Road settlement analysis on improved peat soil in Pekanbaru

. Indonesia is one of the countries that has the largest area of peat soil. Construction on peat soil may cause problems to the structure due to poor physical properties of peat soil such as having low bearing capacity. There have been several cases where roads built on peat experience damages, like uneven road due to settlement. This research presents a study in peat soil with soil improvement where the peatland was used as a location for road construction, which the bearing capacity, settlement and time required for settlement to take place is predicted. The analysis was conducted in a condition where the pavement is placed from the elevation of 0,0 m to -0,88 m, and the pavement is considered as a reinforcement layer. This research presents the bearing capacity, amount of settlement occurred, and the time needed for consolidation of roads on improved peat soil.


Introduction
Indonesia is one of the countries in the world that has the largest peat area, where the country has around 14,9 million hectares [1].Peat soil is known for its low bearing capacity, which is caused by their poor physical properties, which make it difficult to be used in construction.Generally, peat soil is an organic substance which is formed naturally which has plant matter as its main component [2].With the growth and economic development, peat land must be utilized to be used in construction therefor construction on peat lands can no longer be avoided.
One of the infrastructures needed in this time is structures that support the flow of transportation, such as road.However, construction of roads on peat soil results in some problems caused by its poor physical properties which may lead to road settlements and uneven surfaces.Construction on peat soil usually is done with soil improvement to increase the physical properties and bearing capacity of the soil, however with this the cost needed for construction increases as well.This research analyses the settlement of improved peat soil which is used for road construction.

Peat soil
Peat soil can be defined as a soil substance which have a high organic content that comes from plant material.Peat soil is formed when the organic material accumulates faster and then the rate of decaying [3].According to Pd T-06-2004-B also known as the Department of Settlements and Regional Infrastructure, peat soil is a type of soft soil which has plant remains that undergo decay as its main component.Generally, peat soil has a high compressibility and low undrained shear strength, and high permeability.A soil is classified *Corresponding author: jonathan.325190003@stu.untar.ac.id as peat soil when their organic content is > 75%.Peat soil bearing capacity is low, and it has a fibrous texture which makes the soil to have a high permeability Not parallel.Therefore, it can be concluded that peat soil is not great when used for construction, and if it is going to be used in construction soil improvement is needed to increase bearing capacity of the soil [4].Peat soil also has the characteristics like a sponge which make the soil particles to be very loose [5].

Physical properties of peat soil
Peat soil has a relatively low unit weight, where peat soil usually has a unit weight in the range of 8,3 -11,5 kN/m 3 .Peat soil is a highly compressive soil, which made the soil to have quite a high void ratio.Void ratio of peat soil can be around 5 -15 and for fibrous peat can reach to 25 [6].

Shallow foundation
Shallow foundations are foundation structures that are placed on the ground with the depth that is not larger than its width.Shallow foundation consists of pad foundation, strip foundation and raft foundations [7].The bearing capacity of shallow foundation can be calculated using theories such as Terzaghi (1943), Meyerhof (1963), Hansen (1970) and Vesić (1973Vesić ( , 1975)).

Effective stress
Effective stress is stress that is carried by the particles in the soil.Assume a saturated soil element which is applied with normal stress (σ) from Equation 1, which is applied on a horizontal boundary as shown in Fig. 1.
The stress s is called the total stress.The resistance to σ is provided by a combination of the stresses from solids, called effective stress (σ'), and porewater pressure (u) from the water in the pores [8].

Vertical stress
Vertical stress due to a concentrated load can be calculated with Boussinesq method, which use the following Equation 2.
where Δσ: increase in stress in the depth of z (kN/m 2 ), P: load (kN), r: perpendicular distance from the load (m), and z: depth (m).Vertical stress on soil due to strip load can be calculated using correlation table Δσ/qo with 2z/B and 2x/B which can be seen in Das & Sivakugan (2017).

Elastic settlement
Elastic settlement is caused by the elastic deformation of dry, wet and saturated soil without the change of water content.Elastic settlement happen immediately after the construction of a structure or load is applied [7].
Elastic settlement of shallow foundation proposed by Janbu et al ( 1956) on saturated soil with the poisson ratio of the soil μs = 0,5.With A1 and A2 graph proposed bry Christian and Carrier (1978) which can be seen in Fig. 2 [7].
Equation 3, elastic settlement proposed by Janbu et al ( 1956) with μ0 and μ1 values which can be obtain from Fig. 3.
Calculation on the middle of the foundation.
Calculation on the corner of the foundation.
where α: settlement location factor in foundation, B': B/2 for center of foundation, and B for corner of foundation, Is: shape factor, and If: depth factor (Table 1-2).2. Various poisson ratio according to type of soils [10].

