and the design of construction working platforms

Transcription

1 Federation A presentation to the Australian Geomechanics Society Victoria i Branch 11 August 2010 Dr Stephen Buttling Piling rig bearing pressures and the design of construction working platforms 1

2 The Problem One third of accidents in the piling industry result from defects in the working platform that the rig is required to work from. ( Terry Bolsher Chairman of the UK Federation of Piling Specialists). The experience in Australia is NOT dissimilar to that of the UK. Every year piling rigs fall over or are subject to near misses due to inadequately yprepared p or maintained site surfaces all such incidents involve potential fatalities or serious injury to operatives and extensive damage to equipment. 2

3 The effect of poor working platforms Let us look at the effects of some poor working platforms on piling rigs. These photographs are all from the UK where they have maintained more extensive records of such incidents in the past it is often difficult to obtain such graphic details in Australia. 3

4 Crane rig being mobilised on poorly prepared platform 4

5 Bored piling rig collapsing sideways due to soft spot on platform 5

6 Another view of same rig 6

7 New pile driving rig sinking due to poorly compacted platform 7

8 Crane based pile driving rig collapsed across car park & site sheds 8

9 Crane attachment drilling rig collapsed sideways due to soft spot in platform very close to edge of platform. 9

10 Displacement pile rig nose diving off bog mats 10

11 A working platform is the foundation for a piling rig that may weigh anything from 5 tonnes to 200 tonnes, together with all of the ancillary equipment that may be used on the platform ltf to service the rig. Modern piling equipment is becoming increasingly heavy, often with higher centres of gravity, to cater for the demand for deeper and larger diameter foundations to take higher load capacities, combined with the need for more environmentally friendlyinstallation installation techniques 11

12 As in many other industries, experienced operatives are limited, and new operatives are often being trained d on the job To effectively train a skilled operative takes several years not days. The lead time for equipment, accessories and spare parts from overseas is often considerable. For example most European and USA manufacturers have been quoting up to 12 months for supply of new equipment! 12

13 It should be everyone s aim to minimise accidents on construction sites in order to prevent any fatalities or injuries to personnel with the possible long term implications of such injuries. i Delays to projects caused by accidents to skilled operatives, and the downtime whilst equipment is being repaired, are likely to be extensive and expensive. There may also be stoppage time while official enquiries are made 13

14 CFA rig collapsed across main London Paris railway line As this piling machine fell, it brought down the overhead power supply lines, effectively stopping all trains. What would have been the carnage if a train had been passing at the time? 14

15 Another very near miss in the UK This time someone I know well, now living in Sydney, was closely involved. 15

16 There is sometimes also sometimes a lack a of lack of understanding appreciation iti di at about site management tthe & importance supervisory of level preparing of the a effects safe of working excavating platform services capable of supporting through a working the equipment platform to and be used, the need and for whose engineered responsibility it is replacement to design, prepare of any material and that maintain is disturbed such during a platform such excavation. throughout the course of the project. 16

17 The Responsibility The responsibility for the design, construction, maintenance and repair of a working platform should be taken by the organisation that has CONTINUOUS control of the project activities NOT the piling contractor. The platform may continue to exist after the piling contractor has left the site and may be used by other trades, requiring maintenance and repair to continue beyond the end of the piling works. Even during the piling works other trades often operate from the same working platform as the piling contractor. The piling contractor is responsible for supplying details of the plant and machinery to be used on the project including the bearing pressures, dimensions and working space required to operate safely. 17

18 The Responsibility Piling contractor provides bearing pressures Main contractor maintains platform Main contractor engages geotechnical specialist Working Platform Certificate Geotechnical specialist certifies platform Geotechnical specialist designs platform Main contractor builds platform 18

19 Working Platform Certificate The Federation of Piling Specialists in the UK introduced the Working Platform Certificate just over 6 years ago and the systemis now in extensive use throughout the UK. It has proved to be a highly effective way of overcoming the problems that existed prior to its introduction. ti The use of the Certificate is a way of increasing the awareness of working platform safety and the importance of maintaining the platform in good condition throughout the course of the contract. 19

20 Working Platform Certificate Reported incidents have decreased significantly. The latest Certificate states who carried out the design work, allowing the subcontractor to be assured that qualified geotechnical engineers have been engaged to undertake the work. 20

21 The Solution The PFSF has adopted the Working Platform Certificate for ALL platforms from which piling rigs are required to work. This may involve multiple l certificates on some projects such as bridge sites. The Certificate confirms that the working platform has been properly designed and constructed in accordance with the design, and that it will be adequately maintained to ensure that the integrity of the platform is likewise maintained. 21

22 The Solution The Certificate requires the signature of the Principal or Main Contractor on the project and must be handed to the Piling Contractor before piling or other foundation work commences on the site. The Working Platform Certificate does not seek to transfer any responsibility for accidents occurring during the piling contract. Both the Main Contractor and the Piling Contractor have responsibilities under existing OH&S legislation in whichever state the work is taking place and these will remain unchanged. 22

23 Working Platform Certificates What do we write in here? 23

24 Note designer has been added Note no received by signature 24

25 Line added in title Change of headings Track width added Two bearing pressures and two effective track lengths given Derivation given Wording added 25

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28 Certification Visual assessment of a working platform can only check the surface of the platform. It is critically important to also check the underlying material, the quality of the platform fill, its construction and depth, and the application of geotextile material if required. Is certification on the basis of an in situ density test enough? 28

