Geofoam: A potential for Indonesia’s soil problem IV - Update on Geofoam Application

Geofoam is a type of geosynthetic that is used to replace soil fill. Geofoam’s unit weight is only 1-3% of compacted soil. Therefore, it is suitable for constructing embankment over soft soil to reduce settlement. Geofoam can also be used to extend slopes without causing instability. Due to its lightness, construction of geofoam can be done quickly with little to no help from heavy machinery. Hence, geofoam is beneficial not only in terms of solving geotechnical problems, but also in reducing carbon dioxide emission. Geofoam has been used for over half a century in the construction history. However, adoption of geofoam in Indonesia’s construction industry is minimum until recent years. 2022 marked the first use of geofoam for highway project in Indonesia. The highway embankment was to be built on problematic soil, clay shale. Initial attempt to construct the embankment with soil fill triggered slope failure. Finally, geofoam was chosen to be the only solution that can be used to solve the problem.


Introduction
Climate change is caused by increasing amount of greenhouse gases present in the atmosphere, trapping heat from the sun, warming up the earth's temperature [1].Amongst the greenhouse gases, carbon dioxide (CO2) is the biggest contributor, making almost 75% of the total greenhouse gas emission [2]. Figure 1 shows the CO2 emission in Indonesia from 1990 to 2020 [3].In the span of 30 years, CO2 emission in Indonesia has grown almost 300%.The rate of increase in CO2 emission is not equal to the population growth, which grows 49% in the 30 years' time [4].The difference in rate of increase is reasonable.As technology advances, human consumes energy at a faster rate.Figure 2 shows CO2 emission per capita in Indonesia.There is a growth of more than 150% from 1990 to 2020.Also in the figure, percent of world population and percent of global CO2 emission are shown [5,6].Overall, there is negligible change in Indonesia's share of the world's population (3.4-3.5%).On the other hand, Indonesia's share of CO2 emission has increased from 0.7% to 1.64% in the same time period (235% increase).The difference in population growth vs.CO2 emission growth is an indicator of economic growth.This is because economic growth has been considered to have positive correlation with CO2 emission [7].A good example is the United States, world's largest economy [8], having a CO2 emission of 15.32 tons per capita vs. Indonesia's 2.02 tons per capita [9].
In late 20th century, there were increasing awareness in the adverse effects of greenhouse gases.In 2015, the Paris Agreement was adopted by 196 parties at the United Nations Climate Change Conference [10].The agreement aims for 45% reduction in greenhouse gas emissions by 2030 and reach net zero by 2050 [11].Indonesia targets to reach net-zero by 2060 [12].To reach net-zero emission, all activities that contributes to greenhouse gas emission has to be considered.Globally, in 2018, the construction industry contributed to 36% of final energy use and 39% of energy and carbon dioxide (CO2) emission [13].Cut and fill is one of the many construction activities.A study in Australia show that cut and fill can produce between 72 to 2395 kg of CO2-e per m 2 of construction [14].Note: CO2-e is carbon dioxide equivalent.Most activities do not produce only CO2 but also other greenhouse gases.For convenience, the other greenhouse gases are converted to equivalent effect of CO2.The huge range of CO2-e produced (72 vs. 2395) is caused by slope angle, as well as the stiffness of earth-materials excavated.Gentler slope with soft material produces less CO2-e than stiff material with steep slope.One of the ways to reduce CO2-e in the construction industry is by using geofoam.

What is it?
Geofoam is made of synthetic polymer called polystyrene.The raw materials of polystyrene are benzene (C6H6) and Ethylene (C2H4).The raw materials are combined to form ethylbenzene (C6H5CH2CH3).Thereafter, one hydrogen molecule is removed from the ethylbenzene, forming styrene (C6H5CH=CH2).The styrene is then polymerized to form chains of styrene, i.e., polystyrene (C8H8)n [15].
The production of geofoam starts from polystyrene resin beads.Each bead contains expanding agent, usually pentane (C5H12) or isobutane (CH3)3CH.The role of expanding agent is to expand the polystyrene beads.This is also why geofoam is also called expanded polystyrene (EPS).To expand the resin beads, they are placed inside a pre-expander, where they are subjected to high temperature and low pressure.This process allows the resin beads to expand, forming pre-puffs.By controlling the temperature, pressure and duration in the pre-expander, different diameter of pre-puffs can be produced.Essentially, this controls the density of pre-puffs.The longer the duration in the pre-expander, the bigger the pre-puffs produced.Then the pre-puffs are allowed to cool in a silo for 24-72 hours, allowing air to diffuse, making the pre-puffs harder.This process is called aging.The final step in geofoam production is to place the pre-puffs into a molding machine.In the molding machine, the pre-puffs are subjected to high heat (steaming).When the steaming process is completed, the machine is depressurized, and then cooled.Finally, the completed geofoam is ejected from the molding machine.The molding process can take 3 to 30 minutes depending on the machine capacity, volume, and designated density of geofoam [16,17].Figure 3 shows a series of photographs of geofoam block being ejected from the molding machine.Geofoam blocks are usually manufactured as rectangular blocks.

