Drilling Caissons Into Hard Rock: How It's Done and Why It's Measured in Feet per Day

Many hillside foundations in Los Angeles end in rock. The geotechnical report says the caissons need a certain embedment into bedrock, and that number drives the whole job. What surprises many owners and builders is how slow honest rock drilling is, and why a budget built on soil production rates falls apart in the first shaft.

Why an auger isn't enough

A standard drilling auger advances well through soil and soft rock. In competent bedrock, it skids and grinds without making real progress. Hard rock has to be cut, not augered.

How rock sockets actually get drilled

1. Core the rock. A coring barrel cuts a ring into the rock, with a lot of water to cool the cutting edge and carry cuttings.
2. Remove the slurry. The water and rock cuttings are pulled out by hydrovac.
3. Break out the core. The barrel leaves a column of rock standing in the middle of the shaft. Where the shaft size and conditions allow safe entry, a worker goes down with a pneumatic breaker to remove it.
4. Repeat until the shaft reaches the depth the engineer requires.

Why it's measured in feet per day

In ultra-hard rock, our crews have seen progress of only a few feet per day per shaft. That is normal for this kind of rock, not a sign of a problem. It also means the number of feet of rock socket is the most important line in any caisson budget. Quantify it first.

Diameter is a schedule lever

A smaller bore removes less rock per foot. A 24-inch shaft has about a third less cross-section than a 30-inch shaft, so it advances faster. If the geotechnical recommendation allows a smaller diameter than the plans show, that difference is worth raising with the Engineer of Record as a question. The design decision stays with the engineer.

Safety inside the shaft

Solid bedrock is self-supporting, so the collapse risk sits mainly in the soil and fill above it, which gets cased. Entering a shaft is still permit-required confined-space work: air monitoring, forced ventilation, an attendant at the top, and a retrieval system and rescue plan. A budget that leaves out that crew structure is not complete.

Water and waste

Wet coring adds water to the spoils. Slurry is a separate waste stream from dry soil and has to be hauled by vacuum truck, so plan for water supply, containment and disposal from the start.

How to budget rock honestly

• Ask the geotechnical engineer for whatever rock data exists, and for the minimum embedment.
• Price soil and rock footage separately.
• Where rock depth is uncertain, use a unit price per foot of rock socket, or drill a test shaft before fixing a lump sum.
• Write down the production rate the budget assumes, so everyone can see what changes if the rock is harder or softer.

For the bigger picture on hillside foundations, read what drives hillside caisson depth and cost.

How we help

We drill caissons on tight hillside and rock sites across Greater Los Angeles with our own rig, hydrovac and crews, and we tell you up front how rock depth affects the schedule. Call (800) 606-3880 or send your plans and geotechnical report.