Bridge Approach Model with Precast Concrete-cell Box Structure to Overcome Differential Settlement

The bridge approach road embankment has a very important role as a link between the road and the bridge. In the post-construction stage, there are many cases of bridge approach road embankments, especially those with high elevations, potentially experiencing differential settlement. To overcome this, an analysis and planning study was carried out in increasing stability and overcoming the differential decline that occurred in the bridge approach road structure, especially in the approach slab structure with the end of the bridge. So that a bridge approach road model was developed as a substitute for the mass of soil embankment by conducting experiments and making a prototype of the Precast Concrete-cell Box structure model, which was given the maximum repetition and gradual plan load. While the method used in this research is the prototype model method, by conducting loading and settlement experiments on the bridge approach, especially on the approach slab. Furthermore, analysis and simulation are carried out under the condition of reaction force and maximum loading. From the results of the analysis, in the loading test of 2,761.00 kg, 2,717.00 kg 2,974.00 kg on the prototype model, the value of the decrease in LVDT 1-2-3 was 0.00 mm, the loading test of 3,956.00 kg and 3,887.00 kg obtained the value of the decrease in LVDT 1 and LVDT 3 of 0.00 mm and LVDT 2 obtained a decrease value of -0.01 mm which indicates a bending / deflection occurs in the approach slab structure, but it is still safe and no structure is required..


Introduction
Bridge construction is an infrastructure planned to connect traffic lanes or transportation systems that pass through lower contours.The bridge structure and its supporting components are planned according to the plan life, but from several problems there are often cases of decline in the bridge approach structure.especially in this case the subgrade and embankment material as the source of the problem which results in: 1) There is a height difference or what is called a bump between the stepping plate structure and the end of the bridge; 2) Unevenness on the pavement surface; 3) It can even be fatal to the main structure of the bridge.
Bridge approach embankment (oprit) burdened by approach-slab structures and other external loads is a road segment that connects highway pavement construction with bridge abutments, where the bridge approach and approach slab geometry must provide security, safety and comfort for road users IOP Publishing doi:10.1088/1755-1315/1321/1/012039 2 who will move across between the highway and bridge trajectories.In many cases there is structural damage to the bridge approach, including uneven settlement which causes the pavement surface to become bumpy or a decrease in the elevation of the bridge approach which causes the approach slab to break.Both cases are the result of irregularities in the implementation of construction or review of the case in soft soil.The difference in settlement between the bridge end abutments and the bridge approach embankment is a serious problem, and will result in "bumps" that affect driving safety and comfort at the bridge ends [1].
Differential settlement is mainly influenced by regional factors and inundation patterns in the bridge approach area, The effect of land consolidation to changes in the most extensive puddle in Semarang Indonesia, which is related to land subsidence due to inundation and alluvial soil layers in the lower land [2].
Many a rough bridge approach with heavy maintenance requirements is enough to convince the highway agency that there is a serious problem [3].Complaints typically describe 'bumps' that motorists feel when approaching or leaving the bridge.These bumps result in reduced steering response, driver distraction as well as dynamic response to the bridge deck and maintenance operation costs.Approach slabs can lose their contact support due to subsidence and bulging of bridge approach embankments.With the research variable being the approach slab design concept for bridges, and the proposed research method being simulations with Plaxis computer software and field geotechnical studies leading to the test-bed development of a proposed new inter-road bridge in Recees Malaysia [4].
Proper design of approach slabs and bridge approaches requires accurate prediction of the settlement profile of the plan structure behind the bridge abutments.The roughness of the transition between the highway, in this case the bridge approach, and the bridge is a well-known problem [5].When the soil under the approach slab settles, bumps can occur at the bridge ends when there is an abrupt change in the approach slab slope.Unconsolidated embankments and/or loess subgrade are susceptible to moisture damage, especially on embankments at bridge approaches [6].To reduce differential settlement, and reinforce the loess embankment, Caster oil-based polyurethane (COP) was used.The adhesive performance and impermeability mechanism of the loess embankment reinforced with COP utilizes grouting technology.
The observation and explanation, shows an illustration of the approach slab and its interaction with the soil that the fault at the pavement joint with the approach slab (Rigid-pavement/approach-Slab, R/S joint) causes bump-1, the second occurs at the abutment back-wall between the approach slab and the bridge deck (approach-Slab/bridge-Deck S/D joint) causing bump-2.They developed a soil reinforcement system under the R/S joint by designing footings and geosynthetics slabs, namely geogrids [7].The cross drain structure designed under the bridge approach or approach slab is subjected to settlement as a result of being loaded with additional fill [8].The dynamic theory of vehicle line coupling, the numerical model of the spatial dynamic of the vehicle line coupling system to the subgrade, at a distance of +/-5 m from the abutment has the largest deformation, and the stiffness in the 0-5 m zone behind the abutment is specially designed [9].Ahmad R and Yee K 2018 Embankment on a rigid inclusion, using the Controlled Modulus Column (CMC) method planned adjacent to the bridge abutment [10].
Analytical program development study to investigate the shear capacity of precast reinforced concrete culverts.Simulating the experimental results, a three-dimensional finite element model (FEM) [11].Describes the research variables with the design of U-Section Precast Concrete Box Culverts (UPCBC).The evaluation index was analyzed to measure the comprehensive value of box culverts.The research method used a theoretical prestressing model prototype and a numerical model.numerical model [12].
Numerical variables of box-culvert reinforced concrete collapse, with methods of experimental model and numerical simulation, scale test model with system sensors and loading and measurement [13].Investigation of the effect of slope angle and height of topsoil embankment on the live load and dead load characteristics of reinforced concrete inclined box-culvert [14].
Furthermore, from the many cases of differential settlement that occur on the bridge approach and approach slab can be evaluated, namely designing a geotechnical engineering with soil improvement or soil reinforcement.At the back of the bridge abutment structure, a new method is proposed by replacing the embankment mass with a PCcB structure with the assumption that it treads on undisturbed hard subgrade or subgrade that has been repaired or reinforced.Another thing that is taken into account is the footprint of the PCcB base slab plan where it can provide bearing capacity to the subgrade.

