Dear Suresh,

Did you conduct the SPT test or geophysical test during the geotechnical investigation stage, you would able to identify the type of soil or rock based on 'N' value. As i understood from your statement that the rock is very soft and difficult to identify the rock top level. The rock top level can be identified based on the core samples collected during the site investigation, seems it is weathered rock. The weathered rock is usually very soft in nature. The top bed rock depth can be identified by studying the samples collected during the investigations..ok all the best..

Hi Karthigeyan, Thanks for your response. We coducted boreholes awith SPT at 1m interval (especially in the transition zone from soil to rock) and coring initiated with 1m/0.75 m length to identify the samples and to identify the rock top level. As I mentioned earlier the spacing between boreholes is 50 m in longitudianl direction. The pile group consists of 484 piles (241 x 4). Since, the rock top is varying within 50m spacing, the contractor demanding additional stating that the piling in rock is different than the drilling in soil. To check the drilling time, I checked the piling records; surprisingly there is no much difference in time between drilling in soil and rock. This is due to rig capability (using BG-28 & 30; SANY SR285R). Checked the muck retreived from pile boring (contract is using rock bucket with TC bit for both soil & rock) and both parties are ending up with different opinion.

Dear Suresh,

Yes, the boring rate in rock is different than the boring in soil but in your case it is soft weathered rock so drilling will be easy and can extend even by metre without any trouble. The rock top definitely will vary within the spacing of 50 m and accordingly the embedded rock length of the pile is need not to be the same. The higher rate of rock drilling mean it is in unweathered rock, so you may talk to your contractor...ok all the best in your research work..

Dear Suresh,

The problem is interesting and common.

You can go for extra bore logs as you are telling the rock strata depth is varying longitudinal. You can use observational method at your site but it is difficult to implement here because designing and other things will take time.

What is the length of pile here?

Thanks.

it is difficult and very expensive to carry out continuous drilling to determine the bedrock depth, especially where there is no custom of performing such operations. For example in the Scandinavian countries this activity is carried out with specially developed tests (totalsondering) at low cost, which have been calibrated specifically on the land in these areas.

My suggestion is to cover the areas with geophysics survey (seismic, for examples, but attention to the water presence than affecting the Vp, then Vs measurements must be provided).

Bye

Dear Luca,

You are right. Yes, continuous bore logs will be costly. We can go for geophysical testing like MASW but take caution if water is present.

Thanks.