Core Drilling – Site Investigation

Geotechnical Drilling Rigs: How to Choose One for Soil Investigation

By Windrilling Technical Team – Updated September 2026 – 11 min read

A consulting engineer sent me an investigation programme in January that had already been drilled once. Forty boreholes. Standard penetration tests every 1.5 m. Samples to the lab. The lab rejected a third of them – not because the testing was wrong, but because the samples had come up disturbed and the logged depths did not match the ground.

The rig that drilled those holes was quick. It finished the programme eleven days early.

And that is the whole problem with how geotechnical drilling rigs get bought.

I work on the supply side, so take the bias as read. But I have watched enough investigation jobs go sideways to be certain of one thing: an investigation rig is not judged by metres per day. It is judged by whether the data that comes out of the hole can be used in a design.

This is the order I work through it in, with a borehole schedule in front of you and one machine to pick.

What a Geotechnical Rig Is Actually For

Three jobs. Nothing else matters until those three are covered.

• Hold the borehole open at the right diameter, down to the depth where the test or the sample happens.
• Put the sampler in the right place – exactly the horizon the drawings ask for, with material that has not been dragged into the hole.
• Deliver rotation and energy in a controlled way, so the number written on the log means something six months later.

A machine that does those three things slowly is a better investigation rig than a fast machine that does them approximately. That is not a slogan, it is the arithmetic of re-drilling a hole you already paid for.

The Three Investigation Methods, and the Rig Each One Needs

Most soil investigation on a site with structures, roads or mines uses one of three methods. The method is normally written into the specification before anyone picks a machine, which is the correct order of events.

Method	Ground it suits	What the rig has to deliver
Hollow stem auger (HSA)	Sands, silts, clays and soft gravels above the water table	High torque at low rotation speed, an open centre to drop a sampling rod through, and steady depth control. The auger is the casing, so the rig must be able to keep it turning while you sample inside it
Rotary wash boring with casing	Soft soils and gravels, and anything below the water table	A mud pump you can regulate properly, casing handling, and a hole that can be cleaned back to test depth without the bottom filling up
Wireline coring	Rock, and competent soils where a continuous record is required	A wireline hoist, low-RPM rotation with smooth feed, flush control, and a mast stroke matched to the core barrel

You will also meet sonic drilling, covered by ASTM D6914, which takes continuous samples through mixed ground very efficiently. It solves the same problem with a different machine and a very different budget, so treat it as a separate decision rather than an upgrade.

Where a DTH rig fits into an investigation site

Not in the sampling line – but several jobs on the same site are not sampling jobs at all. Slope anchors, rock bolts, drainage holes in a cutting, foundation sockets. Those are drilled by anchor and DTH machines, and on a stabilisation contract they often run alongside the investigation. If your site needs both, that is worth planning together: the mast on a machine such as the WD-ZJ516 full hydraulic DTH rig handles vertical and horizontal drilling, which is what anchor work on a face actually requires.

SPT Is Where Most Rig Choices Go Wrong

The standard penetration test is the most common in-situ test in the world, and also the easiest one to ruin with the wrong machine.

ASTM D1586 sets out the mechanics. A split-spoon sampler 51 mm in outside diameter, driven by a 63.5 kg (140 lb) hammer with a 760 mm (30 in) free fall. The first 150 mm is seating. The blows needed for the next 300 mm are the N-value.

Read that back and notice how much of it the rig controls.

Rig-side factor	What breaks when it is wrong	How to check it
Hole condition at test depth	A collapsed wall or loose cuttings leave the sampler driving into fill, not formation. The blow count records the borehole, not the ground	Sound and clean the hole to exact depth before every test. Do not test on a bottom you have not cleaned
Depth control	Logging the test at the wrong horizon corrupts the profile quietly – nobody sees it until the foundation design is questioned	Mark the rods. Check with a tape, not with the driller’s count
Hammer system	Hammer types put different energy into the rod string. Mixing them mid-programme makes the numbers incomparable	Fix one hammer type for the whole job and keep its calibration record with the programme
Rod string and anvil	Whip, loose joints and a poor anvil seat absorb energy before it reaches the sampler	Keep the string straight, joints tight, anvil seating checked at the start of each shift
Verticality	A leaning hole puts the test somewhere other than where the drawing says it is	Correct or ream early. Do not chase a hole that has already drifted

None of this is exotic engineering. All of it is easier on a rig with fine feed control and a proper rod holder than on a machine built to make hole quickly.

