Recommended Foundation Design For Five Storey Steel Framed Student Accommodation Building

Given the investigations and testing done to date, you are to offer your client a recommended foundation design suitable for the client’s proposed five storey steel framed student accommodation building. This design must be communicated both descriptively and in the form of sketch drawings which depict a ‘typical section’ through the foundation. The design must respond specifically to the following factors:

• The building will have a basement to allow for a student common room and a mechanical plant room only.
• Borehole analysis has shown the site is covered with man made fill material consisting of brick, ash and organic waste up to 1.4 m deep. This lies on top of a peat layer up to 7m deep. Below the peat is bedrock. No strength tests have been done.
• The proposed building is to be constructed within three metres of existing buildings which have their foundations typically 1m deep below ground level and are founded on silty clay.
• A general indication of foundation depth is to be given but not the specific diameter or spacing of any proposed piles etc.This report will be used to help calculate the likely costs of site preparation and the construction of a five storey steel framed student accommodation building up to ground level. The information you give will also help the developer to appraise the impact site that conditions will have upon the erection of the superstructure.

Factors Considered in the Foundation Design

Foundation design is very essential for any type of civil engineering structure such as buildings, bridges, offshore platforms, power plant stations, etc. (Letsios, et al., 2014). The foundation in this project is expected to provide adequate support for the proposed five storey steel framed student accommodation building and transfer the total load of the building to the ground.

Considering the role and importance of foundation in this kind of building projects, it is crucial to conduct relevant site investigation studies so as to gather information that will help in designing the best type of foundation and construction method. The information helps engineers and designers to identify the most suitable foundation type for the project in that particular site, the materials to use, strategies of preventing possible defects, and methods of reducing the cost of the foundation.

The most suitable foundation design recommended for the proposed five storey steel framed student accommodation building in Preston city centre brownfield site is pile foundation. Pile foundation has been selected in consideration of the ground or site conditions, nature of load of the building, constructability, cost, closeness to other structures, sensitivity to vibration and noise, presence of water, accessibility and durability of materials. Pile foundation are usually used in areas where the soil strata near the ground level is not able to support heavy loads whereas the hard rock strata is at a depth of between 5m and 50m below the ground surface.

The key factors that have been considered for the selection of pile foundation as the most suitable type of foundation in this project are: the proposed building is within three meters of existing buildings whose foundations are on silty clay soil and typically one meter deep below hence pile foundation will minimize noise and vibration during construction; the building will have a basement where a mechanical plant room and student room will be situated thus pile foundation will enable provision of these elements;

it has been found from the borehole analysis conducted on the site that there are manmade fill materials on the site comprising of organic waste, ash and brick up to 1.4m deep below which lies a 7m deep peat layer (this means that a depth of about 8.4m below the ground surface is soft or weak strata that has weak bearing capacity thus not able to support the building making pile foundation most suitable); and the groundwater level during winter and summer is 1.4m and 2.5m below ground surface respectively (hence the water level in the subsoil is high and also fluctuates significantly between seasons) . The building is also five-storey hence the foundation will typically be supporting heavy loads.

Pile foundation is one of the types of deep foundation that is most suitable for storey or high-rise buildings with heavy loads or located in areas with weak top soils (Poulos, 2016). The foundation has the ability to transfer heavy loads exerted by the structure to a stable soil or hard rock strata that is deeper into the ground (Fleming, et al., 2009). The foundation basically comprises of a pile, pile cap and then a column built on top of it to carry the load as shown in the sketch in Figure 1 below. The pile cap takes the load from the building’s superstructure and then distributes it equally to the number of piles that transfers the load to the ground. Besides transmitting the load to the ground, the piles also resist uplift, lateral and vertical load.

The Recommended Foundation Design

The depth of foundation piles is very essential in this project considering the information obtained from the geotechnical studies on the site. Approximately 8.4 meters below the ground surface comprises of soil with low bearing capacity. This basically means that the depth of the foundation should be at least 8.4 meters (1.4m deep manmade fill material layer and 7m deep peat layer). Figure 2 below is a sketch showing the section of pile foundation for this project. The sketch shows the column of the building resting on the pile cap of two piles. The pile cap has reinforcement that to increase its strength. The pile foundation extends from the ground surface through the soft soil to the hard rock where the load from the building is distributed.

It is important to note that the building in this project has a basement. This basement is built on top of the pile foundation. This basically means that the basement columns and walls are resting on the pile cap of the piles. The height of the basement is taken as 2.5m and the basement is entirely below the ground surface. Considering this scenario where a depth of 2.5m below the ground surface is taken up by the basement, it means that top of the pile cap will start at this depth. The depth of the pile cap is 1m hence the remaining depth of the soft soil is 4.9m. The depth of the pile in the hard rock or stable soil can be 2m. This means that the required length of the piles to be used in this project is about 7m. The section of the pile foundation for this project is shown in Figure 3 below.

