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The selection of pile for marine work at mangrove area

posted Jan 13, 2018, 4:26 PM by jeffery jim

The selection of pile for marine work at mangrove area is largely affected by its serviceability rather than the ultimate load and the cost involved throughout the structure lifespan.

1) Dimension and design capacity
Pile capacity design plays the main role when it comes to ultimate limit design. The followings are the equation for structural capacity where;
Qall,concrete = 0.25 x fcu x Ac
Qall,steel = 0.3 x fy x As
fcu = 60kN/m²
fy = 250kN/m²
If both have similar dimension where, Ac = As, the overall Q for steel will be much higher than Q for concrete.

2) Exposures
Although steel pile are exposed to severe environment of the mangrove, it is still the best for diagnosis and prognosis of the structure during service period. The aesthetic of the steel pile signals the need for rehabilitation and maintenance. It is harder for layman to detect pitting in a concrete pile or foundation.

Microscopic inhabitants are the main contributor to the deterioration of concrete due to exposure to chemical and physical properties such as ions and the effect of electrolysis. This is caused mainly by carbonation, ingression of sulfide and chloride or worst, if the construction area is also affected by brownfield soil(s).

Aside from exposure, the chemical reaction (eg. pyrite) changes the main material (eg. ferrum) of the structure which causes the structure member to loss its robustness to stress and loading.

Another microscopic issues which many are unaware about mangrove area is the existence of many types of theobacillus such as Th. thiooxidans and Th. ferrooxidans. This also leads to corrosion.

3) The accounting and economic Costs
The cost for steel pile may be higher than a concrete pile cost but the economics behind using steel pile may reduce the long term economic of such selection. Nevertheless, the ease of rehabilitation work place steel pile as a better choice.

Concrete structures rehabilitation usually comes with high degree of difficulty where replacing concrete or its reinforcement bars underground is extremely hard. Usually, the only method of repair is through steel jacketing. This remedial have its downside since the member would behave as hybrid or composite design which requires finite element analysis to conduct its dynamic stability and robustness. The use of thixotrophic mortar and CFRP plate is applicable but it need to be replaced due to deterioration. Cathodic protection is also applicable but it is highly dangerous (when you are assigning anode and cathode to the rebars which may cause a passive rebar to be the anode) and tedious (controlled system which need current supply to regulate).

As for steel pile, it is quite easy to maintain where a simple paint will solve the corrosion issue. The installation of secondary system, the cathodic protection is also simple with self-sacrificial cathodic protection which will protect the steel pile for years).

4) Structural stresses
One of the most important issue for such design is the ability to perform under stress through displacement and deflection. Since the steel element have better range in elasticity and plasticity, the steel pile would perform better when it comes to lateral load or accidental load.

Lastly, the performance of pile capacity (axial load) is very much depending on skin friction plus end bearing capacity. Although skin friction or friction factor and external friction angle between surface of the steel and soil is slightly low (0.20, 11⁰) compare to concrete (0.25, 14⁰) for silty soil, paragraph 1 explained the overall pile capacity is affected by capacity by the f value. For other types of soil, both steel and concrete performs almost similarly.

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