Research methodology
The research was an experimental study that modeled a bridge approach oprite structure on a smallscale model in the laboratory (prototype) by comparing standard embankment materials and replacing the embankment materials by using a precast concrete cell-box (PCcB) structure.Furthermore, the load is gradually modeled as a perpendicular repetition load on the approach slab and a load on the bridge approach oprite with a 4% slope, as shown in Figure 1.This experiment aims to determine the structural stability and deformation or settlement of the bridge approach and approach slab, with analysis using Plaxis 8.6 software, namely the finite element method (FEM/FEA).This section describes the prototype and workload tests on the PCcB structure used in the experiment.The experimental model is made on a small scale and carried out in the laboratory, where the prototype is made of 120x50x2.50mm CNP steel frame test box, 4 mm thick plate, 150x150x120 mm glass wall with a series of culvert box segment models of precast concrete structures hereinafter referred to as Precast Concrete-cell Box (PCcB).The process of making a prototype model with the PCcB model consists of a continuous base plate of 1540x900x70 mm, a culvert-u or inverted u-ditch with a base dimension of 40x40 mm with a wall thickness of 60 mm and a top plate thickness of 70 mm, then between two concrete cell boxes connected by a link-slab (figures 2 and 3).After all materials and supporting instruments were prepared and assembled, the next loading was given to the concrete link-slab, opproach slab and pavement.The PCcB prototype model was analyzed using FEM Plaxis and validated with laboratory results.

Soil preparation
Soil sampling, preparation and extraction of soil required in the test, in accordance with the criteria of the subgrade characteristics.The subgrade used in this study is a soil with characteristics that can be categorized as a hard soil classification.Pada setiap pengujian dengan beban vertikal dan /atau membentuk sudut maksimum 4% digunakan tanah keras yang memiliki karakteristik yang sama, seperti komposisi, gradasi butiran, kadar air tanah, dan kepadatan yang sama.The soil samples used were taken from quarry ex Rowosari Mranggen Demak, Central Java Province.To ensure that the soil used is classified as hard soil, the density of the soil was tested with a Dynamic Cone Pnetrometer (DCP).Testing and treatment of soil samples in this study is schematized into two segments, namely as the subgrade for the foundation of the PCcB structure and the soil as the preferred embankment as in the design of the conventional bridge approach.
During the research and soil sampling, data on soil properties were obtained as presented in Table 1.The subgrade sample has a CBR value of 45.50%, which is a correlation of the soil qc value of 36.4 kg/m2 (0.000367 MPa).The qc value of the soil, according to Look [14] is classified as hard soil as a layer of soil supporting structures and/or foundations.The results of the Atterberg limits test, the soil has a Plastic Limit PL value of 21.08% and Liquid Limit LL of 34.25%, so it is a soil with low plasticity.Based on the direct shear test results, the soil cohesion of 0.47 kg/cm2 (46.09 kPa) is classified as hard soil.From the soil test results, the soil used in the experiment has the same classification as the soil in the field.

Testing procedure
The first procedure was to insert the selected fill sample as hard soil into the test frame.To ensure that the density of the soil in the test bed is in accordance with the density in the field, a DCP test was conducted to determine the level of soil density up to the subgrade layer in the test bed.Next, the subgrade was levelled and the conventional approach slab prototype was tested for settlement, then the PCcB model prototypes (consisting of base, crown unit box and overall link-slab) were inserted into the test bed and each was subjected to loading tests from 0 to 20 kN and the subgrade settlement was recorded on a data logger, as shown in Figure 4 to Figure 6.Experiments were conducted to test two bridge approach structure models, namely the approach slab settlement test and the PCcB structural unit with a slope variation of 0% to 4%.Furthermore, the PCcB structure was retested with full loading of the approach slab up to the pavement layer.

Result and Discussion
Some cases that occur on the bridge approach in this case the preferred embankment at the time of construction are experiencing differential settlement and unevenness of the pavement surface, then a prototype is planned with loading and settlement experiments.

