A Framework for Uncertainty Management in Design and Certifying of Working Platforms for Construction Plants
Ehsan Moradabadi
CEng MIEI | PhD in Civil Engineering (UCD) | Registered Geostructural Engineering Consultant | Technical Design (Geotechnical and Structural) | Feasibility Studies | Team Building | Project Management | Training
Site preparation and safe operation of construction plants at the commencing stage of construction phase of any project are the main priorities of the contractors, project owners and other stakeholders involved in the project including health and safety authority. While one may think that site preparation is only a challenge on the shoulder of contractors, experienced contractors and engineering societies are well aware that a successful site preparation directly relies on an excellent #planning and #engineeringdesign before starting the #construction phase. While an engineer may say that enough data may be available from desk study of permanent designs of a project, an experienced contractor or designer would know that, entering a new site looks like entering a new world, and the main challenge of the contractors at the beginning of any project is dealing with a new unforeseen working environment and unexpected scenarios that generally may be not seen or predicted for the main design elements of a site.
In previous article (https://www.dhirubhai.net/pulse/robust-design-certification-methodology-installing-moradabadi-phd?trk=portfolio_article-card_title), a robust design and certification methodology for geographically distributed infrastructures was discussed in details through an implemented case study relevant to permanent infrastructures of transport Ireland. This article presents the similar methodology for another geographically distributed design, #workingplatforms for construction plants, that are relevant to construction of #windfarms or other #preconstruction activities of similar but very large scale projects such as #tunneling (Figure 1). Having a very simple structure, the working platforms are constructed of granular materials and their vital stability for construction plants makes them one of the important design subjects in #geotechnicalengineering and #constructionmanagement . The design output can be simply a piece of drawing or a temporary working certification, however in consequence of either miscalculation in design or misconception in evaluating the design parameters, especially ground condition, the result can be an engineering disaster. This dictates a harmonised collaboration of plant owners and operators; project owners, contractors, permanent designers, PSDP (project supervisor for design process), PSCS (project supervisor for construction stage) and any other stakeholder involved in construction and design. The similar arrangements mentioned in previous article for interaction between PSDP and PSCS and other stakeholders of the project need to be specified before starting a project, that are not discussed here in details, and this article focused only on the managing uncertainties during design and certifying working platforms considering causes of failure and design parameters involved in design process.
The case study presented here is only one of the successfully implemented case studies that used the design and certifying methodology suggested by the author under directorship of David Lally in Oweninny Wind Farm for RoadBridge. In addition of certifying the working platforms of Oweninny wind farm (e.g. Figure 2), the suggested methodology were successfully implemented for designing crane platforms for?A373 Bypass road?project and advising and consultancy service for working platforms related to two different nearshore marine projects.
Figure 2- A piling platform and a piling rig under operation in construction phase of Oweninny wind farm
In the next section a summery of causes of failure were discussed in order to prepare readers/designers for better understanding what sources of uncertainties need to be treated in design phase to reduce the probability of failure during construction.?
Causes of failure
To better understanding how to manage uncertainties through design process, it is worthwhile to review the major causes of failure in advance.
Failure of a plant can be consequence of different reasons. However, the main cause in failing a working platform is lack of enough bearing capacity in ground.?If the ground bearing capacity was not enough, the failure is guaranteed (Figure 3).
Figure 3- The consequence of miscalculation in design or misconception in evaluating design parameters including ground condition
Experience has also shown that it is far more likely that rigs/plants will overturn owing to localised problems (such as variability in sub grade as generally defined as hard and soft spots) rather than to an inadequate platform thickness across the whole site ( Figure 4) .?
Figure 4- Failure in working platform due to insufficient ground bearing capacity and high level of ground water table
Other causes are positioning of plant in sleep slope (generally more than one-tenth for piling and crane platform) or on a dangerous platform edge, such as a retaining wall with low capacity to resist against the lateral pressure from the plant ( Figure 5).
Figure 5- An example of failure of retaining wall as the main reason of failures of the working platform
Insufficient stiffness of outrigger pads, incorrect positioning of outriggers (Figure 6), and incorrect/inappropriate selection or installation of the plant (underestimating?loadings, pressure or unbalance forces, wind, etc.) can be also causes of failure if they were not properly treated in either design phase by the designers or construction phase by either plant operators or contractors.?
