Geotechnicl Stbility Investigtions on Mobile Construction Mchines Dipl.-Ing. Peter H. Beutinger Institute of Geotechnicl Engineering, University of Stuttgrt, Germny 1. Introduction Nowdys, there is hrdly ny building project, which is relised without the use of mobile construction mchinery like truck-mounted crnes nd concrete pumps. Unfortuntely, misjudgments of the given underground conditions occur gin nd gin, so tht the mchines tumble nd cuse lrge property nd even personl dmges. A review of such ccidents will be presented in this pper. It ppers tht mny such ccidents were cused by insufficient bering cpcity of the soil underneth the support legs of the mobile mchines. During opertion they re usully supported by four support legs nd mchine will loose its stbility when one of the support lods exceeds the bering cpcity of the subsoil. Considering the significnt number of such ccidents group of mnufcturers decided to strt reserch project in order to improve the present sitution. The bsic ide for improving the present sitution is to mesure the lod-displcement behviour of ech support during its instlltion nd to extrpolte this curve for given working lod. Both, the recording of dt during instlltion nd its extrpoltion is to be controlled by bord computer of the mchine. In those cses where predicted displcements exceed the tolerted ones, the computer will indicte tht the footing re of the support needs to be enlrged. 2. Investigtion of documented ccidents The first phse of this reserch project concerns documenttion of ccidents with mobile construction mchines in Germny. Extensive dtbses, covering the period between 1993 number of ccidents 29 30 truck-mounted crnes 25 fixed construction crnes 20 15 14 eril work pltforms 15 rticulting crnes 10 mobile concrete pumps 5 2 1 2 miscellneous 0 Fig. 1. Investigted ccidents by tumbling (primry period 1993-1997)
unknown cuses 14% technicl filure 3% miscellneous 3% soil collpse 23% Fig. 2. Cuse of filure quoted in percent of risen ccidents nd 1997, were obtined from insurnce compnies, news gencies nd some other sources. In this wy we rrived t totl of 63 ccidents by tumbling of mchines. Fig. 1 shows tht 29 ccidents relte to truck-mounted crnes, 15 to fixed construction crnes, 14 to eril work pltforms nd 5 to other mchines, including mobile concrete pumps. In 38 cses people got injured, including 19 hevily injured nd 8 deths. By further inspection of the ccidents division into following cuses hs been choosen: humn filure, soil collpse, technicl filure, unknown cuses nd miscellneous. Unknown cuses re those ccidents, in which no cuse ws indicted in the dmge dossier. The percentl distribution of the risen ccident cuses is shown in Fig. 2. The min cuses of filure re soil collpse with 23 % ppernce nd humn filure with 57 %. Regrding the ccidents by soil collpse different kinds of filure cuses were estblished. Mostly bse filure with 9 cses, this re 60 % of the ccidents by soil collpse, occurred. Slope filure nd the brek in hollows ppered in three cses (ech 20 %) respectively. The im of the project is to develop n expert system tht cn be instlled on the mchines, with which it is possible to prevent both soil collpse nd humn filures tht relte to improper instlltion of supports, i.e. some 40 % of the recorded ccidents. The bsis for sve opertion with mobile construction mchines lies thus in the ccurte substructure of the supports for ech given underground condition. 3. Hyperbolic lod-displcement curve humn filure 57% A sfe working bse hs to be produced for the work with mobile construction mchines. For prevention of ccidents by toppling the development of the lod-displcement curve during instlltion of support hs to be mesured. To predict the continuing progression of the curve the mesured dt hs to be extrpolted. The theoreticl bckground for this extrpoltion is described in this chpter. Fig. 3 is showing lod-displcement curve nd hyperbol fitted to this curve.