Load-induced settlement test
The graph of the relationship between load and settlement from the test as shown in Figure 6 a) is the experiment without approach slab on the compacted bridge approach option prefered bacfill and as a centralized load.Figure 6 b) is the first conventional approach slab load test according to the actual site conditions without PCcB structure.The graph of the relationship between load and settlement from the testing of the approach slab structure of dimensions 520x900x50 mm on the bridge construction prefered bacfill , can be shown in Figure 7.Given a load from 0 kN to 20 kN, there are three changes in the graph of bacfill settlement read by the data logger and the installed LVDT.Three graphs and one more graph were added, thus showing the final results of the laboratory experimental tests (3 graphs) and the Plaxis computer program (1 graph).From the laboratory test results, the approach slab with larger tread area will have lower settlement.From the analysis of the laboratory test results with a maximum load of 20 kN and the analysis with Plaxis software, there is a significant difference in settlement.From Figures 9 to 13, the average value of load testing on the Precast Concrete cell Box structure does not show any settlement readings across the LVDT-1, LVDT-2 and LVDT-3 components.Figure 10 shows a settlement in the LVDT-2 component of 0.01 mm at a test load of 3,956.00Kg, this means that there is bending / deflection in the approach slab structure, but it is still safe.Furthermore, Figure 13, the same condition shows a decrease in the LVDT-2 component of 0.01 mm at a test load of 3,887.00kg, bending / deflection occurs in the approach slab structure.

PCcB Model Prototype Check with Plaxis Software
The following is the Plaxis Software output from the modeling results of the Precast Concrete-cell Box (PCcB) structure prototype to overcome the differential settlement on the bridge oprite approach road.As a result of load-induced settlement tests and inspection of the prototype PCcB model with Plaxis software, this study proves that the differential settlement occurring in all bridge cases can basically be overcome by PCcB construction and structure planned under the approach slab behind the abutment structure.When compared with previous findings and cases, the differential settlement in the bridge approach occurred due to the influence of the soft subgrade.The focus of this research is on the replacement of the selected embankment with a PCcB structure, especially when anticipating construction implementation that does not meet the engineering specifications.

Conclusion
The application of PCcB structure in bridge approach planning is in accordance with its purpose, which is to overcome the differential settlement of the bridge approach embankment above the subgrade.To maintain the stability of the PCcB structure the base plate is designed to be continuous and the geotechnical engineering of the subgrade can be developed with soils improvement and / or / soil reinforcement.From the analysis and graph above it can be concluded that, the approach slab structure with static load test and dynamic load / vibration or repetition load shows a significant and uneven decrease.While the analysis and graphs with the PCcB prototype model, show that the structure is quite stable in accepting the working load.

Figure 1 .
Figure 1.Illustration of bridge approach modeling on site 2.1.Prototype of experimental model 2.1.1.Model and scale.The experimental model is made on a small scale and carried out in the laboratory, where the prototype is made of 120x50x2.50mm CNP steel frame test box, 4 mm thick plate, 150x150x120 mm glass wall with a series of culvert box segment models of precast concrete structures hereinafter referred to as Precast Concrete-cell Box (PCcB).The process of making a prototype model with the PCcB model consists of a continuous base plate of 1540x900x70 mm, a culvert-u or inverted u-ditch with a base dimension of 40x40 mm with a wall thickness of 60 mm and a top plate thickness of 70 mm, then between two concrete cell boxes connected by a link-slab (figures 2 and 3).After all materials and supporting instruments were prepared and assembled, the next loading was given to the concrete link-slab, opproach slab and pavement.The PCcB prototype model was analyzed using FEM Plaxis and validated with laboratory results.

Figure 3 .
Figure 3. Schematic of the research prototype model in 3 dimensions

Figure 4 . 6 Figure 5 .
Figure 4. Basic plan and details of the experimental prototype model Validation of the prototype research model scheme as shown in Figure 5, then installation and observation of the PCcB structural test plan with additional tools in the form of data loggers connected to sensors, cameras and computer units to observe the behavior / performance of the structure under review against load tests.

Figure 6 .
Figure 6.Direct load test on backfill and load test on approach slab without PCcB structure a) Load 1 test on approach slab and backfill b) Load 2 test on approach slab without PCcB

Figure 7 .
Figure 7. Approach slab load test with re-load without PCcB structureTesting of prototype models such as figure8was carried out on the PCcB structure (base plate, cell box, link slab and approach slab).The series of structure is stopped and supported on hard soil such as Figures 9 to 13 with a distance of the 450 mm test point positioned at the edge of the link slab to the direction of the 4% slope on the bridge approach.

Figure 8 .
Figure 8. Test of settlement PCcB structure Carrying out load testing experiments on Precast Concrete-cell Box (PcCcB) structure, devided into several test points with 2-segments box and 1-segments between the box which leads to the multicell box below.

Figure 14 .
Figure 14.Random modeling of PCcB structure prototype