A new pile design

Piles

The literature review for this particular study is conducted in order to ascertain what, if any, problems, solutions and circumstances are present with the manner in which current piles are developed, used and maintained. The literature should provide the researcher with data on the different aspects of current pile technology, as well as information on any new ideas or concepts that might be currently in the works regarding piles and pile technology.

Piles are considered a method for supporting structures in areas where loads are not normally supported. A good example of a pile would be a support structure that transfers load capacity from an inadequate area of support, to an area of adequate support such as in shallow waters or in areas where the soil might not be conducive to providing a good bearing support capacity. A recent report may have put it best when it espoused "the commonest function of piles is to transfer a load that cannot be adequately supported at shallow depths to a depth where adequate support becomes available" (pg.1- pile foundation in engineering). This paper is specifically concerned with the best pile methodology currently in use. This study seeks to determine a more efficient, less costly manner for the use of piles, and in order to do so the current methodology must be thoroughly and efficiently examined. Current literature should provide the researcher with a clear understanding of exactly what, and how, piles are created, developed, built and maintained. After developing a comprehensive understanding of current pile technology, the researcher will then seek to determine an alternative method for building piles that yields a more effective and less costly method that can then be used to achieve the same, or an improved result.

A pile is built with the main purpose of providing support. A good pile support is based upon where the pile is situated regarding 'good bearing capacity'. This can be achieved in a number of ways. Finding good soil conditions into which a pile can be driven is the normal method for building a pile. However, there are times when good soil conditions are not available. In such times a pile may have to penetrate through the stratum of the poor bearing capacity soil to reach a good bearing capacity. When the tip of the pile "penetrates a small distance into a stratum of good bearing capacity, it is called a bearing pile" (p. 1 -- pile foundation in engineering). Another popular methodology is for the piles to be driven into limited soil conditions and develop a carrying capacity based upon the friction on the sides of the piles.

Pile Types

As Pile Foundation in Engineering reports "many times, the load-carrying capacity of piles results from a combination of point resistance and skin friction" (p. 1), when such an event takes place, the piles are called friction piles. There are other types of piles as well besides the friction and bearing piles. Some examples include the batter piles, vertical piles, compression piles and tension piles.

Batter pile and vertical piles are often used hand-in-hand as complementary tools. They are methods for transferring the load capacity from a pile to another structure such as a retaining wall or a sheet pile. The vertical pile, of course, is vertically situated under the structure and it is strengthened by a batter pile that is also situated under the structure but at an angle instead of vertically.

When this type of pile is employed, another horizontal pile can be used to transfer the weight bearing capacity to a sheet pile that is placed nearby in a vertical position near the structure. This type of pile can also be used beneath a retaining wall with the same transferring of capacity.

Another example of a common pile structure is the compression pile or tension pile. As the title states, a compression pile is compressed beneath a structure and is designed to alleviate the problems experienced by moving or swaying structures, such as during earthquakes or other profound events. A tension pile is also used with the same purpose in mind.

Most of the time, classifying the different types of piles is not set to a standard category. There are various manners in which the piles are classified including by material, by installation methods, and by how much ground is displaced when the piles are installed. For this study, classifying the piles will not be a high priority item, so as a general rule, the piles will be classified by the material which they are comprised of. On page 37 of the Pile Foundation in Engineering book, the following classification takes place which coincides with the pile categorization used by this study; 1) timber piles, 2) concrete piles, 3) steel piles, 4) composite piles, and 5) special type piles.

Currently, the two or three most common type of piles include timber, concrete or steel piles. As the Basics of Foundation Design states; "timber, because of its strength combined with lightness, durability and ease of cutting and handling, remained the only material used for piling until comparatively recent times" (p. 1). Recently, however, other materials such as steel and concrete have proven adaptable to the circumstances.

According to the text, both materials "could be fabricated into units that were capable of sustaining compressive, bending and tensile forces far beyond the capacity of a timber pile of like dimensions" (p. 1) and "concrete, in particular, was adaptable to in-situ forms of construction which facilitated the installation of piled foundations in drilled holes in situations where noise, vibration and ground heave had to be avoided" (p. 1). For this study then, a consideration that could be mightily discussed would include whether concrete was the most efficient and effective material for the new pile being developed, or whether some other material might be even more conducive to what is needed. Additionally, reinforced concrete was introduced some decades ago and has replaced (to a large extent) timber as a popular pile. The reasoning behind reinforced concrete is that it works well in dubious circumstances and "its durability was satisfactory for most soil and immersion conditions" (p. 2). The text also states that steel piles are becoming very commonplace, primarily due "to its ease of fabrication and handling and its ability to withstand hard driving" (p. 2).