verticl lod P [kn] 200 150 100 50 mesured dt hyperbol Kondner, Zelsko (1963) hve shown in their work, tht the stress-strin reltion of snd is following hyperbolic lw. Similrly we found from mny plte lod tests, tht the loddisplcement curve tend to be hyperbolic. The hyperbol cn be described by the eqution P = s P s 1 + s P where s is the settlement in the centre of the plte nd P is the verticl force. The hyperbol hs two symptotes s given by the equtions s = s nd P (1) P = (2) where s nd P re constnts. P hs physicl mening, s it is the symptotic filure lod. s is defined negtive. By differentiting eqution (1) with respect to the settlements it is found tht dp P =. (3) ds s=0 s For homogeneous hlf spce nd the theory of elsticity the result cn be seen s P s = 2 E A i ( 1 ν ) f D where E i is the initil modulus of elsticity of the soil, A the loded re, D the width of the plte, f form fctor nd ν the Poisson's rtio. Looking t circle pltes with A = 1/4 π D² nd f = π/4 the constnt s is s = 2 ( 1 ν ) P D E nd regrding qudrtic pltes it yields 0 0 20 40 60 80 100 120 i settlements s Fig. 3. Mesured lod-displcement curve with fitted hyperbol [mm] (4), (5)
rtio of reltive settlements to verticl lod s/d / P mesured dt hyperbol reltive settlements s/d Fig. 4. Plte loding test dt from vn Deventer, Molenkmp (1984) in trnsformed hyperbolic representtion s = ( ) 0.84 1 ν 2 P. D Ei (6) Plte loding test dt conducted by vn Deventer nd Molenkmp (1984) re plotted in Fig. 4. It is shown, tht the lod-displcement trend is following hyperbol, which is in trnsformed representtion liner. The rtio of reltive settlements to verticl lod s/d / P is pplied versus the reltive settlements s/d, whereby the mesured settlements s re normlised by the plte dimeter D. This normlistion hs been crried out to compre the tests performed with different plte dimeters. Verifiction of the clculted prmeters from the tested soil resulted in good conformity with lbortory results. 4. Field mesurements Mounting mobile construction mchines cn be divided in two phses in principle. Phse 1 contins the positioning of the supports nd mounting of the mchine; Phse 2 is during opertion. In the following only one support will be spotted. With the id of severl mobile construction mchine mnufcturers it ws possible to conduct some series of lod-displcement mesurements. One im ws to prove the ssumption, tht the behviour of the lod-displcement curves during mounting nd opertion of the mchines re following hyperbolic lw. An other im is to produce dt bse for different kinds of soils to use in the Fig. 5. Set-up of generl mesuring device
reltive lods P/Pmx [-] 0.8 0.6 0.4 0.2 test dt hyperbol extrpoltion 0.0 0.000 0.005 0.010 0.015 0.020 0.025 0.030 reltive settlements s/d [-] Fig. 6. Test dt nd extrpolted hyperbol expert system which hs to be developed. In Fig. 5 the generl mesuring device is shown. The displcements re mesured with piezometric pth sensors nd hve lter been determined for the middle of the plte. By the pressure in the hydrulic cylinder of the supports the occurring lods re cquired. All dt re recorded in computer were they re sved nd stored for further nlysis. Results of one test re shown in Fig. 6. Reltive lods P/P mx re plotted versus reltive settlements s/d whereby the lods re normlised by dividing the occurred lod P with the mximum possible lod of the mchine P mx. Up to deformtion of 0.018, the positioning of the support is recorded. Un- nd reloding loops re following, cused by mounting of the other supports nd re registered until the cntilever of the mchine is extended nd rotted. The extension nd rottion with the cntilever is demonstrting the opertion phse. During the tests it ws not possible to work with dditionl lods, so tht only bout 65 % of P mx ws reched. Extrpolting the fitted hyperbol from Fig. 6 for the opertion phse, it cn be shown tht quite good results for settlement prediction cn be given. To confirm the mesured results, supplementry geotechnicl field rtio of reltive settlements to reltive lods s/d / P/Pmx [-] 0.05 0.04 0.03 0.02 0.01 mesured dt - - - - - hyperbol 0.00 0.000 0.005 0.010 0.015 0.020 reltive settlements s/d [-] Fig. 7. Prove of hyperbolic shpe of loddisplcement curve
nd lbortory investigtions hve been crried out. Plotting the mesured dt by pplying the rtio of reltive settlements to the reltive lods versus the reltive settlements results in Fig. 7. By dding hyperbol to the plot, it is obvious, tht even the lod-displcement curve of mounting mobile construction mchines re in hyperbolic shpe. 5. Numericl nlysis To mintin the field mesurements, numericl clcultions hve been performed with the commercil FE-code Plxis. In this prgrph the used mteril models nd clculted results re presented for the 2D-finite element nlysis. 5.1 Mteril models To conduct the nlysis two different mteril models hve been used. Model number one is the Mohr-Coulomb Model (MC), which is bsed on idel elsto-plstic constitutive lw. Soil stiffness is ssumed to be constnt with depth. The Hrdening-Soil Model (HS) ws used to compre with. This dvnced mteril model hs lso the Mohr-Coulomb filure criterion, but dditionlly hrdening of the soil is tken into ccount. As well there re different stiffness modules choosen for vrious loding conditions which re stress dependnt. A detiled specifiction of the models cn be found in Brinkgreve nd Vermeer (1998) or Schnz (1998). 