Depending on cost, availability of materials, equipment and personnel, availability of site space and accessibility to the site, the piles can either be concrete piles (cast in-situ or precast), steel piles (hollow or H/I-section piles) or composite piles (steel and concrete). Each of these categories of piles has its advantages and disadvantages hence the consultants will involve the client in making the final decision on what type of piles to use for the project.

After identifying the most suitable type of pile foundation for this project, the next step is to determine the appropriate geometric properties of the piles. This includes the shape of the pile (square, rectangular, rectangular, box, tube, etc.), dimensions (width, breadth or diameter), spacing, etc. (Salem, 2016). Each of these parameters has a significant effect on the effectiveness and efficiency of the piles to perform their intended functions. The piles should also be protected properly to prevent damage by factors such as water penetration (Kaneko, et al., 2018).

Basement

Basements have become very crucial elements of modern buildings both for residential and commercial use. These valuable spaces play a very key role in improving energy efficiency by reducing space heating and cooling costs (About Home Design, (n.d.)). The basement also provides structural stability and support of the building. The basement in this project will be used as a storage space to accommodate larger utilities (mechanical plant room) and also as a functional living space (student’s common room).

The Role of Basement

The basement in this project will have adequate waterproofing properties to protect the internal environment of the building. Considering the site of the project and the findings from studies carried out on it, the suitable type of basement construction for the building is tanked protection or type A basement construction. This construction basically comprises of a water resistant membrane or layer that is installed to the basement’s external face as shown in Figure 4 below The external water exerts pressure on the membrane thus forcing it against the basement structure and creating strong resistance against water penetration (First In Architecture, (n.d.)).

The basement will play a very essential role in protecting the building against water penetration from below the ground. The basement can be constructed from poured concrete, precast panels or masonry or concrete blocks. Poured concrete is the commonest and most preferred type of basement by many people. Construction of poured concrete basement starts by pouring the basement foundation’s footing, installing formwork to hold the basement walls, pouring the basement wall concrete and allowing them to dry. Concrete is considered the best material for basement construction because it is the most durable, provides the minimum number of joints thus reducing the likelihood of water penetration through cracks and joints, it is permanently and totally repairable in case it fails, and it can be made watertight (Goldberg, 2017).

Masonry or block wall is the most economical basement wall. The basement wall is simply made of masonry or blocks that can be reinforced with steel rebars. Construction of masonry or block basement wall also takes very minimal time. Precast panel basement wall is becoming very common today as stakeholders in the construction industry focus on reducing time, wastage and cost of construction and also improving efficiency.

This method involves molding walls in an offsite facility and transporting them to the site where they are installed on ready foundation (My Foundation Repairs, 2018). Timber is also another material that can be used in construction of pile foundation (Civiltoday.com, (n.d.)). The final choice of material will be decided by the client in consideration of the project needs, client’s preference, availability of materials, ease of construction, cost, etc.

References

About Home Design, (n.d.). Building a Basement Do it Right the First Time. [Online]
Available at: https://www.about-home-design.com/building-a-basement.html
[Accessed 28 November 2018].

Civiltoday.com, (n.d.). What is Pile Foundation? Types of Pile Foundation. [Online]
Available at: https://civiltoday.com/geotechnical-engineering/foundation-engineering/deep-foundation/176-pile-foundation-definition-types
[Accessed 28 November 2018].

First In Architecture, (n.d.). Basement Construction Details - Type A. [Online]
Available at: https://www.firstinarchitecture.co.uk/basement-construction-details/
[Accessed 28 November 2018].

Fleming, K., Weltman, A., Randolph, M. & Elson, K., 2009. Piling Engineering. 3rd ed. Londoon: Taylor and Francis.

Goldberg, R., 2017. How to Create an Effective Basement. [Online]
Available at: https://www.houseplanninghelp.com/hph163-how-to-create-an-effective-basement-with-andrew-mcinulty/
[Accessed 28 November 2018].

Kaneko, O; Kawamata, S; Nakai, S; Sekiguchi, T. & Mukai, T., 2018. Analytical study of the main causes of damage to pile foundations during the 2011 off the Pacific coast of Tohoku earthquake. Japan architectural Review, 1(2), pp. 235-244.

Letsios, C., Lagaros, N. & Papadrakakis, M., 2014. Optimum design methodologies for pile foundations in London. Case Studies in Structural Engineering, 2(1), pp. 24-32.

My Foundation Repairs, 2018. Types of Basement Construction. [Online]
Available at: https://www.myfoundationrepairs.com/types-of-basement-construction/
[Accessed 28 November 2018].

Poulos, H., 2016. Tall building foundations: design methods and applications. Innovative Infrastructure Solutions, 1(10), pp. 1-51.

Salem, L., 2016. Effect Of Pile Spacing On The Behavior Of Piled Raft Foundation Under Free Vibration And Earthquake. Australian Journal of Basic and Applied Sciences, 10(12), pp. 240-247.