Preparation for a Safe and Successful Project

There is no doubt about it, ensuring your site is prepared adequately to receive a large piece of construction plant is crucial. The consequences of getting it wrong are potentially so severe that no risks can be taken. Where to start?

When we talk about ‘factors of safety’ in design, we are asking what degree of additional load, if applied, would result in a ‘failure’. Is it twice the design load – factor of safety of 2, perhaps three times the design load – factor of safety of 3?

Makes sense, but what is defined as ‘failure’ when we are thinking about Geotechnics?

For circular test plates, or for bearing piles, Geotechnical Engineers will commonly define the point of failure, or ‘yield’, as the point where the displacement is more than 10 % of the diameter of the loaded area, or where the load/displacement behaviour is markedly plastic.

Site investigation reports rarely provide the level of detail around shallow soil strength and stiffness required for the most efficient design of working platforms at subgrade level after completion of any initial regrading. Owing to this degree of uncertainty, temporary works Engineers are necessarily cautious when designing working platforms.

So how best to determine ‘ultimate capacity’ of soil supporting construction plant, for use in efficient working platform design?

We recommend six steps -

Step 1 – Strip the site to slightly above sub-grade level, initially assuming a piling platform thickness equal to half of the equipment’s track width, taking care to identify any noticeable soft spots.

Step 2 – Prepare a trial area of working platform, with and without geotextile, using a sample of the material proposed to be used for working platform construction.

Step 3 - Take a 300mm diameter test plate, place it on the subgrade and incrementally load it until the deflection exceeds 30mm, or until a noticeably plastic response is experienced. Do this in several areas, being sure to test any areas of softer soil.

Step 4 – Do the same for the trial working platform area.

Step 5 – Use the results from this testing to optimise the working platform design, based on measured strength values.

Step 6 – After completion of the working platform, test the efficacy of the design through further plate load testing, and evaluation of the recorded deflections and load/displacement response.

Then, once all of this is complete, check the working platform every day for signs of deterioration.

Occasionally, but hopefully not too frequently, you will have to dig through the working platform before the piling is complete. Above all else, make sure that the excavation is backfilled to the same standard as the rest of the working platform – if you build your own soft spots, you will have problems.

At Suoloconsulting we have many years of experience assessing site investigation reports and designing safe, efficient working platforms for construction plant.

Come and speak to us at suoloconsulting@gmail.com

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