B.4.1.2 Rate of consolidation settlement p.I

Design assumptions

For the analysis of the rate of consolidation settlement, the drainage characteristics have to be identified (diameter D of hypothetical soil cylinder dewatered by each drain, drain diameter d_w, diameter of zone of smear d_s, discharge capacity q_w) as well as the soil consolidation parameters (coefficient of consolidation c_h, permeability in horizontal pore water flow in undisturbed soil k_h and in the zone of smear k_s), see Figure B.2.

As can be seen from Figure B.2, the value of D depends on the drain installation pattern (1,05 times the drain spacing for drains placed in equilateral triangular pattern; 1,13 times the drain spacing for drains placed in equilateral square pattern). The drain diameter d_w for a band drain can be assumed equal to that of a cylindrical drain with the same circumference as the band drain, i.e. d_w = 2 (b + t)/π where b is the width and t is the thickness of the band drain [17]⁵⁾.

The consolidation parameters of the soil are usually based on the results of oedometer tests where excess pore water dissipation takes place in the vertical direction. This differs from the real case with vertical drains where excess pore water dissipation mainly takes place in the horizontal direction. The difference between the oedometer case and reality becomes important where seams or layers exist with higher permeability than the main body of the soil. For the determination of the coefficient of consolidation in horizontal pore water flow, oedometer tests provided with radial drainage, or CPTU tests as described in B.3.3, can be used.

In some site conditions, the drain installation procedure may increase the soil compressibility and/or decrease the coefficient of consolidation and the permeability of the soil. It may also create excess pore water pressure in the soil. Such perturbations of the initial soil conditions should be considered in the design. When relevant, it is important that monitoring equipment is installed in due time before the drains are installed so that the disturbance effects can be registered and duly considered.

The insertion of the mandrel into the soil during drain installation also creates a zone of smear where horizontal layers are distorted in the vertical direction, followed by a reduction in horizontal soil permeability. The width and the characteristics of the zone of smear are a function of the installation method. The influence of the smear zone should also be considered with due account of the hole created by the mandrel during drain installation. The dimensions of the mandrel are temporarily much larger than those of the drains.

The mandrel used for band drain installation is not usually filled with water during the installation process. In consequence a cushion of air may be left between the drain and the surrounding soil after the mandrel is withdrawn. A cushion of air causes a negative effect on the consolidation process, similar to that of smear. It is taken into account in the choice of smear zone parameters based on experience. The installation may also cause vertical cracks around the mandrel, which in the case of sand drains become filled with sand intruding from the drains [37]. In cases where the un-drained shear strength of the soil is high, the installation may leave an open hole around the drain, which has a favourable effect on the discharge capacity.

Investigations of the characteristics and extent of the zone of smear caused by drain installation have been performed by e.g. [4], [8], [23], [34] and [42].

Key

• 1 plan
• 2 drain
• 3 perspective