Dear Editor,
I have noticed in the print media, that concerns have been expressed by independent engineers on the integrity of timber piles for a project on the East Coast of Demerara, Guyana.
The integrity of splices for any pile type can be compromised during easy or hard driving conditions as a result of tensile or compressive stresses beyond threshold limits respectively. Pile tips can be compromised during hard driving conditions. I suggest the following:
1. A comprehensive geotechnical investigation to determine site specific subsoil parameters
2. Before the commencement of a pile driving programme (assuming this foundation type is necessary), wave equation analyses of piles (WEAP) should be carried out to determine hammer suitability for driving piles for the site specific conditions without compromising its integrity, as well as a driveability analysis to ensure hammer size is adequate for attaining pile penetration. This analysis is applicable to all pile-driving hammers, including the drop hammers which are typically used for the installation of timber piles in Guyana.
3. The conduct of dynamic pile testing (Pile Driving Analyser) or PDA in accordance with the relevant American Society of Testing and Materials specification viz ASTM D 4945-12 on a number of test piles as specified by the geotechnical engineer. The PDA test in real time will determine the stresses along the pile as well as the pile tip. For the spliced pile, the wave forms allow for evaluation of the progressive failure of the splices if they occur and would also conclusively indicate if the stresses at the pile tip are excessive, ie if brooming is occurring. For large scale projects, it may be prudent to test two or more pile types (concrete and timber) so informed decisions can be made from the test programme.
4. During the PDA test, the pile capacity is determined per blow penetration. Because excess positive pore pressures will develop during pile driving in the soft under to normally consolidated clays and/or silts (Guyana’s coastal soil), the capacity of a pile at the time of driving will be significantly less than the long term pile capacity. These pore pressures reduce the effective stress acting on the pile thereby reducing the soil resistance to pile penetration, and thus the pile capacity at the time of driving. As these pore pressures dissipate, the soil resistance acts on the pile increases as does the axial pile capacity. This phenomenon is routinely called soil setup. An allowance for an additional re-drive of 3ins penetration about 2 to 3 weeks after installation would provide the pile‘s ultimate static capacity.
The above methodology is routinely used in the USA and Europe as well as by ourselves on most projects in Trinidad and the wider Caribbean to provide safe and economical designs, and effective quality control.
Yours faithfully,
Andrew Budhram
Engineer
Director
Geotech Associates Limited,
Trinidad
