Driveability Studies - Milestone of Offshore Piling
Conducting a driveability study is an essential part of project planning, not only from a time perspective but also when it comes to choosing proper equipment for doing the works.
This is especially the case in marine construction. As heavier equipment is usually needed for driving piles, a barge of larger bearing capacity will be required, which has a significant impact on the overall project value. Choosing wrong equipment for driving piles can cause many problems for a contractor, i.e. assuming larger equipment than is needed can result in losing the job due to the unrealistically high offer, and on the other hand, underestimating soil resistance can result in a significant reduction in profit margin (due to the additional need to (de)mobilize heavier equipment) and can put the whole planning schedule in jeopardy.
Even though GRLWEAP is not directly based on energy approach, it all boils down to the following energy formula:
Es is total energy available to do the actual work, Ru is ultimate pile capacity and s represents losses in soil. Total energy available can be calculated as total kinetic energy minus all losses in driving system.
Ek is kinetic energy of the hammer, and Eds, Epl, Esl are losses in driving system, pile and soil, respectively. Total kinetic energy depends on hammer system, which consists of hammer, helmet and cushions.
Nowadays, driveability studies are usually done through Wave Equation Analysis Programs - simulating a wave propagation through the pile length while it is being driven. GRLWEAP uses finite difference analysis method to solve wave equation. This system is using series of mass and springs, representing static and dynamic resistance, and consists of several parts:
- Hammer system
The manufacturer usually provides rated energy value, which is the starting point for an engineer in process of estimating final hammer efficiency. Today, there is a wide spectra of them: External and internal combustion hammers, vibratory and impact, free fall and break released, single acting and double acting hammers, etc.
For conducting driveability studies, understanding soil mechanic is of utmost importance. Based on soil investigation, unit shaft resistance and pile top resistance must be determined, as well as dynamic soil parameters (quake and damping). This is implemented by elasto-plastic springs and viscous dashpots for dynamic response.
Soil model in GRLWEAP is based on the Smith approach.
Static resistance is a linear function up to quake limit (qi), and beyond that, limit displacement can increase without an increase in resistance. During unloading (hammer goes up) spring has negative velocity, holding the same stiffness as for loading. Shaft can produce negative resistance, as it is based on friction, while toe can produce only positive values.
The usual way for estimating dynamic resistance is by using the Smith model, which is based on velocity. Very slow moving piles only encounter static resistance - the faster the pile moves, the bigger the resistance. Piles are modeled by dividing the total length to smaller parts, using appropriate parts’ masses and axial stiffnesses.
By interpreting the results, engineers should be able to determine which type of hammer is most suitable for a specific project, taking into consideration that compressive and tensile stresses in the piles are within allowable tolerances during driving and that estimated blow count doesn’t exceed accepted criteria.
Most revealing results are usually hidden in “Ru vs Number of blows” and “Stresses vs Number of blows” graphs. The result should be interpreted in the following matter:
- If number of blows per length is very high, a more powerful hammer is required.
- If number of blows is too low, a smaller hammer should be considered (cost-wise and for more precise installation).
- If pile compression stresses are too high, a smaller hammer can be used, or different type of pile cushion, or smaller hammer drop height, or pile material of higher strength.
- Pile tension stresses should be considered for concrete piles and appropriate measures should be taken if they are too high.
Written by Aleksa Cavic, Technical Engineer at Ecocoast.