ags ground risk: why take the chance?€¦ · ags ground risk: why take the chance? ground risk –...

AGS Ground Risk: why take the chance? Ground Risk – Where do I start? Dr Jacqueline Skipper Senior Partner, Senior Geologist Geotechnical Consulting Group

Ground Risk: why take the chance? A lessons learnt conference

• Where does ground risk begin in engineering projects? • “In the ground, obviously!” – but does it?

We live on a planet that is

• Tectonically active • Gravity • Water • Atmosphere and

weather

We live on a planet that has

• Plate tectonics • Gravity • Water • Atmosphere and

weather

We live on a planet that has

• Plate tectonics • Gravity • Water • Atmosphere and

weather

We live on a planet that has

• Plate tectonics • Gravity • Water • Atmosphere and

weather and climates

We live on a planet that has

• Plate tectonics • Gravity • Water • Atmosphere and

weather • Humans and what we

do to the planet –

This means that the ground is VARIABLE

• If it wasn’t, we wouldn’t be here today • But it is, so we have a primary risk that the ground will vary

Example - sediments Variable sediments can be • confusing • weaker • stronger • less or more cemented • often less stable • more permeable/porous and • at different levels,

– to what was expected (from the information available/provided)

Ground risk based on depositional environment

Fairly low ground

risk

Medium ground

risk

Fairly low ground

risk

Medium to high ground

risk

High ground

risk

Sediments deposited in these areas are likely to be:

Variable sediments • Have a wider range of properties,

and perversely, are:

• More difficult to recover

• More difficult to obtain data for

• More difficult to correlate

• More challenging to design for/construct in

• Thereby increase project ground RISK

Also, these secondary mechanisms increase complexity, often superimposing on one another

1. Environments of deposition 2. Post depositional changes (chemical, biological,

physical weathering) 3. Tectonic changes e.g. faulting, uplift, 4. Quaternary big changes e.g. T↓↑, SL ↓↑

erosion, slope failures 5. Anthropocene effects e.g. mining, construction,

contamination

Ground risk is very expensive. Why do we need to keep having to say this?

• Failing to anticipate ground conditions is a major factor in construction problems Fookes 1997

• 50% of project delays caused by adverse ground conditions Chapman & Marcetteau 2004

• Ground risk = 50% reduction in return on client’s equity Chapman 2006

• 80-85% of (European) building failures are related to ground problems Brandl, 2004

And, even with best EC7 practice

• Even with best borehole spacings/depths, recovery represents 0.008% of the ground

• This is for 100% recovery, rotary boreholes • This % drops with

– wider spacings (e.g. linear infrastructure 0.003%) – other drilling techniques – variable ground conditions/poor recovery

• GIs are frequently not a 100% accurate representation – even in the precise locations in which they are carried out

Despite this, thorough ground investigations are a good idea!

• GIs can significantly reduce ground risk uncertainty • But only 0.2% to 0.5% construction costs are spent on GI! • Why is this?

What blocks our understanding of ground variability and risks?

And is therefore invisible

Here’s a couple of examples of preconceptions…

Most of the ground is

The geology

The Engineer* thinks

Shiny challenging thing, pays our wages!

Perplexing challenging geology to build things on/through.. “It’s all London Clay anyway”

* Very smart engineers who understand geology in detail are also available

The Geologist thinks

Thing being built. Who cares what it is, it pays wages?

Wow! Opportunity to study the geology!

What else affects perception?

Different contract forms + different preconceptions

The Experienced Client thinks

“Shiny thing make money, but maybe a GBR would serve us best?”

“We’ve been stung before by these ground conditions, let’s make sure we get a good ground investigation”

The less experienced Client perhaps thinks

Shiny building make money!

• Don’t understand the ground- pass the risk to the contractor.

• We don’t have time for an extensive GI

• Savings can be made on the ground investigation!

What if the contractor carries the risk?

• ‘We assumed favourable ground conditions to price cheaply and win the work’

• ‘We don’t want to pay for more GI – the client has done a basic Factual Report’

• ‘We can always claim unforeseen ground conditions’

Can this lead to the perfect geotechnical storm? Cheapest ground investigation tender, short deadline Negligible Desk Study (‘what was the exam question?’) Inappropriate investigative methods Poor recovery Unrepresentative sampling Poor logging conditions! Poor descriptions Poor sample care

→ Unrepresentative lab test results • Unsuitable design for ground conditions • Construction problems due to ground conditions • Cost to client – overruns, lawyers etc £££

Sadly

• It is often not until things go wrong that the risk bearer is prepared to get an appropriate GI

Ground data loss = increased ground risk

????

Designer

?????

Contractor Logger

??

Lawyer

!!!!!

Driller

?

Lab

???

And ground risk in the contract comes first

Contract

Unforeseen ground - a landmark case …

“Every experienced contractor knows that ground investigations can only be 100% accurate in the precise locations in which they are carried out. It is for an experienced contractor to fill in the gaps and take an informed decision as to what the likely conditions would be overall”. Mr Justice Coulson, Van Oord UK Limited and SICIM Roadbridge Limited v Allseas UK Limited [2015]

In other words

• If you carry the risk, • You can’t plead ignorance • You must do additional GI if the original was not appropriate

for the probable variability of the ground • Keep up with the current state of knowledge!

Faulting bringing Lambeth Group to near surface position, UCLH Cancer Centre

Two examples of already complex ground (Lambeth Group+ London Clay) related to faulting in London

What happened?

• Faulting only discovered coincidentally • Minor pile failures managed between designer and contractor • Historical data showed previous adjacent site adversely

affected by faulting + sand channels - open bored piles affected • Short paper in Ground Engineering to publicise • Other sites in the area are also affected but lessons have been

learnt

The site is within a zone of unusual ground conditions with complex faulting of London Clay and sand channels within the Lambeth Group

Strike-slip structural ground model.

Explains complex faulted areas of

ground in localised areas

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Site with a ‘silt filled hollow’ and a river across the site

What happened?

• Many previous phases of GI, desk studies • ‘Lost Rivers’ and ‘Silt Pocket’ raised suspicions – faulting? • Reassessment of GIs found evidence of faults • Communication with client and designers about potential risks

led to improved GI • GI – weakened ground, faulting ++ • Discovery of a number of issues for piling

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Complex faulting, silt hollow related to faulting

MOVE structural geological modelling © R. Ghail, Imperial College.

Failure to recognise how others see the ground can itself constitute a risk

• If we don’t understand professional differences, information or advice can be misunderstood or ignored

• Conversely, trying too hard to please (e.g. a client) can cause us to lose trust in our own professional judgement.

What lessons can be learnt? • Relevant competence is essential from the beginning of a project to address ground

risk • Be wary of developers/clients and be aware of who controls risk • Give clients direct and honest, recorded advice, being aware of differing priorities • Do not adopt GI reports without question – do additional GI if necessary • Be wary of how design responsibility slides down the contract chain • Ensure time resources are adequate • Clarify roles and responsibilities • Trust your own professional opinion and STOP when necessary • Communicate, publish – pass on the ground knowledge!