Advantages and Disadvantages -- Piles

The advantages and disadvantages of the three most common type of piles (concrete, steel and timber) are numerous; since this study seeks to discover anything of importance concerning building a new, improved, longer lasting, less expensive and more durable pile, it is important to understand these advantages and disadvantages in order to develop a pile that addresses these issues.

Timber Piles

For example, according to the text, some of the disadvantages of using timber is that the pile can decay due "to fungi, insect attack, marine borer attack, and mechanical wear" (p. 39). Additionally, timber piles are "vulnerable to decay particularly when these are subjected to lowering and raising of the water table" (p. 40). It will be important for this study to determine what uses the pile being developed will be used for. Since timber is subject to decay when used in situations that introduce water, it might be better to stick with another type of material if the developed pile is going to be used there. Some of the advantages of the timber pile include the fact that when treated with creosote, oil-borne preservatives or salts the "life of timber piles above the permanent water tables can be considerably increased" (p. 39). If timbers are used under buildings or as foundation units they may also lose strength "under long-term effects of high temperatures" (p. 40) and "therefore, timber piles are not recommended under such structures" (p. 40).

A recent report on timbers states that "Wooden piles last a very long time underwater but are subject to decay when buried underground" (Columbia, 2011, p. 1). However, some of the problems that timbers face, especially when used for water-based scenarios can be overcome with coatings. According to the text, "problems of corrosion in marine structures have been overcome by the introduction of durable coatings and cathodic protection" (p. 40). Additional methods for protecting timber piles includes; placing fill around damaged piles, armor placement to provide resistance to abrasion, and concrete encasement of the piles. One recent report determined that a recently replaced timber bridge in Maryland was treated with not only creosote but as a dual protective scheme, "the timbers for the new bridge are pressure treated with copper naphthenate as well as being creosoted like the old bridge" (Zeyher, 2005, p. 23).

The project engineer determined that a dual protective approach made more sense than just the creosoting that took place in the past. Technology can certainly help in determining what amount of corrosion or debonding in taking place in timber piles as was recently displayed by a new evaluation methodology used on a West Virginia bridge supported by timber piles. According to the study "The results revealed that infrared thermography can be used as an effective nondestructive evaluation tool for detecting subsurface debonds in structural components wrapped with carbon or glass reinforced composite fabrics" (Halabe, Dutta, GangaRao, 2008, p. 1387). Perhaps, new technology can also assist the researcher during this study in determining exactly what type of pile to create.

Concrete Piles

According to the Columbia Electronic Encyclopedia "concrete piles are generally of two types, the precast and the cast-in-place. They are very strong and durable, do not deteriorate when wholly in the ground, and are immune to the attacks of boring insects" (p. 1). If this is true, then it would seem that when the piles are going to be used in areas where water is a consideration, then the material to be used should be concrete or steel. What is expected from this study is to determine if there is any differences between the materials used for the various piles, and if a new material (or new use of a current material) can be introduced in an improved manner.

Concrete piles, whether they are pre-cast or cast in-situ, are known as durable and long-lasting supports but they are more costly than the timber piles. Some of the advantages to the concrete piles (besides the fact that they are more durable and long-lasting) is that they can be less invasive to the displacement of ground material than with a timber.

Concrete piles can also be shaped in almost any manner, and a recent study determined that "results from the DEM simulations showed that the shape of the driven piles has a significant influence on the development of penetration resistance and particle crushing" (Lobo-Guerrero, Vallejo, 2007, p. 242). Since friction and displacement are major considerations when determining what type of pile to use in any given situation, it is nice to know that concrete piles can be quite beneficial when it comes to penetration resistance. Concrete piles normally come in two different varieties; cast in-place or precast. The manner in which concrete piles are cast in-place is that there are holes drilled and then filled with concrete.

One of the disadvantages to this particular methodology is that there is uncertainty as to how the concrete will end up shaped and in what condition the pile will be in. For example, the integrity of the pile may be compromised in some manner, and it might not be known until a much later date. Some of the advantages of the use of concrete piles is that it is less expensive than other methods, the pile can be varied in length and size, and there is a minimum of vibrations and displaced ground during the installation of the pile. Additionally, concrete piles can be embedded into plates, slabs or blocks of concrete (on the top) to spread weight and support over even further distances. Oftentimes, concrete piles can be supported by reinforced cages, steel casings or rods. Some further advantages to the use of concrete piles it that they can be driven deep into the ground in order to support large buildings and structures. When used in this manner, piles are usually connected to the footers of the foundation(s).

