The following report presents an analysis of a foundation structure following a geotechnical survey conducted before the start of the project. The proposed project is a six-storey concrete commercial building with a basement loading bay. However, according to the specifications from the architect, there were several errors in the design of the foundation and the choice of foundation. The structural requirements for setting up the foundation will require that the type of foundation selected be sufficient to bear the weight of the loading bay and the upper floor slabs in all the six floors. After a detailed evaluation of the proposed design, the in-house foundation design was found to be inadequate hence the need for further consideration before construction kicks off. The discussion outlines two of the main challenges associated with the nature of the proposed foundation relative to the structure. The presence of an uncontrolled fill and a high sulfate concentration in soft clay silts would be a challenge in setting up the structure with the pad foundation. There are several strength-related issues with the proposed design of the pad foundation likely to impede the proper transfer of loads to the ground. In a bid to ensure safety in the proposed model concerning requirements, the report suggests an alternative design for the foundation with consideration of the two challenges. The proposed alternatives are accompanied by detailed drawings of the new foundation design and the necessary adjustments for the proposed plan.

Introduction

While fill is a large factor relative to the construction of residential foundations, it forms a critical role in ensuring the serviceability and strength requirements are achieved. The foundation creates the core control for differential movement due to non-uniformity in subsurface conditions. According to Khan and Kazimi (1993), most foundation failures occur where shallow foundations are prevalent over random fill. The proposed six-storey project involves the set-up of a six-story in-situ concrete story structure with a 100mm thick loading bay and upper columns. The proposed foundation was the use of a pad foundation with the erection of circular columns to the third and sixth storeys. Inline to the standard requirements for structural design, the chosen foundation may be insufficient in accommodating the loading and the stabilization of the structure. The geotechnical survey report indicated the presence of uncontrolled fill and soft clay silts with a high concentration of sulfates. The proposed facility is set to have a loading bay for loading and unloading of goods hence traffic dominated by heavy articulated trucks. Subsequently, the foundation used will require sufficient strength requirements to be met to facilitate this. Besides, the front wing of the loading bay has a minimum slab thickness of 1000mm of concrete. This implies the need for sufficient transmission of axial forces to the ground from the columns holding the building.

Main challenges

Following the detailed assessment of the geotechnical report, it was reported that the proposed pad foundation for the project would not be sufficient. Taking into consideration the load-bearing structure with six storeys above, there were several challenges linked with the structural design of the foundation.

Uncontrolled Fill

The presence of an uncontrolled fill on-site as indicated on the geotechnical report is a risk in meeting the satisfactory level of design. Unlimited fills are affiliated with variable bearing capacities and non-uniform settlement which is likely to cause harmful damage. Common adversarial factors such as small boulders, bundles of brush, drums and toxic substances are often associated with uncontrolled fills (Chapagain, [2016]. These call for control due to uneven collapse, the attraction of termites and corrosion of plumbing pipes, all of which are likely to cause damage. Foundations need adequate compaction subject to detailed investigations on their suitability in supporting a structure as the proposed building.

High concertation of sulfates

Another major threat to the designed foundation is the presence of high sulfate concentration in soft clay silt on site. According to research, sulfate-concentrated soils with aluminium silicate minerals are reactive to lime and water used in construction. These are related to expansion problems where the reaction of calcium and sulfate compounds form calcium-aluminate-sulfate hydrate chemicals (Yao et al., 2020). These can cause swelling due to high pressure during expansion, breaking along the joints of the foundation. Many areas where clay contains sulfides and sulfates have high levels of weathering and leaching, which affect the type of foundations set up.

All available site information is therefore needed for review in making decisions on the type of foundation to adopt before the commencement of all relative design works. Following the geological evaluation, the report indicates that the intrusive samples tested showed a high concentration of sulfates in the soils on site. The problem with these levels is the resultant reaction upon addition of cement which contains lime. The result affects capped layers on structures which causes the stabilized materials beneath ground-bearing floors to wear off. Under wet conditions and a range of temperatures, the reaction leads to the formation of ettringite. The complex compound is subject to uneven expansion and contraction changes which affect the volume of the foundation. After testing the slab foundation for penetration through the fill, the load-supporting elements were found to be insufficient in holding up the structure within the unreliable fill. These conditions prompted the reevaluation of the design and better alternatives for the foundation. Residential constructs need to be approved where pre-existing uncontrolled fill is on-site with consideration of the level of shallowness, presence of trash, age and results of proof rolling.

Foundation Options

Concerning the foundation mentioned earlier challenges, an alternative design will be necessary for the proposed building. The presence of uncontrolled fill indicates the need to change the foundation design to suit the construction. Foundations designed based on bearing capacity and settlement calculations often depend on the fill and quantity of fill in underlying soils. While fills above the surface help in increasing the load on underlying grounds, there is a need to have a controlled fill for foundations. For the proposed project, the following foundation approaches were proposed as alternatives for pad foundation.

Pile foundation

The pile foundation involves the installation of a strong material-based cylinder driven into the ground to hold structures above it. Given the situation on-site where the fill is uncontrolled, the use of pile foundation would help transmit the weight of the building bypassing the layer of uncontrolled fill to the deeper levels (MAITY and CHATTOPADHYAY, 2017). Also, considering the loading bay made of thick material and for heavy traffic, there is a need to transmit the concentrated loads effectively beyond the uncontrolled fill layers. For instance, driving end-bearing piles on layers of rock and compact soil will ensure the better transfer of loads from the building under the principle that the bottom end rests on the surface where the weak and stronger layers intersect (Zhao et al., 2017). Also, the use of friction piles could facilitate the transfer of loads from the building to the soil across the full height of the pile through friction. In the case of the proposed building project, the use of cylindrical concrete piles or steel piles would be suitable to enhance the load-bearing transfer.

