Effect of use of quicklime mix on the slope surface and number of layers on embankment stability

. Common problems that are often found on embankments are soil instability and low soil bearing capacity. To prevent landslides, consideration of the embankment stability and ways to improvement it is essential. This study investigates the use of quicklime as a stabilising material and a number of fill layers on the safety factor of the slope of the embankment in order to find the optimal combination. The type failure or deformation and extreme stresses that occurred in these soil embankments was also studied. Laboratory testing is used to determine soil properties. Three types of soil were analysed; subgrade, fill from original soil and fill from soil mixed with 15% quicklime. This quicklime mix was used as a 40 cm reinforcement layer on the slope surface of the embankment.


Introduction
Civil engineering structures are highly dependent on the soil as soil is used as a building material and as a foundation for buildings. The first step in any construction is to find out whether the soil at the construction site meets the planning requirements with regard to stability, deformation and density [1].
The construction of highways, airports and other large construction works especially in areas with uneven contours often requires excavation and construction of embankments to prepare the site In constructing an embankment for a road fill material is usually sourced from a quarry and transported to the project site. The soil used as fill is usually excavated soil that consists of particles which enable it to exhibit sand-like properties and cannot provide structure [2].
The process of constructing an embankment takes considerable time for the transportation of the fill to the site and its compaction. This is impossible to achieve in one construction phase, so it needs to be done in stages.
The safety factor for the slope of the embankment needs to be determined. Landslides of soil slopes can be very dangerous even leading to the collapse of the road, making it unusable.
The soil used for embankment consists of grains of soil and pores that contain water and air. It consists of solid mineral aggregates which are not cemented (chemically bound) to each other and solid particles of decaying organic matter along with liquids and gases which fill the empty space between these solid particles [3]. Embankment soil contains clay which presents some problems that must be faced by a civil engineer when the location has poor soil characteristics needing some improvements and increase its bearing capacity [2].
Much research has been done on using fly ash as a stabilizer. Low calcium fly ash is a waste material that is widely used in Indonesia along with silica and alumina [4]. However, quicklime is also a suitable choice as a clay stabilizer and it is easily available at a reasonable cost. In this study, the stabilizer used was quicklime.
Compaction carried out in the laboratory is a way of modelling compaction carried out in the field. This study was conducted on a model embankment slope using empirical methods for analysis [5]. The test parameter was the safety factor of the slope. The number of layers in the construction of the embankment was varied and a stabilising layer of quicklime mixed soil applied to the outside of the slope to investigate their impacts on the safety factor.

Geological investigation
The research location was the road in the Faculty of Engineering, Andalas University, Padang. The tests were conducted at the Soil Mechanics Laboratory, Department of Civil Engineering, Faculty of Engineering, Andalas University.

Physical properties of subgrade and embankment
The results of the test of the physical properties of subgrade can be seen in Table 1 showed that the soil could be classified in the MH (Elastic silt) group according to the Unified Soil Classification System (USCS). It was a mix of fine sandy and silty soils, elastic silts, organic silts, clay and silts clay [3]. From Table 1 the properties of embankment show that 14.8% of the fill soil passed through a No. 200 sieve or > 50% was retained in the No. 200 sieve and > 50% of this coarse fraction passed through a No. 4 sieve, the soil was classified as GC (Clayey Gravel) according to the USCS classification system [1].

Mechanical properties test of subgrade
An Unconfined Compressive Strength Test (UCST), gave a q u of 0.43 kg/cm 2 for a original sample, while the q u of a remolded sample was 0.37 kg/cm 2 . This gave a soil sensitivity value (ST) of 1.15 which means the soil tested is slightly sensitive [3].  Compaction test results indicated that the optimal water content was 37.8% at maximum dry unit weight with a dry content of 1.23 gr /cm 3 . This optimal water content was used as a benchmark for water content for the direct shear test and the UCST. The results of the UCST were performed on each variation of the mixture, the q u value was 0.31 kg/cm 2 .

Mechanical properties of embankment soil mixed with stabilizer
q u values were obtained on embankment soil mixed with 15% quick lime as a stabilizer using the UCST of the soil, After the soil was mixed it was cured for five days before testing. The q u of the mixed fill was 0.77 kg / cm 2 .

Subgrade soil
From the results of the sand cone test in the field, unsaturated soil unit weight (γ unsat ) of the subgrade was 1.24 gr/cm 3

Embankment soil stabilised with quicklime
The value of γ unsat from the field test was 1.747 gr/cm 3   The slope cross-section of the model was based on data obtained from field measurements using the cartesian coordinate system having a height of 2.4 m and width 2.8 m

Safety factor
The stability of the slope of the embankment is indicated by the safety factor. Comparing safety factors allows us to determine the relative stability of each embankment slope and the influence of the quicklime mix layer. From this, a comparison table of safety factor Fig.s for each layer of embankment could be constructed.   From the Fig. and table above it can be seen that the largest safety factor for natural soil fill was for an embankment consisting of a single layer at 1.15. When a 15% quicklime mix was applied to the embankment slope surface the largest safety factor was also for a single layer and was 1.90. The lowest safety factor for natural soil fill was 0.99 for two layers and when using the quicklime mix 1.82 for two layers. The safety factors using original soil without the quicklime mix were all below the standard 1.25 indicating that an embankment constructed from this material could be at risk of instability. However, when an embankment was covered with a 40 cm layer of quicklime mix the safety factor increased to above the standard indicating a reduced risk. The increase in safety factor from this quicklime mix layer was more marked when more than one layer of fill was used.  From the table and graph above, it was found that the highest extreme displacement value for the natural embankment was for one layer at 4.2 m and the largest extreme displacement value for embankments protected by the quicklime mix was for four layers at 1.45 m.

Analysis of extreme displacement
The lowest extreme displacement value for the natural soil embankment was for two layers of fill at 1.4 m and the smallest for those using quicklime mix was for one layer at 0.26 m. The addition of a 40 cm quicklimemix layer on the embankment slope decreased the extreme displacement by 37% -94%.

Conclusions
1. The safety factor for the initial conditions at the location was below the standard (1.25) so that the slope can be said to be unstable. 2. The largest safety factor value for layered fill of natural soil was 1.15 for a single layer. 3. The largest safety factor for layered fill with lime mix was 1.9 for a single layer 4. A 40 cm layer of 15% quicklime mixed with original soil on the slope of the embankment increases the stability of the slope as shown by the increase in the safety factor. This increase is most marked in embankments than were constructed of several layers where the safety factor increased by > 85%. 5. The embankment with the lowest extreme displacement value was the one layered embankment with the slope stabilised with a 40 cm layer quicklime mix. The extreme displacement value for this embankment was 0.26. 6. Addition of a 40 cm layer made from 15% quicklime mixed with original soil is effective in reducing extreme displacement and increasing the stability factor to above the standard value for slope stability.