Sample Quality, and the Systems That Protect It

ISO 22475-1 deals with sampling methods and groundwater measurement, and ties the method to the sample quality a design actually needs. The practical translation is short. If the specification asks for samples that can be tested for strength or compressibility, then the drilling method has to be capable of delivering that class of sample. A rig that can only wash-bore cannot, however careful the crew is.

For rock and competent soils that usually means wireline coring. The barrel sizes are standardised, and they decide the machine:

System	Hole diameter	Core diameter	Where it is normally used
NQ	75.7 mm	47.6 mm	The workhorse size. Deeper holes, standard exploration, the default when the specification does not say otherwise
HQ	96.0 mm	63.5 mm	Larger core, better recovery in fractured or soft rock, more material for testing
PQ	122.6 mm	85.0 mm	Shallow holes where sample volume matters, and where the lab needs more than a sliver of rock

Sizes are quoted the same way across public core barrel catalogues, so they are easy to check before you argue about a tender.

Bigger core means more torque, more flush and heavier rod strings. That is what moves you up a machine class – not the depth figure printed on the brochure.

Depth, Hole Size and the Machine Class That Follows

Three numbers in the specification decide the rig before anything else: the deepest borehole, the largest hole diameter at that depth, and the sampling system required there.

For a typical structure, road or dam investigation – boreholes in the tens of metres, mixed soil and rock, NQ or HQ coring once you are in rock – a compact crawler-mounted wireline core rig is the usual answer. That is the bracket our WD-C400 full hydraulic wireline core drilling rig sits in, with the WD-C series going up from there by depth rating. How those depth ratings are written, and how they set the price you actually pay, is covered in our guide to a core drilling rig for sale. If you are new to the method itself, the wireline core drilling field guide explains rods, inner barrels and overshots from the bottom up.

Access Is Usually What Decides the Machine

Nobody buys a compact rig because they like compact rigs. They buy one because the site will not take anything else.

Investigation boreholes are rarely in open fields. They are in road cuttings, on embankments, in factory compounds, between buildings, or on top of a slope someone is trying to stabilise. A machine that cannot reach the location has no value that day, whatever its depth rating says. The comparison between crawler and truck mounted carriers is worth reading even if your work is investigation rather than water wells – the transport logic is the same.

What decides it in practice: track width and turning circle, mast height when travelling under cables and pipework, ground bearing pressure on soft ground, and whether the machine can set up on a slope without a crane. Walk the route to the last borehole before you sign. The first ten boreholes usually sit somewhere easy, and the awkward one at the end is the one that decides whether the contract made money.

Flush, Borehole Stability and the Samples You Lose to Both

A geotechnical borehole is a temporary structure, and it behaves like one. Get the flush programme wrong and the wall sloughs, or the cuttings settle at the bottom just before the test. Either way the sample or the blow count is gone, and you find out after the lab has been paid.

Treat the pump as part of the investigation tool rather than a support item – flow rate, pressure and the mud you choose all show up in the data. The sizing logic is the same one we set out for mud pumps in borehole work. Two habits save a lot of holes: never test on a bottom you have not cleaned, and case the hole through the loose interval instead of hoping it holds until you get there.

What a Soil Investigation Rig Costs

Public listing data is the only honest benchmark I can give you without knowing your programme. Looking at B2B marketplace listings in September 2026: entry-level SPT and geotechnical exploration rigs are advertised from about US$13,880-16,880 for lower-capacity models, a 1000-model portable SPT rig is listed at US$44,000-64,000 FOB, and an 800 m hydraulic rotary rig with SPT capability is advertised around US$22,150. Those are other suppliers’ listing prices, not ours, and the size of the spread tells you more than any single figure.

