JSACE 3 / 12 Research on the Effect of Water Draining Through Vertical Drainage Systems in the Process of Soil Consolidation

Journal of Sustainable Architecture and Civil Engineering Vol. 3 / No. 12 / 2015 pp. 24-33 DOI 10.5755/j01.sace.12.3.12655 © Kaunas University of Technology Received 2015/06/08 Accepted after revision 2015/09/29 Research on the Effect of Water Draining Through Vertical Drainage Systems in the Process of Soil Consolidation JSACE 3/12 Research on the Effect of Water Draining Through Vertical Drainage Systems in the Process of Soil Consolidation


Introduction
Examples of areas where this method is usually used: _ Road and railway embankments; _ Building and strengthening dykes; _ Earthworks made for the construction of the residential, industrial plants, terminals, etc.; _ Preload fillings; _ Coastal and marine construction; _ Land reclamation, ports and airports.
The specific functional requirements to each project, underlying geotechnical design vertical drainage.Steps to approach Fig. 1 Representing sand drainage system a vertical drainage project execution, include achieving a working platform, execution a draining mattresses, drawing drainage network model and the installation of drains, followed by loading and monitoring, Fig. 1.
Installing vertical drains may adversely affect the original properties of the earth (eg, decrease shear strength and coefficient of consolidation), so analysis of site conditions reported to drainage method must be very rigorous.
Due to the excess pressure of the water from the pores created by the load, the ground water is set in motion in the horizontal direction and then is discharged through the drains.An amount of water, generally low, is drained in a vertical direction as a result of one-dimensional consolidation.
For the research was used Plaxis 2D, which is an advanced calculation using the finite element method for analyzing two-dimensional problems of deformation and stability in geotechnical engineering.Plaxis 2D program involves three subprograms called data entry, data calculation program and display the results.Performs analysis in plane or axi-symmetry version with 6 or 15 nodes triangular element.
In order to describe a ground deformations resulting from changes in the current effort, a mathematical model was assigned to the earth.It governs the relationship stress-strain and is called material model.In Plaxis 2D are available a number of materials models.However, this paper will address the Mohr-Coulomb model only.
Consolidation analysis is used in Plaxis 2D when it is necessary to evaluate the dissipation of excess pore water pressure of saturated clay function of time.Here, the important parameter isn't the type of material drainage but rather permeability parameter specified.Is it possible to consolidate with and without additional surcharge.
A saturated soil subject to a total effort/strain will reduce gradually the volume up to dissipate excess pore water pressure, interfering primary consolidation.This process can take a long time and when all excess pressure is dissipated, the land is considered consolidated.
Analysis of the effect of water drainage through vertical drains was achieved by taking into account three assumptions: no drains, with drains placed at 2.0 m between them and sand drains, placed at 2.0 m inter-distance between them.The distance between the two types of drains, was choosen to show the efficiency from the point of view of performance and economic solution.This distance can be modified depending on the needs of drainage, but the 2.0 m between drains is the most common distance applied.The distance between the drains are kept smaller than the thickness of surcharge to reduce radial length of the drainage length.

