1 Introduction
1.1 Background Information
This proposal has been reported in response to the request for developing engineering solutions to strengthen soil to prevent differential soil settlement under structures.
With the Singapore government turning to underground spaces to reduce land demand in Singapore, one can expect the number of underground works to increase. However, underground works have been synonymous with damages to nearby existing structures. This is due to differential soil settlement, a phenomenon that occurs when the soil settles at a different rate. If this happens under a structure, cracks can occur on the structure due to uneven distribution of loading into the soil. This can cost the contractor resources as compensation may be required. To prevent such issues from worsening, measures such as soil strengthening can be put in place when identified. One such soil strengthening method can be to cycle a solution of calcium and biocementation bacteria across the foundation of a structure, with the bacteria providing strength to the soil by precipitating calcite to cement the soil particles together. Successful implementation of the method on 100m3 of sand confirmed that microbially induced calcium carbonate precipitation (MICP) is a potential solution for bulk soil strengthening, and could be of wider acceptance in the future.
1.2 Problem Statement
The ideal soil would be homogenous and share similar engineering characteristics in different parts of the soil. However, soil in reality has highly variable characteristics, and hence reacts differently under various circumstances. Through cycling a solution of calcium and biocementation bacteria through the soil, it is hoped that increased cohesion between soil particles will combat uneven settlement when exposed to agitation from nearby underground works.
1.3 Purpose Statement
The purpose of this report is to propose to the Land Transport Authority on the adoption of bio-cement grout for soil stabilisation for structures which may be affected from the underground works.
2.0 Benefits of Proposed Solution
2.1 Cost-Effectiveness
Biogrout/Bio-cement is calculated to be much cheaper when compared to chemical grouting techniques. Ivanov and Chu (2008) calculated that raw materials in chemical grouts range from $2 to 72 per m3 of soil, compared to bio-grout in the range of $0.5 to 9.0 per m3 of soil, in cases where waste materials are used as the source of calcium for bacterial growth. On top of that, the bacterial enzyme solution can be reused up to 3 times before losing functionality hence decreasing the cost of treatment in the long run.
2.2 Reduce the impact of differential settlement under existing structures.
The lower viscosity of biocement solution as compared to the conventional cement mix will allow seepage of solution into and through the soil, thus enabling a more extensive coverage of land exposed to bulk cementation under existing structures. This increased area of influence then addresses differential settlement by cementing the soil as a whole, thereby reducing the magnitude of damage caused to structures by the phenomenon.
2.3 Environmentally friendly:
Soil stabilization using MICP promotes the concept of sustainability through the use of natural elements such as micro-organisms as the principal source for cementation. Although part of the end product of urea hydrolysis is ammonia, which may be deemed to be detrimental to groundwater, ammonia could be fed back into the surrounding soils as a fertilizer if proper plans and precautions are taken. Moreover, the cementation between the soil particles does not permanently alter the subsurface conditions of bio cemented soils, unlike traditional soil stabilization methods. The mechanism for soil stabilization also consumes CO2 rather than produce it. Making bio grouting a viable solution for stakeholders with concerns about sustainability and environmental responsibility.
3.0 Proposal Evaluation
In this section, the feasibility of the proposed solution will be evaluated and discussed.
The proposed solution of implementing Biocement technology to strengthen the soil of existing buildings. Due to underground construction for example underground Mass Rapid Transit (MRT), nearby existing buildings will be affected by the vibration of the construction and cause differential soil settlement. Differential soil settlement would cause cracks in the existing buildings. Hence biocement could be used to cement all the soil under the foundation together to prevent more cracks from happening at the same time strengthening the soil.
Case study: Bugis, Brash Besar Building.
Bras Basah-Bugis district heritage building and old shophouses have very large and visible cracks on the facade. This is due to the Mass Rapid Transit (MRT) construction of the New Downtown Line which causes differential soil settlement. Figure 1 shows the cracks on the facade at Bras Basah-Bugis district.
Figure 1: Cracks forming at shophouse near Bras Basah-Bugis district.
According to experts, buildings in the southern part of Singapore are more prone to have cracks in their building due to softer soil condition in the region.
Figure 2 shows the geological map of Singapore. At the southern part of Singapore, it mostly consist of Kallang formation.
Kallang Formation is an alluvial, littoral and inshore sediment that have been laid down from late Pleistocene to Holocene period. It is one of the young deposits and could be up to 55m deep. Kallang Formation includes marine clay, fluvial clay, fluvial sand and estuarine. As marine clay/estuarine is very soft and highly compressible, large consolidation and settlement can be expected due to additional loads or water drawdown.
