Improvement Without Addition of Material

Consolidation of clayey and silty soils takes place in the course of time due to its low permeability. Additional excess pore water pressures developing under static loading dissipates over time, and therefore consolidation process is accelerated by means of placing prefabricated vertical drains with a higher permeability in the form of a grid into the soil. Thus, by means of reducing the drainage path within clayey soil, consolidation process can be obtained much more quickly.

There are mainly two types of vertical drains: sand drains, usually installed by drilling and filling with sand in 30 cm to 60cm diameter, and flat prefabricated drains (band drains) with equivalent diameters of to 5 to 6cm. Prefabricated vertical drains are pushed and driven sometimes with vibration into the ground by means of a hollow mandrel.

Prefabricated vertical drains are composed of a permeable inner core wrapped in a filter type geotextile drain (typically composed of PES or PPE based materials).

Prefabricated vertical drains (PVD) are installed by means of drain machines mounted on a crane or crawler-mounted mast and mandrel mechanisms that allows the mandrel to be driven into the soil, depending on the depth of soil to be treated. Depending on the soil conditions high production capacity can be achieved.

Lengths of wick drains are determined in accordance with soil type and the design criteria. During driving of the each drain, an appropriate sized steel plate (anchor plate) is placed at the lower end of the mandrel, providing the wick drains in the mandrel to be driven into the ground; and plate is sacrificed and remains in the desired design depth upon the withdrawal of the mandrel.

Wick drains ate driven with a tolerance of not more than 10 cm in plan positions with a vertical maximum deviation tolerance of 1/50. In case driving a drain is not possible at the planned location due to any obstacle, another drain is replaced with a maximum distance of 45 cm away from the location, then the procedure is carried out with the rest of drain installation with instruction of the employer.

Driven drains should be cut about 15 cm to 20 cm above the surface.

Prior to the construction of pre loading fill drainage ditches should be completed around the improvement area. The results of the soil improvement should be monitored with instrumentation and monitoring and reported accordingly.

Bursa waste water treatment plant soil improvement with prefabricated vertical drains (PVD)

Among other consolidation methods; preloading without the application of vertical drains and waiting under the preloading surcharge can be cited, if there is enough time for the project. An other advanced method is vacuum consolidation techniques which is applied throughout the world. Yet, pre-installment method has a bearing capacity limitation.

For soil improvement with preloading only, the limitation is the bearing capacity of the subsoil, whereas it is mostly limited with time and in most projects waiting time is not sufficient, economically.

On the other hand in vacuum consolidation techniques implemented in soft and weak subsoil conditions where it is not possible to place preloading embankment due to insufficient bearing capacity. In this technique the main principle is to cover the ground to be treated with an impermeable membrane and to apply negative atmospheric pressure together with the support of pumping after special tubes are placed in the soil where poor ground conditions exist, and hence the consolidation is accelerated.

The method; vibro-compaction (or vibro flotation) is primarily about improving the soil by in situ compaction with vibration down to the desired depth. The application process is depending directly on the particle size distribution of the ground to be treated and this method can be applied in granular soils.

The soils of zones A and B in figure below are granular with a percentage of fines (< 0.06mm) of less than 12%. They can easily be compacted by vibration to relatively high densities. To the right of zone A, the soil may be too coarse for the vibrator to reach the required depth. Pre-drilling or the use of powerful vibrators may be necessary.

In zone D (more than 20% fines), permeability is too low for the compaction to work. Vibro-compaction would not therefore be effective and stone columns are needed.

In the intermediate zone C, the soil is too impermeable for vibro-compaction to be fully effective, but the installation of stone columns in silty sand will allow water to escape through the neighboring columns already in place and thus improve compaction.

A vibrator is inserted vertically into the soil following a regularly spaced grid. The vibrations passing through the soil provide a transient state of liquefaction, allowing a rearrangement of the soil particles into a denser configuration.

Vibro-compaction is generally performed using a triangular grid layout. The distance between grid points varies from 2.5m to 5.5m depending on the type of soil and its initial density, the result to be obtained, the type of vibrator used (power, amplitude of vibration, eccentric force) and the detailed compaction procedure (height of successive passes, criteria for completion of each pass such as amperage or hydraulic pressure).

LOWERING THE VIBRATOR

The vibrator is lowered to the desired depth by the help of water and/or air jet.

COMPACTION

Vibration is applied in steps of 50 cm. Compressed granular ground moves towards to the end of the vibrator.

FINALIZING THE PROCEDURE

At the end of the procedure surface of the ground that is settled by the compaction is back filled and compacted.

Among the soil improvement methods, dynamic compaction method is applied for reducing the total and different settlements and for improving the mechanic and engineering properties of the soils or uncontrolled fills in greater depths. This method initially developed and pionereed by Menard in France.

The main equipment required for this soil treatment method are heavy duty cranes and ponders. Most of the cranes are specially designed and may provide the energy to the soil required for improvement by dropping the specific weights from variable heights by means of a number of passes of free fall in accordance with the design criteria.

With this kind of soil improvement method, the density can be increased by the average settlement of the present ground surface.(usually around 5%) and resulting a change and increase in the relative density (20% or more).

The improvement depth is usually directly proportional with the energy delivered for each impact of ponder and can be determined by the following expression:

0.5 x (WxH)1/2 < D < 0.8 x (WxH)1/2

Consequently, the improvement depth D can be determined depending on the design and the ground conditions, and anticipated energy level [in ton.m].

Based on the improvement depth and the energy level given among the dynamic compaction methods, Heavy Dynamic Compaction ( HDC), High Energy Pillars (HEP), and Ironing methods for increasing the post-treatment surface compaction can be used together or separately in accordance with the design requirements.

The quality control and the performance of the soil improvement method should be checked by means of a series of quality control methods. Those are;

  • Systematic pressure-meter tests are performed; considering the dimensions of the area before and after treatment and the planned structures.
  • Systematic pressure-meter tests are performed with 1 meter depth intervals down to the bottom elevation of the soil improvement in the selected areas.

  • A test area is selected in the site that represents the ground conditions. In this test area, heave and penetration tests are carried out to determine the optimum compaction energy, number of drops at each point, drop height, and drop weight. The production starts after determining the optimal energy level and the number of drops for dynamic compaction. Example of the method for heave and penetration tests is given.
  • Additionally, the increase of the shear modulus of subsoil is determined by measuring the Rayleigh wave velocities before and after improvement. For this purpose, geophysical methods can be used to measure the surface wave velocity with seismic CPT before and after the treatment depending on the ground conditions at different points.

Dynamic Compaction Application Method

Dynamic Compaction Application Sequence

 
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