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Sandy layer at 250 ft depth: Some questions from Bangalore!

Recently a borewell was drilled at my site in Bangalore, depth 252 feet. It was dug by Truck mounted pneumatic drilling m/c size 61/2”.  Output of water was good but during the last leg of drilling the driller informed that sand is coming out and it would be good if they stop drilling as sand would collapse and close the already drilled bore. 

I would like to know :

 1) If sand was present at the last leg is their any chance of borewell collapsing? Any preventive methods that can be implemented?

2) How would is sand present at that point so deep nearly 250 feet?

3)Is the presence of sand a natural phenomenon ?

4) How would the water table conditions be down under the sand layer?

N.L.N.Prasad

hintexbgl@hotmail.com

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Comments

1.

Dear N.L.N.Prasad,

It’s good that driller had stopped drilling further. I think the driller reached this sandy layer after pinching out a hard clay layer and sand was coming out below the clay roof. Then its better not to put the strainer and keep open at the bottom of well so that It would behave as a Cavity type Well because cavity would be formed in the water bearing stratum at the bottom of clay roof during drawing of water from the bottom of the well and water from the aquifer will enter the well pipe through this cavity in a spherical pattern of flow. After continuous pumping, the spherical area of the cavity will increase outwards with the result that the velocity of flow will reduce and consequently, the sand particles will stop entering the well pipe. Hence in the beginning sandy water will be obtained from this cavity type tube well but with the passage of time clear water will come out.

Development of cavity well:

A centrifugal pump is usually better suited for developing a cavity well than an air compressor. The cavity should be developed slowly and with great care keeping low discharge. Otherwise, sand will likely to rush into the casing and choke it. When discharging water become clear, the depression may be increase slightly, which may result in further sand being drawn out. This process will be repeated till the sand-free discharge would obtain. The pumping may then be stopped for an hour and then resumed. The discharge after restarting may again contain sand. The pumping may be continued till the clear water would obtain. The procedure may be repeated till the well would be developed fully.

Click here for a detailed illustration
for a detailed illustration

Thank you

S.Halder
Assistant Engineer (Agri- Irri)
Water Investigation & Development Department
Government of West Bengal

2.

Dear N.L.N.Prasad,

As explained in the earlier correspondence, when the pink colored granite is fissured and highly crushed it changes into a formation similar to that of a river sand with pinkish and yellowish tinches, that are due to pyraxinites, felspars and quarts minerals.

If more quartz we get, then the coming out material will have more silica and if the silica has 80% of silica itself and the ton is as costly as Rs 2000.

You can mine it just blowing it out and drying it.
Coming to your point what ever you do the yield will be less and I might yield only for a hand pump with flow of nice sand while pumping.

Alternatively, if we do not have any place in the vicinity beyond 50 feet, the width of the sheared or highly crushed zone has to be verified by geophysical studies, now a day you can do the resistivity imaging, similar to that of Heart scanning. It is available with NEERI and other institutes and I very much doubt weather it is available in Bangalore. NGRI, Hyderabad even does not have one.
You have to tackle the problem in an igneous way like the Gujarat and Rajasthan farmer do.

You drill bore well and blow out the silica rich sand like nice particle. This will come out with moisture and some time with flow of water. You have to blow out a maximum of about a truck load of the particles or more and keep on blowing air until then you do not get any sand.

Before you start the operation you should have 5mm dia or 10 mm dia pebbles readily near the bore well.
When you have pumped out the nice sand like crushed particle from the bore, you stop the drilling and blowing of air and pour the pebbles inside it while slowly the rig is rotating but the air not coming out.

This can be done by many drillers who have drilled in Gujarat, U.P and Rajasthan.
You should replace double the amount or just near to the volume of the sand or silt you removed from the borewell.

Once you do this now collapsing of bore will not takes place and due to hydrostatic pressure the ground water will come to the static water level and the yield will be more than double the yield of the normal bore wells.

If the surrounding bore wells are yielding for 5 H.P and your bore well will yield for 10 to 15 H.P.

Please do not do this if you get clay like silt or very nice black or white colored powder comes out from the bore well and you will get only, hand pump supply.
If you get coarse grained sand gravel like material communing out only then you will get great yield.

This is for your kind information and best practice.

Your quarries are really challenging and I am very happy to part with my knowledge and information at any time.

With regards,

A.Rajamohamed Ambalam
Geophysicist

3.

Bangalore is a hard rock terain and there is no possibility of sand as such at depths of 250 ft. Possibility is that bore hole encountered soft formation at that depth and has a tendency to back fill the bore hole.

It is quite natural to encounter such formations at depths. The chances of bore hole collapsing depends on the rock formations above 250 ft. If the soft formation was met only at 250 ft and drilling stopped immediately it is not likely to cave in.

However if it gets caved/ filled up to the extent of choking the water source, you can flush the well with air compressors, clean it of the soft material, and lower a smaller dia pipe with perforations into it (3" or 4" dia appropriate for the bore well) and the well can be used again.

Drilling rigs crew usually are aware of this method. But make sure that there is actually a case of soft material as they have reported.

M R RAMESH
HYDROGEOLOGIST

4.

Highly fractured Granite or brecciated Granite yields sand like material even at greater depths. Better if you measure the depth to the bottom of the bore well.

Let us assume that the borewell got filled upto 200 feet and the water level is at a depth of, say, 100 feet below ground level. What you can do is to fit a 7.5 HP submersible pump at around 175 feet and test the well for quantifying the yield or the driller may pump the well using the compressor and a V-notch.

If the yield is sufficient for your purpose, you can start using the well. There is no use in drilling further after a collapsible formation is encountered.

With best regards,

C. Udaya Shankar

Geologist

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