5.2 Clcultion results First numericl exmintions hve been conducted in homogeneous soil. The used geometry with xil symmetric conditions nd the generted mesh is shown in Fig. 8. For the clcultion 6-noded tringulr finite elements were used to generte mesh, very fine in loded res nd corse in less deforming prts. Tble 1 contins the used soil prmeters for Tble 1. Soil prmeters Mohr-Coulomb Model E oed = 30 MP ν = 1/3 ϕ = 25 c = 35 kp Hrdening-Soil Model E ref oed = 40 MP E ref 50 = 40 MP E ref ur = 120 MP ν ur = 0.2 ϕ = 25 c = 35 kp m = 0.5 p ref = 100 kp Fig. 8. Used finite element mesh
both mteril models. Investigted ws lom, typiclly ppering in south Germny. For both models the soil weight ws γ = 19.5 kn/m³ nd the dilttion ngel ψ = 0. The chieved clcultion results from 10 both mteril models re shown in 0 0.0 0.1 0.2 0.3 0.4 0.5 Fig. 9. Almost no difference in the reltive settlements s/d [-] progression of the curves re visible Fig. 9. Numericlly determined lod-displcement by pplying the lod P versus the curve with two different mteril models reltive settlements s/d. Becuse of the xisymetric clcultion mode the dimension of the lod P is kn/rd, tht mens the totl rection force cting on circle segment of 1 rdin. In evidence, the simultion with HS-Model fils lredy t reltive settlement of 0.18. This is quite erlier thn the clcultion with the MC-Model. It results out of numericl nonconvergence. In consequence of this, further clcultions were done with the Mohr-Coulomb Model which dditionlly resulted in much shorter clcultion times. In Fig. 10 simultion result from clcultion with hollow in the underground, situted under the loded re, is shown. Displyed is the deformed mesh t soil collpse due to the lod, pplied on the plte. Becuse of symmetric plne strin conditions only the left hlf of the geometry hs been generted. The dimensions re: 2.5 m wide nd 2.0 m deep with homogeneous soil conditions. 0.5 m beneth the surfce lies the top of the hollow; stright under the loding plte with 0.6 m width. The pplied soil is the sme lom s described bove nd the clcultion is performed with the MC-Model. lod P [kn/rd] 70 60 50 40 30 20 Mohr-Coulomb Model Hrdening-Soil Model Soil collpse occurred t bout 0.1 m settlements. Deformtion of the top of the hollow is 0.09 m which indictes lrge influences of the hollow on the lod-settlement behviour. Still hyperbolic lod-displcement curve ws observed. Fig. 10. Deformed mesh t collpse, with hollow in the underground
6. Conclusions The im of the reserch project is to prevent ccidents through tumbling, cused by improper instlled supports of the mobile construction mchines. By investigtion of documented ccidents on tumbled mobile mchines it could be presented tht lmost 25 % of the ccidents were cused by soil collpse. The bsic pproch for the reserch is to mesure the occurring settlements of the supports during instlltion nd to mke settlement prediction for the following opertion phse with extrpoltion of the mesured curve. We could show tht the shpe of our mesured curves re hyperbolic, like plte loding test curves. In numericl simultions the hyperbolic shpe of the curves could lso be proved with two different mteril models. To support the in-situ reserch further simultions with slopes nd hollows in the subsoil will be performed nd nlysed. Finlly n utomtic system hs to be developed nd ttched to the mobile construction mchines to enlrge the sfety during work with these mchines. 7. References Briud, J.-L., Gibbens, R.M. (1994). Predicted nd Mesured Behviour of Five Spred Footings on Snd. In Proceedings of the Settlement 94 ASCE Conference. Texs A&M University. Geotechnicl Specil Publiction No. 41. Brinkgreve, R. B. J. nd Vermeer, P. A. (Eds.) (1998). PLAXIS Finite Element Code for Soil nd Rock Anlyses. Rotterdm. A.A. Blkem. Chrles, J.A. (1996). The depth of influence of loded res. Géotechnique 46(1). 51-61. de Borst, R. (1982). Clculting of collpse lods using higher order elements. In P.A. Vermeer, H.J. Luger (Eds.), Deformtion nd filure of grnulr mterils. Rotterdm. A.A. Blkem. Deventer, J.M.M. vn, Molenkmp, F. (1984). Prediction of the behviour of shllow foundtions by mens of plte loding tests nd trixil tests. I Stiffness. Grondmechnic Delft. Forschungsbericht. Delft. Floss, R. (1997). Zusätzliche Technische Vertrgsbedingungen und Richtlinien für Erdrbeiten im Strßenbu. ZTVE-StB 94. Fssung 1997. Kommentr mit Kompendium Erd- und Felsbu. 2. Auflge. Bonn. Kirschbumverlg. Kondner, R.L., Zelsko, J.S. (1963). A hyperbolic stress-strin formultion for snds. In Proceedings of the 2 nd Pnmericn Conference on Soil Mechnics nd Foundtion Engineering. Volume 1. So Pulo. 290-324. Schnz, T. (1998). Zur Modellierung des Mechnischen Verhltens von Reibungsmterilien. Hbilittion. Universität Stuttgrt. Vermeer, P.A., vn Lngen, H. (1989). Soil collpse computtions with finite elements. Ingenieur-Archiv 59. 221-236.