29 Reference material to assist The FPS in the UK in conjunction with the Building Research Establishment (BRE) have produced a good practice guide to the design, installation, maintenance and repair of ground supported working platforms entitled Working platforms for tracked plant. This gives a rational, if simplified, method of working platform design The FPS then produced a spreadsheet to give the input parameters for the working platform ltf design 29

30 Reference material to assist The PFSF have posted on their web site the following discussion papers from the UK documentation: Rig Loading Spreadsheet Working Platform Design Sensitivity Calculation of Track Bearing Pressures for Platform design Guide to Hydraulically Bound Working Platforms 30

31 FPS rig loadings spreadsheet Note that this was intended for use with the BRE platform design method and does not derive bearing pressures from first principles The simplifications, such as wind dload, are deemed dto have been dealt with in the benchmarking used to determine the load factors used in the design The pressures may not be correct if used with another platform ltf design method from first principles The spreadsheet clearly shows the effect of crowd pressures, such as those generated dby the European rigs FPS rig loadings spreadsheet BG28 kelly 3_33 extended sb 4vi10.xlsx The spreadsheet can be used to quickly check the effects of various changes in configuration FPS rig loadings spreadsheet BG24H SB trials 24vi10.xlsx 31

32 FPS rig loadings spreadsheet It is often difficult to get the necessary information to enter into the spreadsheet, especially on centres of gravity 32

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34 FPS rig loadings spreadsheet When 0.38m is entered into spreadsheet, overall CoG is shown FPS rig loadings spreadsheet BG40 with 3_36 kelly SB 7v10.xlsx This shows the CoG 0.3m in front of the centre of slew. When measured by tracking across some timbers it was found to be 0.2 to 0.25m in front. The difference could be due to fuel etc. In other cases we have used the rated load charts to determine the positionofthe of the CoGofthe of base machine. For any boom length, calculate moment due to load and effective radius about tipping line Plot chart Factor up overturning moment. Subtract known moments such as tracks, counterweights etc. Remainder is base machine times lever arm about ttipping i line Overturning moments BM700.xlsx 34

35 FPS rig loadings spreadsheet For another project we were interested in the maximum bearing pressure using a special drill which, as with a pile driving hammer, would normally be either lifted, or be supported by the ground The rig manufacturers told us the maximum bearing pressure would be 940 kpa Our calculation showed about 205 kpa FPS rig loadings spreadsheet BG25 hammer SB 7vi10.xlsx 35

36 WORKING PLATFORM DESIGN 36

37 Design of Working Platforms on Clay Subgrades 37

38 Design Process Define geotechnical parameters Platform Weight density γ Angle of friction φ Sub grade Shear strength s u If s u < 20 kpa method not applicable If s u > 80 kpa platform probably not necessary 38

39 Design Process Calculate plant track loading Use FPS spreadsheet Masses and dimensions 39

40 Design Process Rig orientation Consider all angles Someangles maynot be physically possible 40

41 Design Process Loading Case 1 When the rig or crane operator is unlikely to be able to aid recovery from an imminent platform failure Standing Standing Travelling Travelling Handling Handling (e.g. (e.g. lifting lifting a casing, a casing, rebar rebar cage cage or or precast precast pile pile 41

42 Design process Loading Case 2 When the rig or crane operator can control the load safely, for example by reducing the line load or reducing power, to aid recovery from an imminent platform failure Installing casing Drilling Extractinganauger, an an auger, bucket orcasing Travelling/slewing with with a foot a foot or or load load (e.g. (e.g. pile pile in in leaders) close close to to the the ground 42

43 Design Process Convert triangular/trapezoidal pressure distribution to rectangular (Meyerhof method) 43

44 Design Process Calculations done in FPS spreadsheet 44

45 Design Process Is a platform required? Check if the sub grade alone can support the track loading Standard bearing capacity calculation Load factor for Case 1 = 2.0 Load factor for Case 2 = 1.5 If OK provide only a running surface 45

46 Design Process If not OK designed platform required Check platform material for bearing capacity Load factor for Case 1 = 1.6 Load factor for Case 2 = 1.2 If not OK, have to use a better platform material (higher h φ ) If OK, design platform thickness 46

47 Design Process Punching shear failure mechanism 47

48 Design Process Punching shear failure mechanism Side shear Track 48

49 Design Process Platform thickness calculation D 1 W d q1 d K p p su d N tan s c p1 s Check minimum platform thickness Must be at least 30 cm or half the track width (whichever is less) c

50 Design Process Carry out a sensitivity check, especially for Platform strength φ Sub base shear strength s u 50

51 Design Process This is all easily programmed into a spreadsheet Working platform calcs clay subgrade.xlsx 51

52 Construction At the end of the design we know that for a given sub base base shear strength s u, we need a known thickness of platform strength with a strength φ and a weight density γ How do we convey that to the constructor? If we are too conservative what happens? UK experience 52

53 Article from Ground Engineering gfebruary

54 Article from Ground Engineering gfebruary

55 Where to from here? Significant changes are already occurring Working platforms can be a very significant cost, especially on large sites on soft gou ground for infrastructure developmente e For Kempsey Bypass Ground Improvement, the Principal Contractor required potential subcontractors to design their required working platforms so that the cost could be added to the subcontract price prior to evaluation The PFSF does not want projects to be won or lost by subcontractors under specifying the required bearing pressures 55

56 Where to from here? We have a Safety Poster dedicated to Safe Working Platforms which has been distributed to all members and is available. We wish to receive support for the initiative from all main contractors and government departments who undertake projects involving piling, and their geotechnical consultants. We call on your support to assist us with this! Thank you for your attention 56