2.4. Borings

A boring is defined as a cylindrical hole drilled into the ground for the purposes of:

• investigating sub-surface conditions
• performing field tests
• obtaining soil, rock, or groundwater specimens for testing
  .

Soil samples

Field test (CBR in situ)

Borings can be excavated by hand (e.g., hand auger), although the usual procedure is to use mechanical equipment to excavate the borings

Hand Augers

STABILIZATION METHODS

During the excavation and sampling of the borehole, it is important to prevent caving-in of the

borehole sidewalls. In order to prevent the soil from caving-in there are many stabilization techniques used in practice such as:

• Stabilization with Water.  Boreholes can be filled with water up to or above the estimated level of the groundwater table. This will have the effect of reducing the sloughing of soil caused by water rushing into the borehole. However, water alone cannot prevent caving-in of borings in soft or cohesionless soils or a gradual squeezing-in of a borehole in plastic soils. Uncased boreholes filled with water up to or above the groundwater table can generally be used in rock and for stiff to hard cohesive soils.
• Stabilization with Drilling Fluid. An uncased borehole can often be stabilized by filling it with a properly proportioned drilling fluid, also known as “mud,” which when circulated also removes the ground-up material located at the bottom of the borehole. The stabilization effect of the drilling fluid is due to two effects: (1) the drilling fluid has a higher specific gravity than water alone, and (2) the drilling fluid tends to form a relatively impervious sidewall borehole lining, often referred to as mud-cake, which prevents sloughing of cohesionless soils and decreases the rate of swelling of cohesive soils. Drilling fluid is primarily used with rotary drilling and core boring methods.
• Stabilization with Casing.  The safest and most effective method of preventing caving-in of the borehole is to use a metal casing. Unfortunately, this type of stabilization is rather expensive. The casing is usually driven in place by repeated blows of a drop hammer. It is often impossible to advance the original string of casing when difficult ground conditions or obstructions are encountered. A smaller casing is then inserted through the one in place, and the diameter of the extension of the borehole must be decreased accordingly.

TYPES OF BORINGS

There are many different types of equipment used to excavate borings. Typical types of borings include:

Auger boring.  A mechanical auger is the simplest and fastest method of excavating a boring. Because of these advantages, augers are probably the most common type of equipment used to excavate borings. The hole is excavated through the process of rotating the auger while at the same time applying a downward pressure on the auger to help penetrate the soil or rock. There are basically two types of augers: flight augers and bucket augers.

• Bucket augers

• Hollow-stem flight auger.  A hollow-stem flight auger has a circular hollow core, which allows for sampling down the center of the auger. The hollow-stem auger acts like a casing and allows for sampling in loose or soft soils or when the excavation is below the groundwater table.

Hollow-stem flight auger machine

Hollow-stem flight auger (hollow core)

Wash boring.  A wash boring is advanced by the chopping and twisting action of a light bit (see fig. 2.5) and partly by the jetting of water, which is pumped through the hollow drill rod and bit.       The cuttings are removed from the borehole by the circulating water. Casing is typically required in soft or cohesionless soil, although it is often omitted for stiff to hard cohesive soil.

Wash boring setup

Rotary drilling.  For rotary drilling, the borehole is advanced by the rapid rotation of the drilling

bit that cuts, chips, and grinds the material located at the bottom of the borehole into small particles. In order to remove the small particles, water or drilling fluid is pumped through the drill rods and bit and ultimately up and out of the borehole. 

A drill machine and rig, such as shown in Fig. 2.6, are required to provide the rotary power and downward force required to excavate the boring. 

Percussion drilling.  This type of drilling equipment is often used to penetrate hard rock, for

subsurface exploration or for the purpose of drilling wells. The drill bit works much like a jack-hammer, rising and falling to break-up and crush the rock material. Percussion drilling works best

for rock and will be ineffective for such materials as soft clay and loose saturated sand.

In general, the most economical equipment for borings are truck mounted rigs that can quickly

and economically drill through hard or dense soil. 

It some cases, it is a trial and error process of using different drill rigs to overcome access problems or difficult subsurface conditions. 

For example, one deposit encountered by the author consisted of hard granite boulders surrounded by soft and highly plastic clay. The initial drill rig selected for the project was an auger drill rig, but the

auger could not penetrate through the granite boulders. The next drill rig selected was an air track

rig, which uses a percussion drill bit that easily penetrated through the granite boulders, but the soft

clay plugged up the drill bit and it became stuck in the ground. Over 50 ft (15 m) of drill stem could

not be removed from the ground and it had to be left in place, a very costly experience with diffi-cult drilling conditions.

Some of my other memorable experiences with drilling are as follows:

1.Drilling accidents.  Most experienced drillers handle their equipment safely, but accidents can

happen to anyone. One day, as I observed a drill rig start to excavate the hole, the teeth of the

auger bucket caught on a boulder. The torque of the auger bucket was transferred to the drill rig,

and it flipped over. Fortunately, no one was injured.

2.Underground utilities.  Before drilling, the local utility company, upon request, will locate their

underground utilities by placing ground surface marks that delineate utility alignments. An inci-dent involving a hidden gas line demonstrates that not even utility locators are perfect. On a par-ticularly memorable day, I drove a Shelby tube sampler into a 4 in. (10 cm) diameter pressurized

gas line. The noise of escaping gas was enough to warn of the danger. Fortunately, an experienced

driller knew what to do: turn off the drill rig and call 911.

3.Downhole logging.  As previously mentioned, a common form of subsurface exploration in

southern California is to drill a large-diameter boring, usually 30 in. (0.76 m) in diameter. Then

the geotechnical engineer or engineering geologist descends into the earth to get a close-up view

of soil conditions. On this particular day, several individuals went down the hole and noticed a

small trickle of water in the hole about 20 ft (6 m) down. The sudden and total collapse of the

hole riveted the attention of the workers, especially the geologist who had moments before been

down at the bottom of the hole.