1) Drop Hammers
Drop hammer without a rigid lead. The guiding rod is not stiff enough. This hammer would deliver extreme eccentric forces and thus, not the best hammer to use.
Drop hammer with a rigid lead.
Drop hammer with a rigid lead.
The more rigid the lead, the better.
Drop hammer weight: Generally 1% to 2% of the required capacity.
The heavier the weight, the better. Weight up to 6% the required capacity would be best, as the drop height can be reduced.
Drop hammers can be utilized to test Bored Piles (Drilled Shafts). Most drop weights are between 1.5 and 20.0 tons.
Drop weights are typically 1% to 2% of the Required Ultimate Test Load (e.g., if Required Ultimate Test Load is 1200 tons, Drop Weights are recommended to be about 12 to 24 tons). Two cautions:
1) Pile tops can be easily broken under hammer impact forces due to several reasons: i) bored piles are not designed to withstand high CSX limits as with driven piles, ii) extreme bending (eccentricity) degree from drop hammers without a rigid lead, which could double the compression stress on 1 side of the pile.
To minimize this risk, Contractor can have a steel casing (shell) confining the pile top concrete. Additionally, plywood is recommended to be used as pile cushion.
2) Skin resistance bonding can be shattered. Thus:
Design Engineers need to consider to design a sacrificial test pile;
Or, if the pile is not sacrificial, consider a proof test (for example, test only to Design Allowable Load) instead of a full load test to the required Ultimate Capacity.
Despite a proof test concept (to Design Allowable Load), concrete-soil bonding along the portion of the pile that experiences movement larger than about 2.5-mm could still be destroyed or weakened, as shown in N_GAPA example #16 sketch here.
For further reading, please refer to FAQ #10.
2) Regular Impact Pile Driving Hammers
Hydraulic
Air/Steam
Diesel
These regular hammers typically deliver more evenly forces (F1/ F2 ratio between 0.3 and 3.0, ideally between 0.5 and 2.0). The ram weights of these hammers vary greatly, from 0.1 to 200 tons (most are between 1.5 and 5.0 tons).