Consolidation settlement
Consolidation is a time-dependent settlement of soils resulting from the expulsion of water from the pores which are contained within the soil [8].Consolidation settlement usually occurs in one direction which is vertical from top to bottom, this is due to the soil around could hold the force or weight that is received horizontally which is usually called (one dimensional consolidation) [11].Consolidation settlement usually occurs in two phases, primary consolidation, and secondary consolidation.In this research will only conduct a calculation in primary consolidation.The following Equations 17-19 are used for calculating primary consolidation settlement: For normally consolidated soil (  ′ =   ′ ): For over consolidated soil: where Sc: primary consolidation (m), Cc: compression index, Cs: swelling index, eo; initial soil void ratio, Hc: soil thickness (m), Δσ': change of stress (kN/m 2 ),   ′ : overburden effective stress (kN/m 2 ), and   ′ : preconsolidation pressure (kN/m 2 ).

Time rate of consolidation
Equations 20-23 are for time factor.where Tv: time factor, Cvcoeffiecient of consolidation (mm 2 /min), Hdr: maximum drainage length (m).U: degree of consolidation (%), and t: time needed for consolidation.
The allowed settlement in this research is taken from Bina Marga, for road pavement which is 100 mm [12].

Soil compaction
Soil compaction is one of the oldest and simplest method of soil improvement.The soil is compacted with force applied from rollers or other equipment [7].
Another compaction can be in the form of dynamic compaction where this method uses a heavy tamper which is lifted and dropped repeatedly with various height and compacts the soil.After very phase of compaction, the resulting crater will be even out with a dozer or filled with granular materials before the next phase of compaction.A lighter tamper with low drop height will result in an improvement to the depth of 3 -4.6 m, for heavier tamper with higher drop height can result an improvement to the depth of 6.1 -9.1 m [13].
CBR values of soil can also affect the characteristics of soils.Where with CBR value of soil, SPT values of the soil can also be determined with the following Equation 24 proposed by Livneh [14].

Research method
First, gathering every reference and theory needed such as journals, books, and articles is crucial.Then undergoing analyses to the characteristics of peat soil, bearing capacity of foundation, type of settlements, as well as bearing capacity and settlement equations.Gathering data and parameters needed for peat soil parameters and searching as well as selecting the type of class of roads for the research.The next stage is designing the load that is used in accordance with the class of the road then calculating the bearing capacity, elastic settlement and primary consolidation settlement that might occur as well as the time needed for consolidation to take place.Compiling summaries and suggestion based on the results of the research is the last stage in this research.

Results and discussion
This research uses soil parameter data from Pekanbaru which is in the form of boring log data.The pavement layer that is used in this research comes from a road in Pekanbaru.The soil parameters used in this research can be seen in Table 3.The pavement is placed on the elevation of 0.0 m to the depth of -0.88 m as seen in Fig. 4. the pavement layer will be considered as a reinforcement layer with the soil beneath it is compacted with a CBR of 15.The pavement layer used in this research comes from the existing road in the location as seen in Fig. 5.
Load calculation can be seen in Table 4. Vertical stress due to concentrated load (vehicle) can be seen in Table 5.

Bearing capacity
The bearing capacity of shallow foundation will be calculated with various method which are Terzaghi, Meyerhof, Hansen and Vesić where the results can be seen in Table 6.

Elastic settlement
The elastic settlement will be calculated with various methods and equation where its results can be seen in Table 7.

Primary consolidation
Primary consolidation will be calculated in every layer, where the settlement in every layer can be seen in Table 8.And for the time needed for consolidation can be seen in Table 9.For settlement in every year can be seen in Fig. 6.

Summary and suggestions
Summary: 1.The total settlement occur is 14.01 cm where the allowed settlement according to Bina Marga to occur is still not fulfilled.2. The elastic settlement that occurred is 0.71 cm. 3. The primary consolidation that occurred is 13.30 cm. 4. The time needed for consolidation if summed is 15.8013 years with the longest time is 7.3639 years.5.The largest bearing capacity is from Terzaghi's method which is 422.4513kN/m 2 .Suggestions: 1.To reduce the settlement occurred in peat soil, it is better to perform soil improvement so that the settlement of roads in peat soil is smaller.2. To improve peat soil, a specific or better method of soil improvement must be used such as compaction or filling the soil pores with grouting.3. Due to the characteristics of peat soil which have high percentage of organic substance, there are a lot of uncertainty in predicting the settlement that occurred accurately.Therefore, it is suggested to undergo laboratory testing to predict the settlement that occurred in peat soil.

Table 5 .
Vertical stress due to vehicle load.

Table 9 .
Time needed for consolidation summary.