Hello I'm Professor Kitch.
Welcome to my series of geotechnical engineering webcasts.
This web cast is an introduction to drilling and sampling.
The purpose of this presentation is to provide an overview of common soil drilling and sampling
techniques.
It is intended to supplement presentations using photographs or videos or drilling and
sampling operations.
While photos and videos of drilling and sample operations can show you equipment being used,
they can't show you what is going on below ground.
This presentation uses animations of drilling and sampling operations to demonstrate what
happens below the ground.
At the end of this video, I'll provide some links to other videos that show actually drilling
and sampling being done in the field.
The presentation will cover common types of drill rigs
The four most common drilling techniques Solid stem auger-open hole method
Hollow stem auger method Wash boring method
the casing method It will also cover the basics of pushed and
driven samplers There are a number of types of drill rigs
available.
Some of the most common are Large truck mounted rigs, sometimes called
a highway rigs Off road track mounted rigs
Trailer mounted portable rigs Different rigs are suited for different site
conditions and different drilling and sampling methods
The large truck mounted rig, is the most versatile and one of the most common rigs used.
However, it can only be used in situations where it can be driven onto the site.
The process of drilling is similar regardless of the type of rig used.
First we positioned the rig over the location where we wish to drill.
Then we erect the drilling boom, or mast, and make sure it is vertical.
A drill head mounted on the boom is the mechanism used to perform the drilling and sampling
operation.
The drill head provided two motions to aid in drilling.
A torque can be applied to rotate drill bit.
And a vertical thrust can be applied to raise and lower drill bits and other equipment.
There are many different drilling and sampling techniques we can use to investigate subsurface
conditions.
One of the simplest is the open hole method using a solid stem continuous flight auger.
The flights on a continuous flight auger run the full length of the auger.
At the tip of the auger is a cutting bit.
The inset photo show two different diameter solid stem augers with two different types
of cutting bits.
The auger is rotated and pushed into the ground by the drill head.
The bit at the tip of the auger loosens soil at the bottom of the hole and the flights
carry the cuttings to the ground surface.
An engineer or geologist can estimate when new soil layers are encountered by observing
changes in the cuttings as they are brought to the surface.
These cuttings can be collected as bulk samples.
However, they are low quality samples because they are completely disturbed by the cutting
process and because soil from upper layers can contaminate soil from lower layers as
the cuttings are carried up the auger flights.
When the boring reaches a depth where a higher quality sample is desired,
the auger is removed from the hole to make room for the sampling equipment.
In the this method the bore hole must be able to stand open without support, which is why
we call it the open hole method.
When we wish to take a sample, we replace the auger bit with a sampling tool,
or sampler.
Nearly all soil samplers consist of some sort a hollow tube which is inserted into the soil
to retrieve a sample.
There are many different designs of samplers to suit different soil conditions, but the
basic process of sampling is similar for most of these samplers.
The sample tool is lowered to the bottom of the boring.
And then pushed into the soil below the bottom of the boring.
When the sampler has reached its full depth, the rods are twisted to shear off the bottom
of the sample, and the sampler containing the soil sample
is retrieved from the boring.
The sample is then stored for later analysis and testing.
The sampling tool is then replaced with the auger
and the auger is lowered back down the hole.
Augering continues until we reach a new layer where we wish to take another sample.
The auger is again retracted from the hole, and replaced with the sampling tool.
Another sample is taken using the same process.
When the soil is very stiff or dense, it is not possible to use a push-type sampler.
In these circumstances we use a heavy walled sampler that can be driven into the soil using
a drop hammer.
The process is similar to retrieving a push type sample, but uses different equipment
and a different sampler.
The open hole method only works when the soil is stiff enough and strong enough to remain
open after the augers are removed.
While it is often possible to use the open hole method, many times the soil is not able
to maintain an open hole for sampling.
If the soil is soft, it may squeeze into the boring when the augers are removed.
This often happens is soft clay layer.
If the soil is a clean sand with little or no cohesion, the soil at the side of the hole
may cave or fall into the boring.
This is particular problem when drilling in sands below the water table.
When squeezing or caving conditions occur we must use a drilling method that will support
the bore hole during the sampling process.
One method that can be used in caving or squeezing conditions it the hollow stem auger method.
In this method, we use a continuous flight auger as the open hole method, but in this
case the auger stem is hollow.
The bit in the center of the auger is retractable and can be removed through the hollow stem.
The inset photo shows a hollow stem auger and its retractable center bit.
For illustration purposes the center bit has been removed and is shown next to the hollow
stem auger.
During drilling the center bit is inserted inside the hollow stem.
The augering proceeds in the same fashion as with the open hole method.
When we reach a depth at which we wish to take a sample, we stop the auger,
detach the center drill bit from the auger and remove it.
The auger remains in the ground and supports the hole.
We can now acquire a sample through the open hollow stem.
The sampling process is the same as we showed before.
After sampling the center drill bit is replace and auguring continues.
The rotary wash boring is another common drilling method.
Instead of using an auger to remove cutting from the hole,
this method uses a slurry or drilling mud to flush cuttings out of the hole.
The drilling mud or slurry is a mixture of either bentonite and water,
or a polymer and water.
The purpose of the bentonite or polymer is to increase the viscosity of the slurry which
makes it easier for the slurry to carry the cuttings to the surface.
Special equipment is needed to handle the drilling mud including a
holding tank a pump
and a swivel head.
The pump circulates the drilling mud from the holding tank through the swivel head into
the top of the drilling rod and then down the hollow inside of the drill rod toward
the toward the cutting bit.
The boring is advanced using a drill bit at the bottom of the hole
The drill bit or cutting bit is turned by the drilling head an loosens soil at the bottom
of the hole The drill bit contains ports that allow the
drilling mud being pumped down the hollow drill stem to exit out the side of the bit
into the annular space outside the drill rod.
The mud flows up the annular space and carries cuttings away from the bit and out the top
of the hole The cuttings settle out of the drilling mud
in the holding tank and the clean mud is recirculated back down the drill rod.
Driving the drill bit of a rotary wash boring takes much less energy than driving a long
continuous flight auger.
Therefore rotary wash borings can be advanced very quickly and reach depths much greater
than auger borings.
In addition to flushing cuttings from the boring, the drilling mud also provides a hydrostatic
pressure along the sides of the drill hole.
This can help to stabilize the hole and allow drilling in moderate caving conditions.
When we reach our desired sampling depth, drilling is stopped and all of the cuttings
are flushed out of the boring.
The pump is then shutdown, the hose disconnected from the swivel, and the drill rod removed
from the hole.
The drilling equipment is replaced with sampling equipment
and a sample is taken directly through the drilling mud.
Another method that can be used to advance a boring in caving or squeezing conditions
is the casing method.
In this method a large diameter pipe is driven into the ground below the depth of the boring
. The hole is then cleaned out using either
the auger method or rotary wash method The casing supports the hole and samples can
be taken in the undisturbed soil below the casing
So let's summarize.
There are many type of drilling and sampling methods.
In this presentation we have presented a few of the most common methods: the solid stem
auger method with an open hole, the hollow stem auger method, and the rotary wash boring
which can be used with or without a casing.
Each method has it pros and cons.
There is no one best method.
The drilling method selected will depend on a number of factors including.
Location and accessibility of the site.
Type of project and its objectives.
Soil conditions Samplers can either be driven in with a drop
hammer of pushed into the soil.
Here are some references and links to drilling and sampling videos online.