Soil settlement analysis in soft soil by using preloading system and prefabricated vertical draining runway of Kualanamu Airport

The method of soil improvement, using the combination of prefabricated vertical drain (PVD) and preloading, was used to accelerate the process of consolidation and the consolidation settlement in the runway of Kualanamu International Airport, which was constructed on the soft soil sediment like silty clay. In this research, the investigated area was the runway of Kualanamu International Airport zone I which had 11 meter-thickness of soft soil. Geotechnic instruments surveyed was settlement plate. Monitoring was done toward the behavior of landfill such as basic soil settlement. The result were compared with the analysis of finite element method of full scale in Mohr-Coulomb model by verifying the vertical drain of asymmetric unit cell and equivalent plane strain unit cell condition. The results of the research showed that there were an interesting behavior between the data in field observation and finite element of Mohr-Coulomb model. It was also found that the result of soil settlement of finite element method of Mohr-Coulomb model was closed to the result of settlement plate monitoring.


Introduction
Due to the rapid development of urbanization and the population increasealong coastal lines, exploitation of undeveloped low-lying areas has became a necessary strategy for many countries [1]. Development of urban transportation and expansion also occurred in North Sumatra, this occurred at the turn of Polonia Airport with the new Airport, known as Kualanamu Airport, located in Kualanamu area, North Sumatra. The airport construction will include runway, taxiway, exit taxiway, apron and other supporting facilities. Given the problem of soft clay soil that is influential in the success of infrastructure development, so must be done soil repairs so that infrastructure is not damaged before the planned age.
Based on above, it is necessary to improve the soft soil conditions. Preloading of soft clay with vertical drains is one of the most popular methods used to increase the shear strength of soft soil and control its post-construction settlement.Since the permeability of soils is very low, consolidation time to the achieved desired settlement or shear strength may take too long [2]. Using prefabricated vertical drains (PVDs), means that the drainage path is shortened from the thickness of the soil layer to the radius of the drain influence zone, which accelerates consolidation [3]. Introduced prefabricated band shaped drains and cardboard wick drains for ground improvement. Typically, prefabricated band drains consist of a plastic core with a longitudinal channel surrounded by a filter jacket to prevent clogging. Most vertical drains are approximately 100 mm wide and 4 mm thick [4]. The soft soil 1234567890''"" improvement by prefabricated vertical drains (PVDs) combined with preloading is one of the most successful applications to this site [5]. Barron [6] describes the area of influence (De) mounting PVD with a function of vertical drains as shown in Fig.1. For triangle pattern values [6]: D e = 1.05S

Equivalent drain diameter of band-shaped vertical drain
The size of the band-drain or PVD is mostly rectangular cross-section. For the purpose of calculating PVD, the cross section of the PVD will be modeled into a circle with the calculation of the equivalent diameter assumed as the circumference of the rectangle as shown in Figure 2.

Figure2. Conceptual drawing of a band-shaped PVD and equivalent diameter well.
Assumption is based on Hansbo's formulation as below [7]: Where dw = equivalent diameter vertical drains, a= wide vertical drains, b = thick vertical drains.

Discharge capacity PVD
Based on the results of laboratory testing Holtz et al [2] suggested for large discharge capacities of PVD ranging from 100 to 150 m3/year.

Smear Zone
PVD is inserted into the ground by using mandrel and at the end of the shoe is given. As a result of this installation, the soil layer stabbed with mandrill will be disrupted. The disturbance that occurs is called smear zone while the result will be a reduction in the coefficient of permeability of soil radial (kr).Smear zone effect is decrease permeability coefficient value for clay soil near PVD or PVD diameter used is minimized, this is due to process of remolding during installation of PVD.

Modeling Verification of PVD
The planeting pattern of PVD installed in the local field, at a certain distance, while in the FEM program the facility of implementing PVD is plane strain as shown in Fig. 3. To be able to implement PVD installed in the field into the program, it must first be verified in the form of plane strain that will produce a new soil permeability coefficient (k), then with the new soil permeability coefficient (k) the new simulation process in the FEM program can do.  [8,9].
According to Hird, et al [8,10]. The equivalent process can be done in several ways: 1. The distance between PVD at plane strain condition can be changed (geometric change), with permeability made fixed at axisymmetric and plane strain conditions (kax = kpl). 2. The permeability of plane strain conditions can be altered (permeability change), with the same geometry being created. 3. Combines geometry and permeability changes.

Field of measurements
Settlements that occured during construction are observed with the help of settlement plate tools. In this study the decrease was reviewed at three points of observation as shown in Figure 4.

Finite Element Modeling
Predictions were made using plane strain finite element analyses (FEA). The embankment and soil condition were in model two dimensional.

Unit Cell Analysis
In the field the pattern of PVD planeting is done locally with a certain distance, whereas in the FEM program the implementation of PVD is continuous (plane strain). Using Table 1 and 2, the first stage was verification of the axisymmetric cell unit with the coefficient of permeability of axisymmetric conditions and then equate the same water discharge with the condition of the plane strain in the form of unit cell plane strain ( Figure 5).  In the full-scale analysis of the disturbed area effects considered is kh / ks = 2.8 this value is taken from the value of the near-field matching. Geometry and mesh model of the Sta 0 + 750 runway of landfill is shown in Fig. 6 and Fig. 7. The soil parameters used in the analysis using the finite element method is shown in Table 3.   Figure 7. Mesh runway at Sta 0+750

Deformation pattern
Points reviewed in the calculation of the finite element method are two points, namely point A (0,0) and point B (30,0) indicating the position of the settlement plate location points shown in Fig. 8. Each point has a varying magnitude of decline but the required consolidation time is the same for all points.

Discussion
The settlement comparisons between the field observations and Mohr-Coloumb soil model are shown in Table 4.