Variable diameter pile foundations – advantages and disadvantages

Variable diameter pile foundations can be an efficient foundation solution used for special structures, mainly for the ones with increased heights. For economic efficiency in land use, especially in urban areas, there is a tendency to build very high constructions with very small footprint related to their height, which transmit large loads to the foundation soil. Based on these aspects, it is necessary to consider pile foundations in order to ensure the desired safety and stability requirements. The technologies for piles constructions evolved a lot and a main objective was to better use the interaction between the soil and the pile. From this point a view the paper presents a comparison between the bearing capacities of a constant diameter pile and a variable diameter pile, considering different soil conditions. All the calculations were performed using finite element analyses.


Introduction
Due to the technological evolution around the world, new ideas for sustainable development are needed.The fast increase of human population leads to less available construction sites which can be solved by extending buildings in their height instead of area.Especially in highly dense urban areas, constructions are built on very small footprints related to their height, and for that transmit large loads to the foundation soil.This comes with a challenge for all types of engineers, especially for civil engineers.In order to provide new and sustainable solutions for different types of structures, we can interfere in the construction of the infrastructures where more than 40% of the total costs of a structure are assigned.One of the purposes of the researchers is to find new technical solutions for deep foundation, aimed to reduce costs, by work technology, possibilities to increase the bearing capacity and reduction of the material consumption.Therefore, to be able to create less environmentally damaging infrastructures, the variable pile foundation, piles that have variable diameters (as increased cross section) along the length and at the base, come as a solution.Constant diameter pile foundations are the usual design solution when it comes to tall buildings.The large number of piles; the increased length of piles in order to reach the good foundation soil; the large diameters of piles, all of these related to loads and to the type of soil can become disadvantages in using such foundation solutions.The variable diameter piles solution, allows an increase in the bearing capacity of the piles for a smaller diameter, by using increased diameters along the pile and at the base (joints), positioned in the good foundation soils.This allows increasing both the friction resistance, by increasing the pile lateral area, the resistance in compression and tension due to the created enlargements.This paper presents the theoretical results obtained by a comparative calculation, using numerical modelling, for a pile foundation in two situations: constant diameter and variable diameter for the piles considering also different soils conditions.The analysis of the results allows us to highlight the benefits of using variable diameter piles for deep foundations design.

Theoretical background of pile foundations
A pile is basically a long cylinder made of a strong material such as reinforced concrete that is pushed into the ground to act as a steady support for structures built on top of it.Pile foundations are used when there is a layer of weak soil at the surface and this layer cannot support the loadings from the building, so it is needed to bypass this layer and to be transferred to the good foundation soil or rock that is below the soft layer ore when a building has increased concentrated loads, such as in a high-rise structure, bridge, or water tanks and they have to be transfer to a soil with high bearing resistance.One of the strongest points of a pile foundation is the increased load carrying capacity compared to spread footings.There are two types of pile foundations which are commonly used and each of them is working in different manners.One of them is the end bearing piles, which have the bottom end of the pile placed on a layer of special strong soil or rock.The load of the building is transferred through the pile onto the strong layer.In a sense, this pile acts like a column.The principal key is that the bottom of the pile bypasses the weak layer and the load therefore it is safely transferred to the strong layer.The second one is the friction piles that transfers the load of the building to the soil around on the full length of the pile, by friction.Therefore, the entire surface of the pile, which is cylindrical in shape, works to transfer the forces to the soil.Figure 1 is presenting these two types of piles and their work principles.Variable diameter piles are considered an improvement of the constant diameter pile.The variable diameter piles solution allows an increase in the bearing capacity of the piles with a smaller diameter, by using increased diameters along the length of the pile and at the base (joints), positioned in the good foundation layers.This allows an increase in both the friction resistance, through the enlarged lateral area of the pile, and resistance in compression and tension, giving the created enlargements.Figure 2 is presenting a view of the variable diameter piles.

Technological innovation for variable pile foundations
The construction of variable diameter piles needs a particular innovative technology.The devices used for building the variable diameter piles are more complex than the ones that are used for constant diameter piles.
In order to build variable diameters along the pile length and at the base, it was needed to improve the usual boring devices with some particular extensions to laterally expand at certain depths in the borehole.Therefore, based on these classic techniques, an innovative technique for building variable cross section piles was developed.The main steps in building either constant or variable diameter piles are drilling, reinforcing and concrete pouring.

Drilling process for variable diameter piles
Drilling is a mechanical process performed with a special boring device.One of the main steps in creating a borehole is to establish the precise location of the longitudinal axis of the future pile and after that a special attention must be paid in positioning the drilling machine in accordance to the precise angle described in the design process (Figure 3).In the step of drilling and soil extraction, for the constant cross section areas of the piles the technological process is the same as for regular pile construction (Figure 4).For preventing the walls from collapsing while the soil extraction is performed, bentonite mud is poured in the borehole to generate hydrostatic pressure.Care should be taken when using the bentonite mud in the borehole, as it is necessary to have special equipment to collect this material after its use in the concrete pouring stage.
For drilling and soil extraction in the variable cross section areas, to the classic drilling device a special element is attached for lateral expansion at the required depth of the position of the increased diameter.
The device is made of two parallel blades which are extended on a certain angle at its sides and through a circular rotation the excavation of the bulb (increased diameter) is created.Figure 5 is presenting the described device.At the base of the pile the blade device is creating another enlargement in the same manner as the intermediary ones were drilled.After drilling the entire length of the pile bentonite mud is poured in the borehole while the drilling device is extracted.