Figure 6-?Correct selection and proper positioning of outrigger pads and examples of improper selection and positioning of outrigger pads
Limit state function
From a Geotechnical designer’s perspective, the platform is rationally stable if the factored design bearing resistance was greater than the factored loading characteristic value. This relationship between design bearing resistance and loading characteristic is called ‘limit state function’ in Geotechnics (e.g based on Eurocode 7). Considering the formula of Figure 7, it can be found that there are generally four parameters need to be defined for limit state function of the platform: the design bearing resistance, loading characteristic value, and their relevant partial factors.
Figure 7- General limit state function for designing a working platform
Figure 8 shows how to define the characteristic load for defining the pressure implying?to the platform. The relevant values for pressure either can be specified by the plant contractor or can be found in the plant’s manual. However in case that the characteristic pressure was not available, the designer completely relies on a detailed calculation regarding the?type of plant, the weight of objects, the phase of operation and external loading condition such as wind load.
Figure 8- Characterisation of the pressure from construction plants to working platform?
In case of having a piling rig, different operational conditions of the plant may be considered by the designer (Figure 9) such as standing/travelling, drilling and extracting. An excel sheet?such as what was shown in Figure 10 may be used for calculation of each load case.
Figure 9 - different operational stages of a piling rig
Figure 10 - An example of spread sheet used for calculation of pressure of a piling rig to a platform
For crane rigs/ trucks, the pressure/load for traveling/standing scenarios?resulted from calculation of maximum axle load, and also the calculation of outriggers’ forces during lifting may be needed. The detailed dimensions of the plant (Figure 11), especially the positions of outrigger pads, and counterweights and heavy parts of the plants for calculation pressure under different characteristic load-distance scenarios (Figure 12)?of the crane are usually necessary to be defined for the designer. In case of availability, the designer can also use a calculator provided by the manufacturer or other relevant commercial sophisticated software (Figure 13). Same procedure may be used for calculating forces/pressure to the platform from an excavator during either digging, pulling,?travelling, handling or parking.?
Figure 11- Example of?a fully dimensioned crane rig, usually can be found in engineering manual of a plant provided by the manufacturer
Figure 12- An example of load distance diagram of a nominated crane, generally can be found in the plant manufacturer’s engineering manual
Figure 13- An example of sophisticated software used for calculation of pressure of plants to the platform
Worthwhile to mention that for calculating characteristic loads, the effect of wind can be significant ( Figure 14), being necessary to be treated by the designer in preliminary design phases.
Figure 14- A scenario example that wind load can be significant during operation of the plant
In terms of partial factors, notably, if the operator is unlikely able to aid recovery from an imminent platform failure (especially for cases such as standing, travelling and handling), greater safety factors are usually used. On the other hand, if it was likely to be able to control the load safely by the operator (such as Drilling, piling, extraction), it is allowed to use lower safety factors. Examples of partial factors suggested by BRE manual 470 can be found in the table of Figure 15.
Figure 15-?Acceptable partial factors according to BRE manual 470
On the left side of the limit state function there is?bearing resistance. Providing the dimensions of the platform were defined,?the bearing resistance of?the platform on granular sub-grade can be conservatively estimated based on?the formula presented in Figure 16. As you can see here the uncertain factors in this equation are punching factor and bearing capacity factor which are needed to be defined by site investigation being mentioned in later paragraphs.
Figure 16- Bearing resistance of platform on granular sub-grade and relevant parameters
For cohesive material, a conservative approach was mentioned in Figure 17, where Cu is the shear strength of the ground, and Td is the design tensile strength of the geogrid, if the design shows the necessity of using of it. Similar to the previous formula, the characteristic parameters of the soil and the platform are uncertain.
Figure 17- Bearing resistance of platform on cohesive sub-grade and relevant parameters
For Cu?it is possible to use?a very cheap cone penetrometer to find characteristic value of undrained shear strength (Figure 18). For punching factor it is possible to use either of a cone penetrometer or CBR test (Figure 19). Experienced engineers may conservatively use predefined values mentioned on the table after a detailed site visit.
Figure 18- An example of using cone penetrometer to find characteristic value of undrained shear strength of the sub-grade
Figure 19-?An example of using CBR test?to find characteristic value of punching factor
Worthwhile to mention that, when dealing with working of plants close to the edge of a slope or adjacent retaining structure is the matter of interest, it?should be considered that the simplified formulas do not work. It is possible that new condition dictates either?a more complex analysis of the stability for the slope or controlling the capacity of the retaining structure for fulfilling the extra lateral pressure from the plant that may be not designed for. It is seriously advised?to?designers and contractor to be working far enough from the edge of the slope or retaining structure to ensure?that the mentioned simplified formulas are valid for calculation (see Figure 20).
