There's a lot of confusion in the development industry about the added cost and expense of building a pier over water or on filled land. As development and developers interest begins to focus again on major metropolitan areas, few sites remain in premium locations that use standard construction techniques. In order to secure a particular market, developers will have to think outside the box and consider sites that had been passed over by previous rounds of development.

One of these types of sites consists of the air rights situation. Whether this type of site is built entirely over a transportation use, such as a rail line in the case of the proposed Trump project in upper Manhattan or a highway in the case of Boston's anticipated Columbus Center, or on filled land in the case of Battery Park City in lower Manhattan, all these types of projects share a common construction thread. The construction must take place using piles and a deck over piles. This article will take a close look at this technique and explore some cost differentials in terms everyone will understand and then examine a few projects, both past and present, that use this technique.

Perhaps the greatest cost differential in constructing a pier is the lack of support that the deck would receive had it been built on dry land. Consequently, the space between piles would have to have a form in place, either removable or permanent, upon which to pour the reinforced concrete deck. The alternative is to use a crane to hoist prestressed, preengineered concrete panels into place. The cost of the deck would likely increase the price per square foot of the building foundations by up to 25% greater than a similar deck created in the usual fashion allowing the prepared ground to be used as a form. Given our present technological base and cost of capital versus skilled labor, there's likely a slight cost advantage to using the crane to place preengineered concrete panels on the piles. There are other permutations to this scenario that will be addressed later in this article; however, at this point I'd like to present some background information for those that are unfamiliar with these techniques.

The piles themselves are a consideration as well. Piles may consist of wood, reinforced concrete, or steel. Piles are used where the prepared surface cannot support the weight of the overbearing structure. Piles are driven by a piece of equipment known as a pile driver. This equipment consists of a long pair of rails parallel to and straddling the pile to be driven. The rails support and guide a pneumatic weight, which is raised and allowed to drop on top of the pile – using the force of gravity upon the weight to drive the pile into the ground.

Engineers calculate the weight of the structure and the required soil compaction to support the structure without subsidence. Test borings or cores determine the soil consistency over the entire site to be prepared for construction. Engineers perform this work in order to know what underlies the entire site – an important consideration with large structures – to prevent subsidence. Subsidence occurs when the structure slowly sinks into the ground. An example of uneven subsidence is the Leaning Tower of Pisa. Almost all subsidence occurs unevenly in nature. Besides causing the structure to tilt in one direction, it may also tilt in two or more directions causing fractures in the structure that may even make it uninhabitable.

The piles are driven to the predetermined soil compaction weight and cut to the proper length or height. The type of pile used depends upon the setting it will be used in. In areas where they may be replaced easily, wooden piles are used. Wood has excellent properties for resisting horizontal shear. You'd see wood pilings on fishing piers where ships rub up against the pier. These may be called wearing or fender piles since they wear out and defend the pier from the vessel. The piles carrying the weight are called bearing piles. In this type of structure, all the piles would be treated wood, usually white oak. Steel piles are used in situations requiring some horizontal shear yet more vertical weight, but in relatively dry conditions. Concrete piles are used more for bearing than horizontal shear but are better for wet conditions. Concrete piles are actually concrete filled steel piles. All piles may be steel tipped so as to be driven through small intermediate obstructions. Piles are sold by the running foot. As you might surmise, the longer a pile that is necessary for the job, the more costly it becomes

The cost of piling an upland site with poor soils adds between 10% to 15% to the costs of the deck and foundations. Certain soil groups in the western states are silty and unable to bear the load of a structure and must be piled. In the east, these soils are usually classified as peat and muck, which are associated with wetlands, or made land, which is associated with filling. Any landfill, whether it is good, sandy fill or bad, containing excessive amounts of organics (that will decompose and leave voids) must be piled. The only exception to that rule is made land that is periodically compacted as it is filled. Interstate highway cloverleaves are made using this methodology. It would not be possible to compact fill placed in water with any great success. Furthermore, in order for this compaction process to function optimally, unsuitable substrata, like peat or silty materials, need to be removed.

The proximity of bearing piles to each other is another factor, which will greatly effect the cost of the project. In order to get the required load carried for the structures to be built, piles will be placed in very specific patterns or designs. Essentially, if the piles are to carry the proper weight, they will be placed as close as possible underneath the weight to be supported. This would mean that a multistoried building would have a concentration of piles directly underneath the footings, which support the walls. Because some of the weight may be carried by reinforced concrete beams or footings from one pile or cluster of piles to another, this perimeter of piles will not be a solid line of piles. A lesser number of piles will be required to carry the floor load for the non-bearing space in between. The proximity and pattern of piles required is a function of the weight to be supported and the soil compaction.

