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Bilaspur, Chattisgarh:On varying diameter borewells:Construction, Implementation Methods:Advice

I am planning to implement a 6" bore well with submersible pump assembly. Location: Bilaspur , Chattisgarh.

Boring with vehicle mounted machine is not possible due to space limitation.  Accordingly, the boring work is done using diesel operated machine mounted on tripod. They informed me that for a bore of 6", initially a bore larger than 6" has to be  made till rock is found. After getting rock large size pipe will installed and be rested on the rock to support the  earth materials which may dislodge and fall during drilling in rock due to vibration. However after that, onward 6" hole will be drilled and hence 6" pipe will be installed upto required depth till water stream is found.

I have sone doubts regarding the same.

- Please go through the problem at hand and let me know if the correct form of action is being undertaken.

- Does one need to use higher size bore for primary bore,  than the final bore size?

- Do we need pipe after rock is hit?

- Can PVC /  HDPE / HDPP pipes be used for long term purposes in borewells?

- Please outline the right procedure for boring, borewell assembly installation.

G. Singh

singh.g@hotmail.com

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Comments

1.

Dear Singh,

You appear to be receiving lot of correct guidance in your proposal to construct a bore well. But for a well to become a success, it is necessary to make sure of the availability of groundwater. Hope you have taken the correct advice from a geologist/geophysicist to know the subsurface conditions including the thickness of soft formations such as soil, weathered rock and soft disintegrated rock requiring blank casing and the maximum depth up to which water-bearing pipeline structures occur within hard rock thereafter.

You may buy the best quality heavy-duty plastic pipe available at your place as the casing pipe. To install a 6"-diameter casing pipe, it is sufficient to make a 6.5"-diameter bore hole.

Blank casing pipe has to be installed against the soft formations occurring above the hard rock. You may ask your driller to install the casing pipe to as much depth as possible to snugly fit into the hard rock. You may note that once hard rock is met, it is difficult to install casing pipe deep within it.

Good luck.

Dr. R. Jagadiswara Rao
Retired Professor of Geology
Sri Venkateswara University
Tirupati, AP 517502
rjagadiswara@gmail.com

Dr. R. Jagadiswara Rao, Professor of Geology Retired, Sri Venkateswara University, Tirupati, AP 517502, India rjagadiswara@gmail.com

2.

Dear G. Singh,

Usually the pipe is fitted when there is soft material, in hard rock the bore it self create a pipe like hole, so PVC or any kind of pipe is not required, I had some experience in hard rock of Deccan Basalt and never found anyone using pipe in hard rock.

Regards

Vivek Mishra
vivekmishra_2000@yahoo.com

3.

Dear G. Singh,

A larger diameter drilling is essential to install casing pipe of the required diameter (i.e. 6” in the case explained by Mr G. Singh).
As you struck the hard rock formation, the drilling agency will install casing pipe to avoid collapsing of overburden i.e. soft formation (soil cover and weathered formation), casing pipe should be of good quality and strong - PVC/HDPE/HDPP.

You need not to install pipe in the hard rock formation.
In case you are getting a layer of soft formation in between, than there is need to plan for further reduction of diameter by installing the casing pipe of smaller dia than the above layer. There is also need to install perforated pipe at the depth where water bearing formation (aquifer) exist.

It is hoped that the above points will clarify the doubt.

With best wishes

S C Jain
Dy. Programme Coordinator
Action for Food Production (AFPRO)
25/1A Institutional Area,
D Block Pankha Road, Janakpuri
New Delhi -110058

4.

Dear G. Singh,

1. It is advisable to do so. Simply because one is never absolutely sure of what lies below, till you have reached there.

2. Often a pipe is NOT necessary after rock has been hit. But this method is used only if one knows the geology of the area well or there is enough experience with drilling in that area to be sure that there will be no loose formations below the rock. In case a loose formation is struck below the rock then the casing pipe is essential. Here we have two problems.

The start of drilling in the rock is of a smaller size than the upper part of the hole. So if a loose formation is hit below the rock the first option is to "ream" the hole to the larger diameter of the upper part of the hole. Most Indian drillers do not do this and do have the necessary reaming bit.

The second alternative is to use a second casing of a smaller diameter in the rock to get past the loose formation till rock is struck again, or to use well screen, if the loose formation has water and continues for some depth.

Generally what happens is that such holes get abandoned, because the solution is always quite difficult and one needs a "combination rig" for the job. Also, the casing pipe has to be SMALLER that the bore for the casing to go in. A 6" hole CANNOT use a 6" casing, it can use a 5" casing.

3. Yes, but with qualification. The pipe to pipe joints take up space in the well diameter. Therefore, either the well diameter has to be large (an 8" hole will accommodate a 6" casing with bell joints- threaded or glued), or the joints have to be "flush" with no change in diameter (which means the wall thickness of the casing must be high enough to cut threads both on the inside and outside to make a flush joint).

Rigid unleaded UPVC of a minimum of 6 mm wall thickness should be used. HDPE tends to bend and snake in the hole. There is Bureau of Indian Standards specifications for UPVC casing and well screens, but not many people know about it or pay attention to it.

4. There are books on the subject. What you need is an honest driller. And they are rare birds.

Raj Kumar Daw
Consultant, WASH
The United Nations Children's Fund (UNICEF)
Abuja, Nigeria

5.

Dear Mr Singh
The driller has given you the correct advice.

Generally the hard rock is overlain by a veneer of weathered rock mass and soil and its thickness varies from a few meters to even 40 to 50 meters. Hence a Blank pipe of either Mild Steel Pipe of 7 inch dia or PVC pipe of * inch dia and 80 to 120 shedule pipe is used to case the overburden to fall into the well. Once the well is well cased further drilling in the hard rock is resumed with 165mm bit (6.5 inch). More over please dont forget to measure the amount of water coming out of the bore well during washing.

If it is 3 inch in the V notch it amounts to around 2 liter per second which is necessary reqisite for putting a submersible pump of 2 HP capacity. However it depends on the water level in the bore well.

Please note that in hard rock you cant be sure of getting good amount of water if you are going deep. If all dug well options are non fruitful and you can have sufficient money then you take this option.

Nirad Ch Nayak
CGWB(SER), Bhubaneswar

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