Prebored socket H-pile - Sand goes into casing

You have not provided enough information to answer your question. Are you drilling below the ground water table? How are you trying to install the casing? What drilling method and equipment are being used?

Thanks for your reply!

We use ODEX drilling below groundwater table. The groundwater level is around 5m below ground. Alluvial sand is at 40-60m below ground. Pile length is designed to be around 70m below ground.

Are you drilling and flushing with air or water? Without water or a drilling fluid filling inside the casing pipe, the ground water will cause the sane to run up into the casing pipe. If the ground water has sufficient artesian pressure, the water may still want to run up into the casing pipe carrying sand with it.

Flushing with water. But it seems that the pressure is high and the sand runs into the casing bottom.

Jet grouting the alluvial sand before drilling is one of the solutions we are thinking. But the concern is the water pressure is high, not sure if jet grouting works.

Do you have better suggestion to share?

I assume that your drilled-in H-piles (soldier beams?) will not penetrate too far below excavation subgrade. Correct? If you have artesian pressure that flows water out of the top of the casing pipe and sand into the casing pipe, I don't know how they will build the building(?) without significant dewatering. If they need to dewater to build the building, do it before your drill the soldier beams. Don't expect to fight water that someone will then need to remove later. Do it now or you will lose your shirt. Someone needs to check the base stability of the excavation with the upward artesian pressure. Will there be enough impervious soil cap below subgrade to prevent the subgrade from heaving or blowing up or turning to quicksand?

The H-pile is 70m long socket into rock which is foundation of new viaduct. Dewatering may not be a good choice since there is existing viaduct (supported by H-pile too) right next to my new pile. Afraid to affect stability of existing viaduct. Unfortunately, there is not enough impervious soil cap.

You haven't fully described the soil layers, their depths, and ground water elevations and pressure. Is it possible to first "screw in" a casing pipe to rock or to a less permeable soil layer above rock but below the bottom of the sand layer with the artesian water? If so, then you could clean out inside the casing pipe without water shooting up into the casing.
How was the adjacent bridge foundation built? Were similar, deep, socketed piles also used?