Hence when there is underground construction at the southern part of Singapore, it is more likely to have differential soil settlement hence affecting existing buildings. Bio-cement can be implemented at these areas where soil settlement are more likely to happen.
Figure 2: Singapore Geology map
4.0 Limitations
4.1 Lower Strength
Biocement concrete will typically have lower strength standalone compared to conventional concrete cement. In the research article “Geotechnical Properties Of Biocement Treated Sand and Clay”, Li Bing (2015) states that a 5 days of biocement gives a range of unconfined strength, from 10 to 1400 kPa with 2 to 9% of calcite. (Refer to appendix 1a) However, a typical concrete cement has 16250 kPa in only 7 days. This limitation is very important if biocement concrete was used as a stand alone as it has a huge difference in strength.
4.2 Production of Ammonia
As biocement uses Microbially Induced Calcium Carbonate Precipitation (MICP) process for bacteria to process the cementitious material. This process is driven mainly by an enzyme, urease. Urease will produce an excessive amount of ammonia into the environment. According to Newyork state health, too much ammonia will cause burning to eyes and lung issues. As a result, despite being environmentally friendly in the long term, sometimes it will still produce a much higher amount of ammonia compared to traditional cement which will be harmful to the environment. Thus, the production of ammonia is also a limiting factor for the use of biocement.
5.0 Problems in stakeholder’s point of view
5.1 Adaptability of Bio-Cement
Bio-cement has not been implemented in Singapore, more research needs to be done, general public and government can perceive this technology as it yet to have any successful usage. More manpower will also be needed to implement the technology.
5.2 Immaturity of technology
Bio-cement, first discovered by Hendrik Marius Jonkers in 2015, is new technology. Founded barely 5 years ago, there are still too many uncertainties and too little time with the technology to be able to assure stakeholders that bio-cement is able to reliably provide the benefits that it offers. For instance, Portland cement took approximately 26 years from its inception by Joseph Aspdin in 1824 to widespread use in France between 1850 to 1880. While advances in science and technology shorten the time needed to understand the product sufficiently for safe usage, the industry is still not very keen on a relatively new and unproven material to which lives are to be entrusted to.
5.3 Profitability
Bio-cement is cheaper than conventional concrete cement by a few cents despite having a very large amount of compressive strength difference. However, it will still be profitable. It is because due to the scales of the construction industry, a few cents can end up to a very big figure.In the long run, bio-cement will help the building to be more sustainable and this will reduce the maintenance cost of the buildings.Stakeholders and main-contractor do not need to worry about only saving a few cents but end up forking out a large amount of money for maintaining the structure. In contrast, they might save more money due to the cut on maintenance cost.
6.0 Market Research
6.1 Interview question
What do you think of implementing biocement in strengthening the soil underneath existing structures
Does the construction of underground MRT lines affect buildings from your experience.
During soil assessment and building damage prior to the commencement of the mrt lines, has there been instances where certain structures were highlighted to have a possibly large damage.
6.2 Professional Opinion
The interview with Dr.Kum was done to get feedback on the feasibility of our solution. Soil settlement issues as well as the measures in place by LTA during the construction of MRT lines was covered.
After having an interview with Dr.Kum, a trained engineer with experience in the construction industry, we came to discover a lot regarding the underground built environment. Differential settlements due to mixed foundations were covered alongside issues relating to the feasibility of the project.
Throughout the interview, we had a delightful exchange with Dr.Kum and discussed extensively the possible issues that could affect our project. After we explained our modus operandi, Dr.Kum was quick to point out potential problems and ended by stating that our project might be feasible if the issues mentioned were addressed.
This interview provided our group with valuable information regarding the operations and measures in place by LTA before the commencement of construction.
Real-world failures not generally shared with the public was discussed and highlighted the possibility for our solution to address specific issues of existing structures during the construction of MRT lines. In all, the interview reaffirms the need for our project, helped demystify possible areas for the implementation of our solution as well as highlighted specific issues that were previously overlooked.
7.0 Conclusion & Recommendation
In conclusion, despite having numerous limitations when it comes to using bio-cement, there are more benefits that outweigh the cons. For instance, even though there will be a high amount of ammonia being produced during the process, it is still considered environmental friendly as it does consume carbon dioxide. In addition, with the help of Dr Kum’s professional opinion, there will be some challenges when implementing biocement. However, he is positive that this will be feasible if we are able to solve the potential issues.