Methods
Based on the results obtained in the three hypotheses considered, they were analyzed as graphics, consolidation process characteristic parameters such as displacements, times of consolidation and pore water pressures.
Modeling was performed on a layered soil profile, with high compressibility clays and incompressible.It was executed a precharge step, serving to highlight the neutral pressure dissipating way and settlements-time curve shape.In Table 1 it is presented the type stratifications and geotechnical characteristics, with an increased interest on very compressible soft plastic clay layer.
Considering the chosen stratification, the greatest interest in analyzing the consolidation process it is represented by the very compressible clay layer.Hydrogeological conditions generated by the model (groundwater level at -2.0 m depth) are modified by arranging vertical drains, lines along which the neutral pressure reduced to zero.At the ground surface has represented a drainage layer (mattress), with thickness of 0.5 m, which will ensure water drainage.Above this draining layer it is made the embankment of ground filler (preloading equivalent).Execution of fill in embankment, will realize in two stages, each stage of each 1.5 m height.
Contour conditions (the model limits), involves locking the horizontal displacements, while in vertical direction the displacements are free.Drainage is prevented from lower limit of the model and Fig. 2 The layout analyzed stratifications to the side, open only to the top by drainage mattress.Finite element mesh network with integration type 15 knots in 12 Gauss points.
The model is conducted in three-phase construction, different, to the hypothesis that involves carrying out the drains and the sand columns, in the following order: After running calculation, were obtained the results shown in the following.Figure 4 highlights deformed mesh network, the scale deformations is 50:1.The maximum total displacement obtained at the end of the consolidation process, it is 4.8 cm for vertical drainages, and 5.2 cm, for sand columns, otherwise it represents, as shown in Figure 5, the maximum settlement to share common ±0.00 m (natural ground surface).
The average effective effort is illustrated in Figure 6, in the color code.It notes the increase with its depth, but also the inside of the model.The average effective effort is one of compression and has a maximum value of 157.53 kN/m 2 for vertical drainages, and 166.49kN/m 2 , for sand columns, but the value is found after the -15.00 m depth.The excess pressure of pore water (neutral pressure) is highlighted in Figures 8 during the consolidation process (last step of the pre-load) and 9 at the end of consolidation process.In Figure 8, there is an immediate impact by reducing drains along their neutral pressure.Figure 9 illustrates the end of the consolidation process, a neutral pressure concentration in the area outside the influence of drains, but the value is very low, about 0.72 kPa for vertical drainages, and 0.69 kPa for sand drains.

Results
Fig. 4 The total displacement after consolidation (finite element mesh), for 2 cases Fig. 5 The vertical displacement (settlement), representing the contour lines, for 2 cases Fig. 6 The average effective effort, code color representation, for 2 cases Based on the results, comparative analysis was made in the calculation assumptions, the main parameters consolidation process characteristics such as displacements, consolidation time, effective and total efforts, pore water pressures.
Improving by the drainage is observed since the analysis of the evolution settlements reported on time, Figure 10 maximum land surface settlement in the cases considered, varies between 2 and 4 cm.Alura-time settlement curves are following stages of construction, with a considerable increase in the precharge phase followed by slow levels, representative for consolidation phases.The influence is shown by drainage time consumption of settlements, which is 1733 days under free drainage, 616 days when using sand drains and 622 days for vertical drains.

Fig. 7
The total effort, represented by the main directions, for 2 cases Fig. 8 Neutral pressure during the consolidation process, color code representation, for 2 cases Fig. 9 Neutral pressure at the end of consolidation process, color code representation, for 2 cases

Discussion
The same phenomenon can be observed in depth at -2.4 m (Figure 11) and -8.3 m (Figure 12).
As regards the time evolution of the lateral displacement at the ground surface, Fig. 13 shows a more rapid increase in lateral movements in drained conditions, but it is dictated by rate of consolidation in each case separately.At the end of the consolidation, the maximum lateral displacements in the three hypotheses considered and measured in section A-A (Fig. 14) reaches following values: _ without drainage: 23 mm; _ with vertical drains 2m: 16 mm; _ with sand drains: 10 mm.

Fig. 10
The settlement recorded in the share ± 0,00m Fig. 11 Settlement at -2.4m depth Fig. 12 Settlement at -8.3m depth Fig. 13 The lateral displacement recorded at the share ± 0,00m, section A-A It can be concluded therefore that the consolidation process of compressible clays, drainage it has influence in reducing vertical and lateral movements.

Fig. 3
Fig. 3Reporting safety factor to the evolution of displacements in 2 cases: a) vertical drainages; b) sand drainages

Figure 7
Figure 7 is represented by the main directions, the total effort, with maximum value of 382.82 kN/m 2 for vertical drains, and 401.53 kN/m 2 , for sand drains.
Fig. 14 Horizontal displacements distribution with the depth: a -Section A-A*; b -Section B-B*