The piles then act as distributors by ensuring that the weight of the building or structure is supported in a manner that reduces the risk of a collapsing building or structural failure. Oftentimes, concrete piles will be used in the case of buildings that need deep foundations due to the size of the building or it can also be due to the underlying ground that might not be conducive to supporting large, heavy structures unless a supporting foundation is set. Concrete piles can be that supporting foundation or the support of that foundation. Another reason for using concrete piles is that the area might be in a seismic zone, and concrete piles are able to withstand seismic activity to a much higher extent than timber piles. A disadvantage of concrete piles is that they are limited as to the drilling equipment that is being used to install them (Hussein, Likins, 1993, p. 2).

This study will seek to add to the high durability that is associated with concrete piles when developing a new pile. The use of high quality concrete results in minimal cracking and provides high resistance to moisture penetration which is a very great advantage when compared to timber piles. The researcher should understand these advantages and attempt to incorporate the advantages of one pile over another, while limiting the disadvantages. Since concrete piles are so durable, even when exposed to extreme conditions, this quality will also be sought in the newly modeled pile. Another aspect of the concrete pile that will be attempted to emulate is the high load capacity of each single pile. Since each pile bears more capacity, one of the resulting advantages is that fewer piles are needed, which means less work, less maintenance and lower cost.

The concrete piles are also highly adaptable to various conditions and since they are often pre-cast, the pile composition can be adjusted according to the needed specifications. Project engineers can use their own discretion as to the composition, strength requirements, size and shape of the specific pile. This means that the engineers can consider such items as the soil displacement and composition, the environment that the pile will be in, and the specific construction situation. Since the concrete piles are also easy to handle, simple to transport and install, using this type of pile ensures the most effective and efficient manner for employing piles.

Additional advantages to the use of concrete piles includes the fact that they are economical, can be driven in long lengths and yet are easy to join together, they are easy to install and are noiseless (for the most part), are strong and corrosion free, and they have large axial and bending moment capacity.

Steel Piles

Steel piles are quite similar to concrete piles regarding the advantages and disadvantages. Additionally, steel piles offer a number of advantages that even concrete does not offer. For example; a steel pile can reduce driving costs and the H-section steel piles have a low displacement volume which can be a nice advantage when soil displacement or upheaval is a problem. Steel piles are known to be more expensive (for the most part) than are concrete piles, and steel piles also have the risk of corrosion that is not normally associated with concrete. In natural non-contaminated ground, the risk of corrosion still remains low.

Another advantage to the steel pile is that it can come in different forms such as; H-sections, tubular sections, box piles and sheet piles.

The different available forms allows the project engineer a certain amount of flexibility that might not be available with the concrete or timber piles. Installation of the steel pile is a much easier consideration as well, since steel piles can be driven at a much higher impact than timber. The bearing capacity of steel piles can also be increased with certain procedures not available with the other types of piles. If the steel pile is driven to a hard stratum, it can have an even higher carrying capacity and after it has been driven, it can be cut down to size or reshaped for further driving. A steel pile can also be extended quite easily, and if it happens to be too long can easily be cut down to size and the cut off portions can be used again or sold to recoup expenses.

Some of the disadvantages of steel piles are that they are subject to misalignment during the driving process. The idea during the driving process is to ensure that the steel pile is aligned in a proper fashion and there is currently no easy way in which to confirm that steel piles are aligned correctly so far under the surface. If the piles happen to be misaligned, then they really cannot be used as a support column with any degree of certainty. Other disadvantages are that a proper and valid soil test is usually required (an added expense), steel piles have a much smaller footprint than do concrete piles, and steel piles are normally more expensive than are concrete or timber piles.

Choosing Piles

On page two of the text a comparison of the different piles explains that oftentimes the choice of which pile to use in specific projects will most likely contain a consideration of what can be easily obtained in whatever country the project is taking place.

The text states that "timber piles are suitable for light to moderate loadings in countries where timber is easily obtainable and that steel or precast concrete-driven piles are not as economical as driven or bored and cast-in-place piles for land structures (p. 3). A number of other factors for choosing the different types of piles should also be considered with any project. Certainly one of the main factors is how the pile will be installed. As the text relates; "the method of installation of a pile may have profound effects on its behavior under load and, therefore its load carrying capacity" (p. 3).

A look at the pile installation methods will likely yield information of interest to the researcher who is seeking to develop a new and improved pile. Pile installation include a number methods such as; driven precast or driven cast-in-situ, bored cast-in-situ, jetting, jacking, spudding or screw methods. Quite often, the researcher is considering not only the effect of installation on the project's building or structure but on nearby buildings and structures as well. Installing piles can result in building or structure movements and vibrations that can result in structural damage to other buildings. Pile behavior is often based on what type of soil the pile is being driven into, and whether the pile will experience either static or dynamic resistance.

