CUURENTLY I AM HANDLING A PROJECT WHERE 200X200 RC PRECAST PILES HAS BEEN CHOSEN. IT'S PREDICTED THAT THE FOUNDING DEPTH OF HARD LAYER IS AT ABOUT 45M. THEREFORE 200X200 PILE WITH 7 JOINTS IS TO BE USED SINCE THE SMALL PILE SIZE COMES ONLY WITH 6M LENGTH.
MY CONCERN IS 7 JOINTS IS A PILE SEEMS TOO MANY AND CAN ANYBODY LET ME KNOW HOW TO FIGURE OUT THE MAXIMUM ALLOWABLE NUMBER OF JOINT IN A PILE.
The current practice in Penang Island, Malaysia is a reduction of 5% (I normally practice 7%) for every joint in the structural axial capacity. As far as I know, there is no limit on the number of joints in a pile point and the key issue is the slenderness ratio. Please note that the slenderness ratio permissible for piles is different for above and below ground. In the ground, the pile can be assumed to be fully braced (somewhat) to the intensity relative to the stiffness and of the soil.
Where is your project site? Do you know that 9m piles are already currently available in Malaysia? API (maybe known now as MDC), Hume, SG and PKM all produce 9m 200x200 RC Sq. piles. In terms of quality Hume and PKM are the only real choices but PKM is quite different. Hume's quality is very good but the reinforcement cage is hand-tied and the concrete is only OPC but PKM's reinforcement cage is fully welded using a welding robot, their concrete is PFA cement (better durability and long term strength) and because of the welding effect, they are the only manufacturer in Malaysia able to produce 10m piles! I have witnessed pitching and driving of their 10m and 9m piles and they are surprisingly more rigid than compared to the conventional market's pile.
What's your project location? If in M'sia I can probably check my database for the suitablility of a foundation. Also what's your working load (and FOS)? With depth of 45m, I would recommend normally 250x250 mm piles as the smallest size. 200x200 mm piles are too small in dimension for such a depth. Maybe you can redesign the pilecap. You can contact me if you want to obtain contacts for PKM, Hume or/and Api (or MDC).