Figure 20-?A conservative guideline for distancing plants from the edge of slopes
Another important issue that the designer needs?to be aware of is drainage. Drainage is very important especially?when the sub-grade permeability is very low. Drainage design and managing runoff?can be a serious part of?the design especially if?the temporary works are scheduled for a rainy season.?On the other hand, cohesive soil are very sensitive to weather and a long drought or a long period of raining may affect the characteristic values of the bearing capacity of the ground, that need a separate consideration in design.
Depending to the service life of a platform, the passage of truck/ plant can be significant, especially when the platform needs to be used as a haul road as well. This may dictate a temporary road design calculation for the platform.?Moreover,?there are some situations that the calculation of settlement of the ground is important especially when the assessment shows that the differential settlement is possible. Thus, the designer need to have a comprehensive assessment of the needs of contractors and provide per-solved solutions for different possible scenarios.
Worthwhile to mention that to?be sure about the suitability of the platform, most of the time, the?bearing capacity of the platform is post-validated by the contractor using enough numbers of plate load tests (Figure 21).
Figure 21- Plate load test for post validating a constructed platform
Managing the process from site investigation to post-validation and operation?
As mentioned earlier and illustrated in Figure 1; a robust design and construction of a project rely on a harmonised collaboration of all different parties involved in the project. This is not achievable without having a systematic approach in design and to arrange the required interaction between all stakeholders of projects and involving Health Safety and Environment (HSE) concept in all design and construction steps of the project.?
Regarding the number of platforms need for a large project and their distribution in a large geographical area, collecting all design input data and prediction of geotechnical behaviour at the beginning of the project is usually difficult. To overcome this, according to Eurocode 7, my general suggestion for these kind of projects is employing an ‘observational method’ (also known as ‘learn-as-you-go method’ in geotechnics). This helps that the project economically executes due to increasing knowledge of the properties and behaviour of the ground.?
The flowchart of Figure 1?shows that how the design process goes forward during different stage of the projects, and the uncertainty in parameters decreases upon the successful progress of the project. The project generally starts with reviewing?received information, an effective communication between all parties involved in project may be needed for collecting available data for the design. If basic information was sufficient, a desk study is necessary then. This may include but not limited to:?
During desk study a preliminary site investigation may be planned and a pre design process is?executed. This may consist of:
All of these process should be based on acceptable standards such as Eurocode 1, Eurocode 7; BRE 470; CIRIA SP 123; EN 16228
After pre-design process,?the main design process is done that consists of developing modelling scenarios, analysis and presenting design outputs. Generally the design process needs a design risk assessment?that?includes hazard identification, hazard elimination and risk reduction, control measures for safety and health plan, and planning health and safety.
If safety was ensured, the design will be issued for construction. During construction period a new site investigation may be needed to confirm the appropriate design alternative for the site. If the new site investigation shows that the design is sufficient, the construction of platform will be executed.
Implemented Case Study
The case study presenting in this section summarises the?implementation of the design?process mentioned earlier for a crane platform in Owenniny wind farm.
The project was about positioning a suitable crane for?demolishing the previous bridge and mounting a new bridge across Oweninny river.
The contractor in this case wanted to use a grove mobile crane.?The stiff soil strata was at least 2 m lower to the ground level, and the contractor decided to remove this ground and back fill with granular material. Thus, in first glance there should not?be any issue in bearing capacity of the ground?in general.
The first question was?asked from the contractor was about the maximum load they wanted to lift.?Considering the?safe distance to the retaining wall, the allowed load of the crane was controlled to ensure that it fulfils the maximum load?they needed to lift and to handle for crossing the river. There was?also a buried cable that mounting of platform on that was not possible and needed to be considered for positioning of the crane.
Thus the?the critical outrigger forces and relevant?pressures were calculated for the project, and an analysis have been done then for the controlling the structural safety of retaining wall to double check whether the general and internal stability of existing abutments are satisfied. Due to analysis result, there was a need of?site visit (Figure 22) to ensure about the height of backfill behind the abutment, and also to assess?that whether the passive pressure is enough for stabilising the retaining wall against the pressure on the platform. The summary of design provided to contractor is illustrated in Figure 23.?
Figure 22- Photographs of site visit
Figure 22- Drawings and the details of the case study that successfully implemented according to the suggested methodology
To conclude, the design of platform is a very simple but serious task and harmonised collaboration of different parties involved in the project are vital to?guarantee the safety in design. Site investigation?is the most significant factor in design and to reduce the cost, an observational method is suggested.?