ratio is low. The potential for ground disturbance could be minimised by using augers with steeper flights. H ow ever, such an auger would probably require a higher torque to penetrate to the same depth. 3. D uring auger penetration, the transporting action o f the auger will be activated if the penetration rate is too low. I f the transported volum e o f sand exceeds the volum e displaced by the auger, the result will be soil loosening as sand is fed from outside the bore to fill the flights. T herefore an assessm ent o f the volum etric efficiency o f the auger during penetration at a particular rate is required in order to determ ine the optim um rate o f penetration o f the auger. 4. T he lateral earth pressures w hich develop around the periphery o f the bore during penetration o f the auger are greater than those which occur during over-rotation o f the auger. Therefore the use o f Terzaghi's m ethod for determ ining the lateral earth pressures may under-predict the lateral earth pressures during the boring stage, since the m ethod calculates the lateral pressure required to support an unlined bore. 7 R E FER EN C ES England, M. & J. H arding Instrum entation o f pile installation as a m anagem ent tool. Proc. o f 2nd Int. Geotechnical Seminar on Deep Foundations on Bored and Auger Piles: Balkema: Rotterdam. Fleming, W.G.K T he understanding o f continuous flight auger piling, its m onitoring & control. Proc. o f the institution o f Civil Engineers, Geotechnical Engineering, 113, July: Rad, N.S. & M.T. Tum ay Factors affecting the sand specimen preparation by raining. American Society for Testing & materials, 10(1): R oberts, A.W Analysis o f screw conveyor performance. Proc. o f Conference on Powders and Bulk Solids' Chicago: USA. R oberts, A.W. & A H. Willis Perform ance o f grain augers. Proc. Instn. o f M echanical Engineers, Manipulative & M echanical Handling Machinery Group, 176(8): Singh, G., J. E rgatoudis & B.S. Siah A laboratory m ethod o f m easuring the in-situ density distribution in dry sand. American Society for Testing & Materials, 2 (3 ):

6 Terzaghi, K Theoretical Soil Mechanics. N ew Y ork: John Wiley. Thorbum, S., D.A. G reenw ood & W.G.K. Flem ing The response o f sands to the construction o f continuous flight auger piles. Proc. o f 2nd Int. Geotechnical Seminar on Deep Foundations on Bored and Auger Piles Balkema: Rotterdam. U nsw orth, J. M. & W.G.K. Fleming C ontinuous flight auger piling instrum entation. Proc. Conf. on Geotechnical Instrumentation in Civil Engineering Projects : Thom as Telford: London. Viggiani, C Further experience with auger piles in the N aples area. Proc. o f 2nd Int. Geotechnical Seminar on Deep Foundations on Bored and Auger Piles: Balkema: Rotterdam. 1090