Installing Bored Piles in London Clay — Site Practice
What the designer's calc actually looks like when you're standing in the box at 3am. Bentonite slurry management, when the bucket isn't lifting clean, why the head matters, what to do when the pile sat open overnight. The site agent's view of every assumption the designer made — and the ones you can't make on Friday afternoon when the contractor wants a call.
Tom Mearns
EngTech MICE · 18 yrs · Senior Site Agent, Ferrovial Construction
The design has its first principles — the physics. So does delivery. These are the three a good practitioner carries onto every job; get them in your head and the rest becomes easy. The read from Tom Mearns on this topic:
the basics that make everything else easy
Know your water table, your bentonite and your overbreak before anything goes in the ground. Get the slurry discipline right and the bore looks after itself — get it wrong and nothing downstream saves you.
the order of operations — what comes before what
Casing, bore, clean, cage, tremie concrete — in that order, no shortcuts. The pour follows the dig within the hour; you don't bore a hole Friday and concrete it Monday. People skip the clean-out because it's the boring bit — that's the one that fails the integrity test.
what to look ahead for — anticipate, don't react
Read the arisings two metres ahead of where you are. Sandy lenses in the clay mean you ring the designer now, not at the load test. Have the next cage and the tremie ready before the bore is open — an open bore is a clock running.
On site — the practice points
- 1
Bentonite slurry head discipline
Keep slurry head 1.5m above water table — never less. The day you cheat this is the day the bore collapses. Document head at every 2m of advance.
- 2
QC observation during boring
Watch what comes up in the bucket. Sandy lenses in stiff clay = designer needs to know NOW. Don't wait for the integrity test.
- 3
When to call the designer
Anything sitting overnight. Any collapse during boring. Any loss of bentonite head. The pile load test will tell you 6 weeks later. The designer needs to know today.
Site walkthrough video
Installing Bored Piles in London Clay — Site Practice
Mock — live video activates when content arrives
The single thing new starters struggle with most: turning a drawing into work on site. This is how Tom Mearns reads the pile schedule and the geotechnical design calc — shaft friction in London Clay:
On the drawing
α = 0.45, 18 m embedment
What it means on site
the designer is trusting the clay's shear strength along the whole shaft — so your job is shaft cleanliness and slurry head, not just hitting depth. A dirty shaft throws away the friction the calc assumed.
On the drawing
s_u rising with depth
What it means on site
the deep piles carry more per metre than the shallow ones — don't treat every pile the same; the deeper bores are the ones to baby.
On the drawing
no negative skin friction noted
What it means on site
the calc assumes the made ground isn't dragging the pile down. If you are boring through recent fill that's still settling, that assumption may not hold — flag it before you pour.
This is the delivery half.
Same topic, two credential chains. The chartered engineer sets the intent; here's how it's made real on site.
Delivery knowledge is verified by a Master-Trade or chartered validator and passes the AI pre-publication check. Codes are the floor; judgement is the value.