Raft foundation

The use of raft foundation is appropriate where soils are weak and have poor compaction. A continuous slab resting extensively on the entire footprint of the building supports the weight of the building and transfers it onto the ground reducing the stress on the soil. Also, the use of raft foundation will be suitable for the basement construction where foundations will be created in a strong compact and controlled soil depth. The use of raft foundation will help in adjusting the concrete poured according to the desired thickness as much as the loads within. A waterproofing membrane would be necessary to ensure the protection of the concrete (Sinha and Hanna, 2017). Also, waterproofing would help to chemically isolate the building since the soul contains chemicals that could affect the concrete negatively. This would be essential considering the high sulfate concentration in the fine clay silts within site. The development of raft foundation would be critical to the basement construction and the support of the tall structure.

Additional Protective Measures (APMS)

Studies have indicated that the design of foundations based on bearing capacity and settlement constraints can be carried out while observing fill requirements. The bearing capacity of underlying foundation soils determined the compaction requirements for fills and provided the stricture is designed to accommodate settlements (Das and Dey, 2020). The site preparation processes are essential in ensuring proper compaction and placing for fills. Also, site-related problems, as identified in the report, should be addressed through criteria with environmental considerations being prioritized. Adverse weather conditions often affect the costs of operations in compaction and filling hence there should be mitigation measures

Several important factors must be considered in designing foundation structures overfilled grounds as on the proposed site. Firstly, delineation of fill extent and condition through test borings performed at different depth levels and other conditions (Myburgh, 2018). Additionally, the potential of contamination depending on the hazardous nature of the soil in its environment. The various foundation options available will be determined by the conditions of filling and compacting in the area. The architect’s knowledge of the foundation requirements will influence the choice of material and the type of foundation constructed. For this reason, there should be qualifying recommendations based on the variability of existing subsurface conditions and specifications. Uncontrolled fills on grounds on-site require certain conditions in foundation compaction so that the foundation soil is enhanced to the degree necessary. This often requires that all differential movements are controlled, giving the foundation the advantage of distributing the foundation loads. Concerning the high concentration of sulfates, the use of sulfate-resistant cement would be helpful. This would help stabilize the soils against reaction with lime. This would reduce the volume change of clay soils due to physio-chemical properties.

Solutions

Following the above analysis, the design of the alternative foundation should encompass both pile and raft foundation. Pile foundation will ensure efficient transfer of axial loads from the columns supporting the storeys to the ground beneath the loose soils in the sub-surface. For the pile foundation, pile reinforcements would be driven into the ground as the concrete is cast on-site while ensuring that the depth goes beyond the subsurface. The end-bearing piles would then be allowed to cure before laying the tie beams upon which columns for the building will be erected. Also, the loading bay would require the set-up of a raft foundation which would stretch throughout the basement. Due to the poor conditions of filling, the raft foundation extending throughout the footprint of the loading bay would help support its weight and transfer the forces to the ground. The raft foundation would require adequate reinforcement to accommodate the 1000mm thick concrete floor slab supporting the bay.

Conclusion

Given the above evaluation, the discussion successfully identifies alternative designs for the proposed building foundation. Conclusively, the use of pile and raft foundation designs would efficiently tackle the design challenges. The poorly compacted fill would be insufficient in transferring the loads from the building hence the need to sink piles into the substratum to help increase the resistance. Also, the use of raft foundation would successfully carry the loading bay slab throughout the basement. The basement should be thoroughly waterproofed to prevent the entry of water. Also, the use of sulfate-resistant cements will be essential in countering reaction with lime due to the high concentration of sulfates. This would protect the foundation from irregular expansion of clay which would cause damage to the structure. The use of the two foundations with observation of the protective measures as observed earlier would help counter the challenges in the design. The two alternatives are effective control mechanisms in dealing with the challenges identified in the geotechnical survey.

References

Chapagain, N.K., REFLECTIONS ON THE BUILT ENVIRONMENT AND ASSOCIATED PRACTICES, VOL. 2 [2016].

Das, M. and Dey, A.K., 2020. Use of Soil Cement Bed in Improvement of Load Carrying Capacity of Stone Columns. Geotechnical and Geological Engineering, pp.1-22.

Khan, I.H. and Kazimi, S.M.A., 1993. Failure of buildings founded on fills.

MAITY, J. and CHATTOPADHYAY, B.C., 2017. GROUND IMPROVEMENT TECHNIQUES. PHI Learning Pvt. Ltd..

Myburgh, K.S., 2018. The minimum site investigation requirements needed to define site conditions considering the results of ground investigations and its true reflection of actual site conditions found during construction (Doctoral dissertation, Stellenbosch: Stellenbosch University)

Sinha, A. and Hanna, A.M., 2017. 3D numerical model for piled raft foundation. International Journal of Geomechanics, 17(2), p.04016055.

Yao, Y., Wang, W., Ge, Z., Ren, C., Yao, X. and Wu, S., 2020. Hydration study and characteristic analysis of a sulfoaluminate high-performance cementitious material made with industrial solid wastes. Cement and Concrete Composites, p.103687.

Zhao, H.Y., Jeng, D.S., Liao, C.C. and Zhu, J.F., 2017. Three-dimensional modeling of wave-induced residual seabed response around a mono-pile foundation. Coastal Engineering, 128, pp.1-21.

CONSTRUCTION TECHNOLOGY & STRUCTURES

Pssst… we can write an original essay just for you.

Executive Summary