The use of polymer slurry as an economic and sustainable way to build diaphragm walls – a case study of a construction work in São Paulo/Brazil

The Civil Construction Industry is one of the first economic sectors to react to the stimulus of favorable economic conditions of a country. In such a scenario, new tools for handling and reusing solid waste from construction sites have been extensively discussed in technical conferences. This paper is a case study of a construction work in the city of São Paulo, Brazil where a polymer slurry was used instead of a bentonite-based one as stabilizing fluid for excavation work on diaphragm walls. Based on the evaluation of the data obtained in the cited case study, it was possible to conclude that the use of polymer slurry in the excavation of diaphragm walls is technically and economically feasible, besides presenting several environmental advantages in comparison to the use of bentonite slurry.


Introduction
According to the Brazilian Institute of Geography and Statistics (IBGE, 2011) the Brazilian Civil Construction sector showed a growth rate in 2010 of 11.6% -the best result for the last 24 years. Furthermore, this sector generated more than 329,000 formal jobs. It is estimated that it accounts for investments surpassing U$ 45 billion a year. The demand for labor has also grown accordingly, generating 62 indirect jobs for every 100 direct jobs.
Nevertheless, although civil construction has an important social role, helping to directly reduce the home construction and infrastructure deficits, which are crucial factors for a country's development, it is also responsible for the high consumption of natural resouces, once most of the raw material used in construction is extracted from natural deposits. According to Dias (2004), around 45% of the resources in Brazil extracted from nature reserves are utilized in construction.
Although not always obvious for ordinary citizens, the quantity of waste generated by construction activities represents a highly significant proportion of urban waste. It is estimated that construction and demolition waste (CDW) may account for 50% of the waste generated by some municipalities in Brazil (ULSEN et al., 2010). In the city of Salvador (Bahia), CDW represents 45% of the total volume of urban waste (UW) generated daily (AZEVEDO et al., 2006), whereas in the cities of São Paulo (SP) and Rio de Janeiro (RJ) it is only 21% (GOMES et al., 2008). Morais (2006) presents data related to a few medium-sized and large cities in Brazil in which the mass of CDW, as a percentage, varies from 41% to 70% of the total mass of urban solid waste.
Currently, it is increasingly common to hear discussions about sustainability in the construction industry and about environmental manage-ment tools for this sector. The concept of sustainability extends beyond practices related to the preservation of natural resources; it also comprises several different areas such as sociology, economics, politics, ecology, culture, and environment, to name just a few.
Due to the growth in the construction industry and the recognized value of management tools to reduce its impacts on the environment, sustainable processes are becoming increasingly widespread and gaining more relevance in current construction sites. The fact of choosing sustainable practices to build a facility can affect positively all the other sectors involved in the construction process, resulting in lower costs of services and materials, as well as providing a better quality of life for the labor force and everyone else who benefits from the building itself.
In building construction there are several sustainable technologies and products available in the market that are increasingly being used in Brazil. The use of polymer as a substitute for bentonite to stabilize excavations in the specific task of building diaphragm walls could be cited as an example.
It should be stressed that the practice of building diaphragm walls is widely used in Brazil, once most buildings feature several floors which are below ground level, and that this kind of walls is needed to resist the thrust of the soil against the structure and to ensure the stability of the underground floors.
Even though it is more frequently used than polymers, bentonite slurry causes severe impacts on the environment, because the waste generated from its use, besides being a contaminant, is very difficult to be disposed of. On the other hand, the polymer has the advantage of being biodegradable, which makes it easier to dispose of and reduces the environmental impact caused by the excavation of the diaphragm wall.  During the process of building a diaphragm wall some special equipment is needed, such as a cutting auger, an auxiliary crane, slurry reservoirs, and suction and injection pumps.
According to Saes et al. (1998), diaphragm walls present better performance than other kinds of contention for the following reasons: a) they can be implemented in almost any type of terrain without needing to lower the groundwater table; b) they form a watertight piece, therefore avoiding the percolation of water into the excavated ground; c) they adapt better to the perimeter of the containment and can be used without any type of shoring.
As the soil is excavated, the drilling mud is introduced into the trench and pumped to the reservatory. The drilling mud efficiency will depend on its physical and chemical properties, that is, density, viscosity, gel consistency, control of the filter and plaster, and inhibition of the hydrating clays (LUMMUS; AZAR, 1986).
Among the drilling muds used to stabilize excavations, bentonite slurry is the most used.
The purpose of this drilling mud is to counter the thrust from soil and water, avoiding the collapse of the faces of the trench during the excavation work. Bentonite slurry has a stabilizing characteristic because it forms a layer similar to a gel, called The use of polymer slurry as an economic and sustainable way to build diaphragm walls -a case study… "cake," that penetrates into the voids of the soil, making the wall more uniform and offering better stability (CARDOSO; SHIMIZU, 2002).
Although the stabilization of perforations using bentonite slurry is the most commonly-used method, currently there are several restrictions to its use by norms from environmental and labor health governmental agencies. In addition to causing environmental impacts, such as soil contamination, bentonite can also affect the health of people working directly with this kind of clay.
According to Brazilian Technical Standard    Figure   3 shows a view of the construction site.

