Foam Mortar Layer in Soft Subgrade Embankment: A Review

Soils are the basis of the construction of civil engineering structures that are designed to support and withstand the weight of the structures above them. The main problems of soft soil include low bearing capacity and large settlement. In road pavements, the sub grade as the bottom layer of the road must have a good bearing capacity. Subgrade is a part that must be considered because of the weight of the pavement structure and the embankment above it. Soft soil conditions cause the need for reinforcement of the sub grade layer. Solving the problem of road construction is done by engineering so that the sub grade as the bottom layer of the road is safe and stable and has a good bearing capacity. The effort made to increase the bearing capacity of the subgrade is by reinforcing the soil (soil improvement). One of them is with lightweight material mortar-foam is an alternative construction material in soil reinforcement for road construction. Mortar-foam is formed from a mixture of foam, cement, sand and water. Research on foam mortars has been conducted in various places and countries in various ways and methods. This article will discuss the latest developments in the use of foam mortars as backfill in soft soils in the subgrade layer presented based on information provided by the author.


Introduction
Soil problems are very diverse and varied, problems arise if it is estimated that the soil conditions on which construction will be built are categorized as soft soil.Soft soils that require special treatment where the soft soil has a large consolidation settlement, soft-very soft consistency, high moisture content, small shear force, large compression and low bearing capacity of the embankment and construction above it.[1] Soft soil structures that have road construction on them are inseparable from studies that require standardization according to what has been determined to support the load on them to remain stable and sturdy.[2] The road construction supports its own weight and the weight of the traffic on it..The subgrade layer found at the bottom layer as the base layer of the road must have a good bearing capacity.However, the soft soil condition requires reinforcement of the sub grade layer.[3]Solving the problem of road construction is done by engineering so that the sub grade as the bottom layer of the road is safe and stable and has a good bearing capacity.[4] Foam mortar is considered to be one of the alternative construction materials to be used as a lightweight embankment material that has its own advantages over other materials used as reinforcement in road construction.[4] The lightweight characteristic of foam mortar is due to the fact that it can expand 4 times from the initial volume condition, which can reduce the use of other materials without using foam mortar.Foam mortar is formed from a mixture of materials such as cement, water, sand aggregate, and foam.The addition of foam made from vegetable and chemical protein bubbles into separate stable bubbles makes the foam mortar material lightweight.[5].
In geotechnical applications foam mortar is widely used in road embankments reduction of settlement that occurs on the embankment as a soft soil filler behind the retaining wall so that the thrust is reduced.[6] This research is similar to the research of foam mortar installed as backfill in the subgrade layer with a thickness of 0.5 m which is expected to be given the loading above it, either in the form of vertical loading or cyclic loading of 650 kg/m3 so that it can be stable for the long term in the subgrade layer.[7] Foam mortars in soft soil embankments ranging from 0.5 to 3 m of soft soil layer thickness which translates to less than 25%, of the total soft soil height.The resulting qu values were 1.3 -1.8 times higher than the qu values in the laboratory tests.[8] So that the compression value is more than 100 kPa at 7 days of curing and more than 200 kPa at 28 days of curing.[7] The stability of construction on soft soil is a study related to the importance of using foam mortar.[9] Discussions on the use of foam mortar have been carried out by several countries related to the problem of settlement due to the self-weight of road embankment, the load of vehicle traffic on it.
[3].This article examines foam mortar in the subgrade layer as embankment against the settlement, bearing capacity and safety factor that occur.This study is expected to solve geotechnical engineering problems, especially in soft soils.The use of foam mortar in various countries in several civil building constructions is also studied.

Material Properties of Foam Mortar
The constituent materials of foam mortar resemble common lightweight concrete.Specifications for the use of foam mortar materials are governed by job mix requirements that are designed based on the needs of the construction and the loading of the structure above it.[10] In Indonesia, several construction work controls have been carried out on the prolific use of foam mortar to overcome settlement and deformation occurring in soft soil.[5] Foam mortar is a mixture of several ingredients, namely : [5] a. Cement In mortar mixes, cement serves to bond one material to another to create a solid bond, tailored to the design composition.The cement particles bind the aggregate grains together to form a solid and rigid mass so that air voids can be filled by the layer.b.Sand aggregate The sand aggregate used for mortar design is sand that comes from local materials or can be imported from other places.The composition of the sand aggregate is adjusted to the design weight content to be made.The sand used is of good quality, fulfilling the general requirements in accordance with the provisions of c. Foam agent Foam agent is a foam-forming liquid consisting of vegetable liquids and non-plant liquids, including 1-dodecanol, methoxyacetic gcid tridecyl ester and 1-tetradecanol.This liquid, which looks like water, has chemical characteristics.The purpose of this foam agent is to create a stable consistency in air bubbles when the weight content is designed to get the mixture according to the plan.Mixing between foam liquid and water is done using a foam generator and compressor, so that over time foam is formed with the specified composition to get the weight content according to the design and use.

d. Water
The water used for mixing the foam mortar design is the water used in normal concrete.The water must be in accordance with established standards, not contain chemical elements and

Reinforcement of Subgrade with Foam Mortar
Several studies have used foam mortar as reinforcement in several types of rocks and soils, addition to soil reinforcement with residual materials slope reinforcement, performance improvement of drainage composite layer stability and utilization of lightweight foam mortar material to lighten structural loads on bridge oprites and retaining walls [6] Geotechnical investigation of the use of foam mortar in the subgrade embankment in order to obtain the amount of settlement that occurs, installed tools in 4 accordance with the layout, so as to be able to describe well and clearly the rotation of the settlement that occurs.In the construction of flexible pavement roads built on soft soil.The construction load on the pavement layer must be considered to be an important factor, such as settlement, deformation, load recurrence through it, such as cyclic loading, the effect of strain on soil stiffness, so that the consolidation and settlement process when the pavement layer is passed can be taken into account.
Research on toll roads on the efficient use of foam mortar compared to selected materials was carried out in order to minimize the occurrence of compression on the subgrade.Variation of the percentage of lightweight material to the backfill material with a composition of 0%:100%, 25%:75%, 50%:50%, 75%:25%.results in the more the percentage of lightweight material of foam mortar the lower the settlement and an increase in the safety factor.

Use of Foam Mortar in Earthworks
Tables 2 and 3 below define previous research studies relevant to using foam mortar in soil reinforcement with different case studies under various conditions.These studies were conducted to overcome settlement and improve the bearing capacity of soil as reinforcement.The research includes laboratory scalatical testing, field testing and numerical analysis with Plaxis application using foam mortar.

Conclusion
This article concludes that foam mortar can be used as an alternative soft soil backfill material for soil reinforcement.The characteristic properties of foam mortar can minimize deformation in the form of settlement and increase the bearing capacity and have the required safety factor which is greater than 1.2.The research considered in this review article is in the form of laboratory testing, field and numerical analysis accompanied by case studies, several countries have used foam mortar as reinforcement of road layers, backfill, retaining walls, bridge oprites, tunnels, and soils.

Acknowledgments
The author would like to thank the Civil Engineering Doctoral Study Program of Sultan Agung Islamic University Semarang, the Civil Engineering Study Program Sekolah Tinggi Teknologi Pekanbaru, as well as the contribution of the parties involved in this writing.

Figure 3 .
Figure 3. Approximate Placement of The Settlement Prediction Tool.

Table 1 .
Application of Foam Mortar in Relevant Previous Research.

Table 2 .
Use of Numerical Analysis in Foam Mortar Research.