Analyses of Hope Outfall construction should be beneficial

Dec 06, 2012 Letters

---

 

 

Dear Editor,
The analyses of the ongoing construction activities related to the Hope Outfall, by Messrs Sohan and Alli, have identified issues which should lead to discussions on the economics of engineering construction activities in Guyana.
I have opted to stay away from commenting on Government of Guyana projects since our firm has a policy of not working on those projects. I would however like to attempt to clarify and expand on some of the concerns raised by those two experienced engineers.
Our firm, Ground Structures Engineering Consultants undertook a geotechnical investigation, for a private developer, for another facility originally proposed for the Hope Area. That investigation included drilling six boreholes over an area which straddles the area of the Hope Canal Outfall. We experienced refusal to drilling at depths of approximately 20 metres (m). I was consequently significantly amazed to determine, based on your article, that piles are being driven to depths of approximately 36 m for construction of the outfall. My amazement increased exponentially when your article went on to point out that the outfall itself, constructed at some point below ground, was to be supported by spliced timber piles.
The engineering design of the facility, if evidenced by the facts above, cannot stand up to engineering scrutiny. All piles are subject to stresses during driving. These stresses can lead to failure of the pile while being driven and prior to imposition of the design loads. Spliced piles have a weak zone at the splice location.
The integrity of the splice can be imperiled and the splice can fail when piles are driven in areas with hard driving conditions.
This phenomenon is not easily recognized during pile-driving since hard driving conditions will result in the pile tip breaking into several splinters, similar to that of a coconut broom.
The blow counts, recorded during pile driving, will continue to be low since it is now a record of the advancement of the splinters into the soil and not a record of the actual pile tip advancement.
Hard driving conditions at the Hope Outfall would begin around 20 m. The fact that the piles have deviated from the design position is probably reflective of “brooming” and/or failure of the splices due to hard driving conditions.
The best option, if piles were preferred, would have been to test load a pile and to develop pile capacities based on the pile load test.
We conducted similar work for several private sector clients and have load tested 20 m long piles in conditions similar to those encountered at depth of 20 m at the Hope Outfall. In some instances the piles did not experience failure, evidenced by excessive settlement, when loaded to 100 tons.
Another perspective is to question the need to support the outfall on piles in the first place. While I have paid no attention to this project and have very little knowledge of the design, we have undertaken similar work for Booker Tate for several pump stations, some in soil conditions worse than those prevailing at Hope.
These structures are founded 6 m below existing ground level.
None of the pump stations, built by Booker Tate, are supported on piles. The fact is that the soil removed weighs less than the structure to be built at the site.
The structure consequently imposes no load on the soil.
This is obviously a repeat of the mistake made on the aquatic centre construction. The challenges associated with that type of improvisation relates to the management of the negative pore water pressures which develop during the construction stage. The costs of managing negative pore water pressures pale in comparison to the cost of countless piles.
I should point out that the structure will most likely perform as proposed, since consideration of the engineering factors indicates that the piles are redundant. However, considerable savings would have resulted in terms of both construction material and time, if a more technically defensible approach was applied to the engineering design of the structure.
The Guyana Association of Professional Engineers would be strongly advised to use this as a case study to benefit the development of recent engineering graduates. It is only through recognition and acknowledgement of failures that the profession will move forward in Guyana.
As an association you cannot seek legal recognition of engineers when engineers are failing to provide defensible engineering designs. Legal recognition and respect will come when engineers stop providing layman solutions to engineering problems.
I would like to applaud Messrs Sohan and Alli for attempting to engender some discussions on these issues. Hopefully it would lead to more discussion on the technical merits/demerits of engineered solutions in Guyana. Taken objectively that will benefit the development of our engineers and will ensure the optimization of resources expended on engineering projects.
Charles P. Ceres, PE
Geotechnical Engineer & Groundwater Hydrologist

---

---