Achieved results
Before building the diaphragm wall, a study was performed to identify the best alternative to attain the stabilization of the excavation that had to be done. For this purpose, two alternatives of drilling muds were considered: bentonite and polymer slurries. The costs were obtained based on market tenders in São Paulo in 2011 during the construction process.
The Tables 1 and 2 show the comparison between the two alternatives that were evaluated for the present study.   Total volume of contaminated soil to be disposed of (m 3 ) 836,07 Total volume of non contaminated soil to be disposed of (m 3 ) 2,508.20 Total cost of non contaminated soil to be disposed of (US$) 53,675.50 Total cost of contaminated soil to be disposed of (US$) 45,983.80 Total cost of the total generated soil to be disposed of (US$) 99,659.35 Source: The authors. Total volume of contaminated soil to be disposed of (m 3 ) 0.00 Total volume of non contaminated soil to be disposed of (m 3 ) 3,344.27 Total cost of the total generated soil to be disposed of (US$) 71,567.38 the final disposal site are lower, as can be seen on Table 2 that follows.
Comparing Tables 1 and 2, it is noticeable that the disposal of the soil waste generated by the excavation using polymer slurry is 28% lower than the cost calculated for the disposal of the soil when the bentonite slurry is used. This is due to the fact that the soil resulting from the excavation using polymer slurry is not considered as contaminated material; therefore, it does not require a special landfill to be disposed of.   Table 3.
On Table 3, it can be seen that, considering the volume of excavation needed for the construction work, the consumption of bentonite clay was 18 times higher than the consumption of polymer.
The study also contemplated the costs comprised    amounting to US$ 31,904.42. It should also be pointed out that even presenting a cost five times lower than that of the bentonite slurry, the cost of the transportation and disposal of its waste is much higher than for the polymer slurry. Figure 5 presents a comparison between the costs for both alternatives presented per area of diaphragm wall to be built. b) The data that were obtained indicated that the use of polymer slurry as a drilling mud provided an economical alternative, even though it is a little more expensive (6%) than bentonite slurry, due to the environmental advantages it offers. Being a biodegradable product is a relevant aspect due to the environmental issues that are increasingly being faced by the construction industry; and in the case of the use of the polymer slurry, it was possible to reduce the quantity of excavated soil that needed to be disposed of and transported. In addition to this, for the same volume of soil to be excavated, the amount of material consumed was lower compared to what was needed if using bentonite slurry. It is important to highlight that the implementation of practices and the use of products that cause a low environmental impact, such as the use of polymer slurry, is one of the requirements to be met for a construction site to get Green Building certifications, such as LEED and AQUA.