Physical and chemical properties
Figure 4 shows what geofoam looks like under microscope at different magnification level [19].It can be seen that majority of geofoam is made of void, i.e., air.In fact, 98% of geofoam volume is made of air!This molecular structure is what contributes to the low density of geofoam; typical geofoam is manufactured in density ranging from 11.2 to 45.7 kg/m 3 [20].Compared to water, geofoam is 50 times lighter (20 kg/m 3 vs.1000 kg/m 3 ).When compared to traditional compacted soil fill, geofoam is 80-90 times lighter (20 kg/m 3 vs.1600-1800 kg/m 3 ).Despite having void as most of its structure, the molecular structure formed when the polystyrene is expanded form a relatively stiff material.A 22 kg/m 3 geofoam has similar stiffness to loose sand (Young's modulus of 5000 kPa) [20].In addition, the molecular structure makes geofoam water-resistant (does not absorb water), and chemically inert to most chemicals, like acid and alkali [21].However, since geofoam is essentially made of hydro-carbon polymer, geofoam is soluble in organic solvent or oil-based liquid [22].Being made of hydro-carbon also made geofoam flammable [23].The weakness of geofoam to flame is easily solved by adding flame retardant (typically Butadiene styrene brominated copolymer) [24].Another weakness of geofoam is ultra-violet (UV).The expanded polystyrene molecule can degrade under ultra-violet light.However, a long time is required before degradation starts.An existing study exposed a polystyrene bar under a UVlamp which emitted UV-light 5 times the sun's UV intensity [25].Only slight yellowing can be observed after 200 hours of constant UV-light, while minor break down of polystyrene particles were only observed after 800 hours, and major break down after 2000 hours.Figure 5

Advantages and disadvantages of geofoam in construction
The obvious advantage of geofoam in construction industry is its density.A block of geofoam with a 2 x 1 x 1 m dimension weighs only 40-50 kg, meaning they can be carried and installed manually with 2 worker without any heavy machinery, e.g., forklift, bulldozer.Furthermore, installation of geofoam does not need any compaction, unlike traditional soil fill.Therefore, using geofoam to replace traditional soil fill is environmentally friendlier, lessening CO2 emission from heavy machinery.
Being very light also holds the advantage when dealing with problematic soils.When soil experience a change in stress, soil will deform.In very soft soils, the soil can deform as much as 30-40% of the embankment height to be built.As geofoam is very light, geofoam embankment induces very low stress change on the soil, minimizing its deformation [26,27].Another problem with stress change is instability.When embankment is built on an existing sloping ground, the additional shear stress caused by the embankment can induce slope failure.Even though the embankment is built on a flat ground, slope failure can still occur on the embankment slope.Thus, embankment is usually constructed with a relatively gentle slope, typically 1V:2H or gentler.The gentle slope is to ensure stability of the embankment.As geofoam is very light and self-standing, instability issue can be minimized [28][29][30].
Geofoam-built embankment can even be built vertically, reducing the volume of soil as well as land space used [31].
Of course, geofoam is not without its disadvantages.Besides the aforementioned properties such as solubility in organic solvent, flammability, and deterioration under UV-light, another disadvantage of geofoam is its cost.The raw materials of geofoam, i.e., resin beads are quite expensive.The price per unit volume of geofoam vs. traditional soil fill can range from 3-4 times globally [32], 5 to 20 times in Indonesia.The feasibility of geofoam has to be evaluated as the overall project cost and environmental impact.Geofoam can reduce cost of installation, cut and fill volume, heavy machinery usage, construction time.Table 1 summarizes the advantages and disadvantages of geofoam, as well as ways to overcome the disadvantages.Cost has to be evaluated as overall cost, rather than material vs. material

Geofoam application: Global vs. Indonesia
Since geofoam first highway application in 1972 [26], geofoam has rose in popularity worldwide.Other than highway application, geofoam can also be used for railways embankment, EPS sandwich panels for buildings as well as void filling, e.g., stadium and theatre seats.In 2022, the total geofoam market has reached 900 million USD.More than 55% of the market is attributed from the road and railways sector [32].Up to 2021, there was little to no application of geofoam for geotechnical related problems.The main market for geofoam in Indonesia is for EPS sandwich panels and void filling.The main issue lies in geofoam price when compared to traditional fill.Compared to overseas, the cost of soil as well as labor in Indonesia is relatively cheap.This gives rise to the large discrepancy in geofoam to soil fill cost ratio in Indonesia vs. the world.
One of the few geotechnical related application of geofoam is to stabilize a failing retaining wall.The problem and solution have been detailed in Gunawan [33].To summarize, an existing 4 m high retaining wall was to be heightened to 11 m high.However, when the fill had reached 7 m high, cracks started to form on the retaining wall.To stabilize the retaining wall, it was designed to replace the fill from 7 to 10 m high with geofoam, while the last meter can be continued with soil fill.The owner decided to take a more conservative approach and replaced the leftover fill height with geofoam.Figure 6 shows the construction photographs of the project.
Finally, in 2022, the first application of geofoam for highway construction was carried out in Indonesia.The instability of the slope, coupled with the imperative for highway operation to begin, necessitates the utilization of geofoam.More details will be given in the next section.

