Drill techinique to go through existing earth dam

[Tatsu]

Greetings,

I am going to drill into/through existing dams to install some instruments (mainly pore pressure meter). I wonder if you can tell me your experience in order to choose appropriate drill method. The dams are earth dams sitting on residual soil/weathered rock. I have to drill into dam core at some locations, and foundation ground through sandy blanket drain at some other locations. The program is undertaken in a country of Africa, and I am trying to comply with US Army Corps of Engineers Regulation No 1110-1-1807 PROCEDURES FOR DRILLIN IN EARTH EMBANKMENTS.

DRILL METHOD:
Drill equipments/techniques available on site are:
a) Tricone:
My driller told me that this is only for soil formation.
b) Percussion:
The driller switches from tricone to percussion for rock formation. I am afraid of that vibration from drill may disturb fill.
c) Diamond Drill (wire line core barrel type)
I guess that diamond drill can be called a core drill. Coring type roller is not preferred by the regulation. It may be because of high rotation rate? I believe that it can advance in soil and rock formations without using inner tube.

Casing should be introduced in any case to protect hole.

I do not have hollow/solid stem auger anywhere near me. The regulation bans using compressed air and water as circulation media. However, the regulation allows using rotary drill if engineered mud can be prepared. Assuming I have the engineered mud, which drill technique does suits for the purpose, tricone(soil)+percussion(rock) or diamond ? As I mentioned, I am afraid of vibration from percussion. Thus I prefer to using diamond drill.

DRILLING FLUID MIX (engineered mud):
The regulation introduces drilling fluid mix as below in case of that rotary drill is employed:
100 gallons of water, and
20 lbs HY-SEAL
20 lbs MICATEX
10 lbs QUICK-GEL
5 lbs JELFLAKE
1 pint CON DET

I don’t think it is possible to obtain all listed additives where I am currently working. I wonder if you can suggest simpler combination of additives or an additive.

Any comment will be much appreciated.

 

[cvg]

I'm not a drilling expert, but I know sonic drilling has been used successfully on dam projects around here. Do a search for "resonant sonic drilling"

 

[GeoPaveTraffic]

You should not use any method in the core of the dam that requires air, drilling mud, or water. The risk is that the core of the dam can be fractured by these fluids, thereby leading to failure of the dam.

Sonic is the prefered method, but my luck with it has only been fair to poor. Other options include hollow stem augers and a bucket auger rig.

You need to be very careful and the value of these instruments needs to be considered with the risk of installing them. A lot of the risk depends on the depth to be drilled and the size of the dam.

 

[oldestguy]

Of the various methods, driving casing and cleaning it out periodically with water and a reverse jet with fish-tail bit is a common method where little water leaves the zone of the work. No downward jet of water is involved. This is time consuming and has pretty much been eliminated from usual jobs, but is a definite advantage if you are going to rock and then core.

Backfill could be a combination of bentonite and cement.

 

[BigH]

Agree with oldestguy that wash boring techniques - casing and washing out is a proven method of advancing boreholes with little to no expectation of collateral damage. Just need to be very careful on how you close down the hole after you are done - to ensure that you leave no "seepage" paths.

 

[Tatsu]

Thank you for the replies.

> cvg
Unfortunately, sonic drill is not available anywhere near me. Lack of tool/personnel is always an issue when working in developing countries.

> GeoPaveTraffic
If I don't find any method convincing myself, I may have to talk to my seniors and client to give up the instruments.

> oldestguy, BigH
Reverse jet would be a great help. My driller has bit which jet circulation media upward (this is bit for reverse circulation rotary drill). I will see if I can use it just for clearing. I guess simple bailer should work as well, it may take forever to drill though.


I wonder if anybody can comment on percussion drill at the foundaiton rock formation, and drill mud for rotary drill.

 

[Tatsu]

wait... the bit for reverse circulation jets circulation media upward inside of dualpipe rod. It cleans up only the inside of dualpipe, instead of cleaning between rod and casing. I have to use a single pipe rod with some mechanism at the bottom of rod in order to jet circulation media upward.

 

[BigH]

In the old days we had a chopping bit that went on the end of an "A" rod that squirted the water out sideways - not down/not up. Casing would be driven to a specific depth and the chopping bit would be sent down and with up and down actions of the cat-rope, the bit would be raised and dropped (thereby "chopping") - the material would then be forced to the surface by the squirting out water - of course, heavy gravel sizes are a problem. I would suggest you look at Terzaghi Peck & Mesri, Article 11 on Methods of Exploration (see Figures 11.1 and 11.2). Article 11.2.2 explains Wash Borings. Article 11.2.3 explains Rotary Drilling.

 

[dgillette]

Do you really need the instruments? That is, do you have identified failure modes that the instruments can alert you of? Dr. Peck once lectured us about the folly of drilling holes in a well-functioning dam to put in instruments we don't absolutely need. Leave well enough alone is what he said.

That said, absolutely no air or foam ANYWHERE in a dam (because they can store a lot of energy). Mud and water are often used without problems, but you need to be very vigilant about pressures and fluid loss, or plugging of the annulus around the rod. Location in the dam matters - don't drill where there are high gradients, because that's where there is the greatest potential for harm.

DRG

 

[GeoPaveTraffic]

Tatsu,

As has been stated above, do not use air drilling methods anywhere in an existing dam. The issue is, if the hole becomes blocked, just for a second, the air pressure will jump and you will cause fracturing of the dam. Depending on the location and amount of fracturing, this can lead to loss of the dam.

As dgillette, pointed out, look closly at why the instruments are being installed and determine if they are REALLY needed.

 

[oldestguy]

In using the wash boring technique and casing, my experience has found that using old fashioned extra heavy pipe (Sked 80?) with couplings drives and pulls easier than flush coupled casing.

Rock coring with diamond bits requires clean water. I'd never resort to percussion drilling in the typical bore holes used for exploration and instrument installation.

 

[Tatsu]

> BigH, dgillette, GeoPaveTraffic, oldestguy

Thank you for your comments. I will study/investigate a little bit more. I often end up doing geotech drill with exploration drillers and equipments. It is sometime difficult to explain to them what I am trying to achieve.

Have great holidays.