With a specific site in mind, an engineering design would have to come after the architectural design. However, the architects should be made very aware of the specifics in order to craft a cost-effective plan. In the instance of building on filled or made land, of which there is quite a bit on the waterfront in most coastal cities, the cost of foundations, etc. would be 10% - 15% more than a conventional site. If the site were completely over water, the cost differential would rise to 20% - 25% more for the reasons cited above with sites requiring longer piles to be at the higher end of the scale.

Once a deck over water is constructed, a parking lot is essentially completed except for lighting and line painting. This actually acts as a cost reduction compared to traditional sites where preparation of the land can be extensive. At current rates, this is a saving of $10-20 per square foot. Moreover, in the instance of a building built over water, the footings and foundation are complete as soon as the deck over the water is done.

Pile Costs

1. one story building (2) two story building (3) three story building

Source: R.S.Means 2003 Building Construction Cost Data

The only other consideration effecting the costs of the alternatives reviewed in this outline is the cost of maintenance and repair, which is a variable or ongoing operational cost rather than a fixed cost as in the construction of the structure. The actual costs of maintenance experienced by projects range from a low of $0.57 per year in New Hampshire for estuarine situations to a high of $0.65 per year in New York City in tidal situations. In both instances, small particles suspended in water abrade the piles requiring the piles to be parged or refinished every few years. Because these costs are essentially for maintenance of what would have been an item not requiring any repairs had the structure been built entirely on land, these costs would reduce the value of the riparian land underlying the water or increase the size of the contemplated development to support the added costs. Some maintenance of exposed piles, piers or abutments is required for parking garages, bridges, etc. This work is more in response to specific damages from human activities or accidents than an ongoing force of nature making them virtually impossible to predict in time or severity. Piled buildings constructed completely on land are not subject to these forces and, consequently, do not have this additional maintenance cost.
 
If the site were completely over water, an architect should give serious consideration to placing the parking in an elevated garage so as to maximize the efficiency of the site by reducing the amount of single story deck to be created. If ferry service or water taxis are expected to use a portion of the site, then that portion should be improved with replaceable, wood fender piles on the exposed or wear surfaces.
 
Other than what has been outlined above, there's no real limit to the height that a structure built on piles may attain. In New York City, the World Financial Center at Battery Park City is built on made land as is most of the rest of Battery Park City. Piles support all the structures in this area. Those buildings soar up to 53 stories. The residential structures average seven stories. The filled area extends out to the Pierhead Line as established by the Federal Government at Battery Park City.
 
In Boston, the proposed Columbia Center project expects the costs of decking over the six lane Massachusetts Turnpike to be between $250 and $700 per square foot. Other than the cost of this deck, construction will proceed in the usual manner and cost the same as built on land. This construction process is similar to building a deck over water since the highway will remain in operation during construction. This extra expense would require that the land be 10% - 15% more intensively developed than typically the norm for the area. One tower of the project rises 35 stories above the deck over the highway. The greatly higher costs anticipated with the Columbia Center project are a direct function of the necessity to span three lanes of traffic to the median in each direction. This is a function of the spacing of the piles being very far apart, except in this situation the supports will be a virtually solid bridge-like abutment due to the length of the span and the weight of the structure to be supported. If the area to span had merely been one lane of traffic, rather than three, the cost of the decking for the project would have been reduced by over two thirds as it is a greater than equal cost increase to fight gravity over the wider distance. If the spanned area were shorter the supports would be more evenly spaced columns or piers – much more like that in a multi-story parking garage. Perhaps the construction cost range for the smaller span would be more like $80 to $250 per square foot.
 
In closing, the added cost and expense for constructing a deck and foundations in the process of developing land under water either through filling or piling (10-15% or 20-25% respectively) needs to be offset by the greater value of the property in situ and/or the lower cost in acquisition as well as the intrinsic value of its location, visibility and proximity to services and infrastructure or the deficit must be overcome through increasing the intensity of the use of the land – as is the case with the Columbus Center project that is being developed at a rate 10-15% in excess of what otherwise would have been allowed.