What moves the price	Why it matters on a bid
Depth rating and hoist capacity	A deeper rating is mostly a heavier hoist, a longer mast and a stronger rod string. If the specification says 60 m and you buy 300 m of capacity, you paid for steel you will never use
Sample systems supported	NQ only is cheaper than a rig set up for HQ and PQ. The specification decides this, not the sales sheet
Feed and rotation control	Variable, smooth feed and low-RPM torque are what protect samples. They are also the parts that are quietly deleted from a cheap machine
Rod handling and mast stroke	Handling rods by hand is slower, harder on the crew and worse for hole condition. A stroke matched to the core barrel saves a trip every run
Tooling in the package	Core barrels, diamond bits, reaming shells, casing and shoes are a large share of a small rig’s invoice. Buyers routinely forget this and are surprised at the total

That last row is where most investigation budgets go wrong. The consumables have to match the rock, and they have to come from somewhere you can reorder from. We list them individually so you can price them against your programme: diamond core bits, diamond reaming shells, casing shoes, and the rest of the drilling tools. Send us the borehole schedule and the ground conditions and you will get a real number. A price without those two things is a guess dressed as a quote.

Nine Checks Before You Sign

1. Can it deliver the sampling class the specification asks for? Write the answer down. If the rig cannot do it, no amount of crew skill fixes it.
2. Is the feed infinitely variable, and can it hold a slow rotation under load? This is the difference between a sample and a smear.
3. Does the mast stroke match the core barrel? A mismatch costs you a trip on every run for the life of the machine.
4. Is there a rod holder and a breakout system? Manual rod handling is where good hole condition goes to die.
5. How does the machine clean a hole to test depth? Ask for the procedure, not just the pump flow figure.
6. Which hammer system comes with it, and is it calibrated? Fix one type for the programme and keep the record with the logs.
7. What are the travel dimensions and the ground pressure? Check them against the site, including the last borehole on the list.
8. Can the same supplier support casing, barrels, bits and rods? Mixed-supply tooling is the most common reason a hole gets abandoned halfway.
9. What is in the delivery inspection? Get the function test, the paperwork and the shipping photos before the balance is paid, not after.

FAQ

Can I use a water well drilling rig for soil investigation?

Sometimes, for shallow holes in easy ground, and plenty of contractors do it. The limit is control. Water well machines are built to make hole and hold it long enough to set casing, not to stop at 1.5 m intervals and leave the bottom clean enough for a valid test. If the programme includes SPT work or undisturbed sampling, check that the feed control and the cleaning method can actually deliver it before you hire the machine you already own.

Do I need wireline coring for every investigation?

No. If the job is soil boreholes with SPT and disturbed samples, an auger or rotary rig is the right machine and a coring rig is money you did not need to spend. Wireline coring earns its place where rock coring is specified, where rockhead has to be proven, or where a continuous record through a mixed profile is required.

What size rig do I need for a typical investigation programme?

Start from the deepest hole and the largest diameter at that depth, then add the sampling system. Most structure and road investigations land on a compact crawler core rig with NQ or HQ capability. If the programme also needs slope anchors or foundation sockets, that is a second machine or a machine specified for both jobs, and it is worth saying so early rather than discovering it on site.

Does a hire rig ever beat buying one?

For a single short programme, usually yes – if you can get the right machine. The problem is availability and condition. Hiring gives you no control over how the last crew treated the rod string and the core barrels, and a worn barrel with poor feed control is exactly how samples come up disturbed. If you hire, inspect the tooling, not just the rig.

Before You Send the Tender Out

Send us the borehole schedule, the depth and diameter at each hole, the sampling and testing requirements, the ground conditions you expect, and the access constraints on site. We will tell you which machine fits and, where it matters, which part of the specification is asking for something the budget cannot buy. If a water well rig will genuinely do the job, we will say that too – a machine that fails your first programme costs us a repeat customer, not one sale.