The first highway application of geofoam in Indonesia
The Cisumdawu highway is a 62 km long highway covering 6 sections, namely Cileunyi, Rencakalong, Sumedang, Cimalaka, Legok, and Ujung Jaya [34].Cisumdawu highway was initially planned to be operational by 2018.However due to delays from land acquisition as well as COVID-19, the target operation date was delayed to 15 th April 2023 [35,36].By November 2022, 3 sections were already completed, 1 section was nearly completed, while section 4 and section 5 had only reached 80.85% and 65.78% completion [37].The cause of delay in section 5, the highway covering Legok to Ujung Jaya, was caused by the difficult terrain of the skirt of Mount Tampomas.Major cut-and-fill operation was required.
In 225 m of section 5 (STA 40100 to STA 40325) [38], difficulties were encountered when more than 10 m high embankment has to be built on top of sloping terrain.To aggravate the situation, the soil encountered in the area is clay shale, which can be categorized as problematic soil or rock.When fresh, clay shale behaves like a rock.However, when exposed to the air, accelerated by wetting and drying cycle, clay shale quickly loses its strength.Figure 7 shows the shear strength of clay shale at different weathering grade.It can be seen that when clay shale reached residual state, it only retains ~15% of its original strength [39].On March 14, slope failure occurred during construction (figure 8).Due to presence of clay shale, the only plausible solution is either piled embankment or geofoam.Since there were only a month left before the targeted highway operation, only geofoam can achieve this construction speed.The detail of the design is presented by Rahardjo et al. [40].After the design and procurement process, the construction began at the end of March 2023.Figure 9a shows the progress of construction on 4 th of April 2023, and Figure 9b shows the progress the week after.It can be seen that the construction process is very quick.In fact, the geofoam production is up to 800 m 3 per day.Despite the geofoam manufacturer and contractor effort, it was not possible to completely finish the whole embankment within 3 weeks.Therefore, it was decided to construct half of the embankment first.This was to allow the highway to operate for Eid al-Fitr mass exodus which took place in the mid of April.Figure 9c-f

Discussion on Geofoam Status in Indonesia
Despite geofoam worldwide usage since 1980s, up to 2020, geofoam has received little to no attention in Indonesia.However, ever since the geofoam embankment fill in Cisumdawu highway, the potential for geofoam is recognized by Indonesia construction sector.Geofoam has the potential to solve many problematic soil conditions, such as slope instability and soft soil.There may be resistance by owner or contractor on the cost of geofoam per unit volume, but it should be reminded that geofoam is not simply a soil replacement.Geofoam usage can reduce cost of soil improvement, construction time, land usage, heavy equipment, and labour.In addition, the reduction in heavy equipment use will contribute to reducing CO2 emission.The author hopes that geofoam can be used more frequently, as the benefits do not only affect construction cost and time, but also extends to environmental benefits, contributing to Indonesia's target to reach zero CO2 emission by 2060.

Acknowledgement
The author would like to acknowledge PT Dinar Makmur for providing the photographs and information of the project.

Figure 6 .
Photographs of geofoam application to stabilize a retaining wall: (a) beginning of construction; (b) first 1 m of geofoam; (c) after 3 m of geofoam; (d) levelling of geofoam

Figure 9 .
show the construction progress on 13 th April 2023, 2 days before the targeted operation date.The placement of lean concrete was nearing completion.Construction progress of Cisumdawu Highway Section 5: (a) 4 th April 2023 [42]; (b) 11 thApril 2023[43]; (c-d) 13 th April 2023[44]; (e-f) 13 th April 2023 (courtesy to PT Dinar Makmur)After the Eid al-Fitr exodus, the other half of the embankment was resumed and completed by the end of May 2023.Figure10shows the progress of construction on 24 th May 2023.Soon after, the Cisumdawu highway was completed.The Cisumdawu highway was officially inaugurated on 11 th July 2023 and has been fully functional ever since.With the completion of Cisumdawu highway, the travel time from Bandung to Kertajati airport is shortened from 3 hours to 1.5 hours and the travel time from Bandung to Cirebon is shortened from 4 hours to 2 hours.

Figure 10 .
Figure 10.Construction progress of Cisumdawu Highway Section 5 on 24 th May 2023 (courtesy to PT Dinar Makmur)

Table 1 .
Advantages and disadvantages of geofoam, as well as ways to overcome the disadvantages