In Chapter one of Bengt H. Fellenius, Basics of Foundation Design, he states that "before a foundation design can be embarked on, the associated soil profile must be well established" (p. 1). Additionally, he states that "all foundation designs must start with determining the effective stress distribution of the soil around and below the foundation unit" (p. 1). Fellenius also states that the method of installation of a pile may have profound effects on its behavior under load, and therefore, its load carrying capacity (p. 3).

It is important then to take a look at the different installation techniques in order to determine how the newly developed pile is going to be installed in order to achieve lower expenses and an improved pile.

Page 11 of Pile Design and Construction Practice states that choosing a pile is based on three basic factors; 1) the location and type of structure, 2) the ground conditions, and 3) durability (Tomlinson, Woodward, p. 11). A brief synopsis of the specific pile that would work well based on each of these three factors is contained herein. First, the location and type of structure is a concern because each of the most common piles was developed to address each of the above listed factors. If the structure is one that will include a water-based approach then according to Tomlinson and Woodward; "a solid precast or prestressed concrete pile can be used in fairly shallow water, but in deep water a solid pile becomes too heavy to handle and either a steel tubular pile of tubular precast concrete pile is used" (p.12). Of course, timber piles can be used in shallow waters as well, with an excellent example being the Corpus Christi seawall built in 1939. The seawall consisted of approximately 2,000 tons of steel sheetpiling, 77,000 linear ft of timber piles, and 22,000 cu yd of concrete (Massengill, Moore, Garza, Hayes, 2008, p. 48). It is also an example of how even early project engineers were using a variety of piles and methods to create long-standing structures. The article states that the original purpose of the seawall was twofold: to protect the downtown area from storm surges and to stabilize and beautify the shoreline. However, the seawall was deteriorating and in the late 1990's an engineering firm was hired to assess the deterioration. What was discovered was that the timber piles were corroded (some severely) while the steel piles were relatively unscathed.

The article espoused the fact that "excavation work revealed voids in the fill material…up to 50% loss in the diameters of the timber piles, severe timber pile decay, and general honeycombing on the sides of the pile caps" (p. 48). Additionally the article states that the "dowels connecting the sidewalk to the seawall were severely corroded, sometimes to the point of nonexistence" (p. 48). One good point for the evaluation was that "the reinforcing steel in the sidewalks and seawall was in fair to good condition overall" (p. 48).

The steel piles were also evaluated, and what was discovered was that there was "various levels of deterioration, primarily in the form of surface corrosion, pits, and pitting holes in the steel sheet piles" (p. 49) and that the resulting corrosion "reduced the sheet pile's bending capacity by up to 36%" (p.49). It would be interesting to determine what effectiveness and corrosion levels were present when comparing the timber to the steel piles, especially in factoring in the costs of each. The question remains then as to how cost effective the timber piles were as compared to the steel piles?

When the seawall was reconstructed the engineering firm recommended the following; a new continuous steel sheet-pile wall option was recommended because it would provide both lateral stability and rear support for the seawall, supplementing the function of the deteriorating timber piles (p. 51).

It seemed as if, according to the article, that the timber piles had deteriorated more quickly than the steel piles, and that using a steel pile wall option to shore up the deteriorating timber piles was much more conducive and effective an option than replacing the timber piles.

As the Tomlinson and Woodward book states "timber piles are used for temporary works in fairly shallow waters" but that "large-diameter steel tubes are also an economical solution to the problem of dealing with impact forces from waves" (p. 12).

According to Tomlinson, using a pile for a structure on land is "open to a wide choice in any of the three categories (steel, timber, concrete). The least expensive piles are the bored and cast-in-place ones where "unlined or only partly lined holes can be drilled by rotary auger" (p. 12). Augered piles are also suitable when ground heave, noise and/or vibration is to be avoided. Tomlinson says that timber piles are suitable for light and moderate loads, while heavy and seismic loads would lean more towards the concrete or steel piles.

Determining the ground displacement and upheaval is a key component to determining what pile will be the most effective in specific cases. When developing a new pile, it will be necessary to determine where and when that pile will most likely find a niche. The developed pile is not being developed to replace any or all of the common piles now in use. Instead the new pile is being created to augment or complement the current offerings. Therefore, understanding what factors will play a role in where and when the new pile will be used, is most definitely important. According to Tomlinson, the ground conditions are the second factor that need to be considered in order to determine the correct pile to use. Tomlinson et al. states that "ground conditions, influences both the material forming the pile and the method of installation" (p. 12).