Reinforcement of variable diameter piles and concrete
The reinforcement of the piles is designed in accordance with the European design codes prescriptions.The geometrical shape of the reinforcement is the same as for constant cross section piles. 4 The process of concrete pouring is the same as for constant cross section piles.After the reinforcement is being placed in the borehole, the concrete is poured from bottom to the top, while the bentonite mud is extracted gradually.

Analysed cases
The attention of this study is focused on evaluating the maximum bearing capacities of constant and variable cross section piles evaluated in different types of soil.For these analyses a 20 m long pile with a constant cross section of 1 metre in diameter placed in a uniform soil layer was considered (Case 1).Three types of variable cross section piles were considered in the analyses, the piles having the same length and soil depth as in the case of constant cross sections.For the variable cross section pile cases the same constant diameter equal to 1 m was considered and, in the areas where the bulbs (enlargements) were created the diameter reached 2 m.The variable cross section cases are the following: Case 2 the pile has 1 bulb at the base of the pile, Case 3 the pile has 2 bulbs one at the base and the other one at the middle of the entire length and Case 4 the pile has 3 bulbs, one at the base and the other two equally distributed on the length.In all cases studied the piles were made of C20/25 concrete.
For each of the four studied cases 3 types of soil conditions were considered but the soil is uniform on the entire depth.The foundation soils were modelled using the Mohr-Coulomb model with the input geotechnical characteristics presented in Table 1.

Finite element modelling
All the numerical simulations were performed using the finite element software Plaxis 2D.A nonlinear FEM analysis with standard settings for the iterative procedure was applied considering a gradually increasing load in a range between 0 KN to 800 KN.Due to the symmetry of the structure the models were analysed only on one half.The computation was continued until plastic failure occurred.Figures 6 to 9 are presenting the finite element models with the corresponding foundation soil for each case study.The mesh of the models was defined with a fine dense and refined step in the close surrounding of the pile, since high stresses are expected there.The mesh is further refined at the end points of the variable diameter pile foundation element representing the bulb.Interface elements are drawn beneath the piles to smooth the mesh.Figure 10 is presenting one of the model's mesh.

FEM analyses results
Based on the FEM analyses the maximum bearing capacity was determined for each of the case studies.
In table 2 the results of these analyses are presented.From the table above a comparison in percentage of loads increasing was performed.Therefore, Table 3 is presenting a percentage increase of all case studies of this study focused on evaluating the maximum bearing capacities of constant and variable cross section piles.11 it is noticed that apart from the differences in results influenced by the variable cross section also the foundation soil has a great impact in all analyses cases, as we all already know.Variable diameter piles can be a solution of improving the constant diameter piles.The analysis results from computer calculation with Plaxis 8.2 shows an increase of the bearing capacity of 10 % for a pile with one bulb, 20% for a pile with 2 bulbs and 30% for the 3 bulbs pile compared to the constant cross section.Due to these results the economic efficiency is considerable.This foundation solution could reduce the number of piles needed, their length, and the material cost, without interfering with the strength of the structure.The multiple diameter-expanded parts are expected to also reduce the risk of building settlement [3].
Because of the variable diameters piles are increasing both the friction resistance, by enhanced pile lateral area, and resistance in compression and pull, due to the bulbs, they can be used also for bridge's pile and wind turbines.

Conclusion
• The field of civil engineering is one of the greatest pollutants of the world starting from the material manufacture and to the construction itself.Therefore, the sustainable development of the construction area is needed.It is necessary to realise that natural sources are limited, as foundation soils, and civil engineers have to develop better technological and ideological solutions.It is known that the cement industry is one of the most damaging ones for the environment.The variable diameter piles come as a solution for less concrete use due to the high bearing capacities which leads to a smaller number of piles on the total foundation area.• In this paper it is studied the efficiency of using variable cross section piles compared to traditional ones.From finite element analyses it was noticed that an increase of approximately 10% for each enlargement on the pile is produced in terms of load bearing capacities.Based on this observation one can say that the technological innovation of variable cross section piles brings economical, ecological and technical improvement for a safe environment.• From the economical point of view using this solution leads to a smaller number of piles which implies a small amount of concrete and reinforcement usage.• From the ecological point of view, it brings a reduction of carbon emissions due to the decrease of the amount of resources and building materials.Another important aspect is the decrease of the urban expansion on sites but creating an extension on height.• From the technical point of view it can offer an easier building process due to less boreholes created.

Figure 10 .
Figure 10.Mesh of the model

Figure 11 .
Figure 11.Graphical comparison of bearing capacities

Table 1 .
Geotechnical characteristics of the foundation soil

Table 2 .
Bearing capacities of the case studies

Table 3 .
Comparison in load increase for all cases