METHODS OF GROUND IMPROVEMENT - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 222

- Biogrout. In-situ cementation of permeable soils for strength improvement with
retention of permeability.
- Black Clay. Active carbon/clay organic barrier for isolating material from the
environment.
- Drillmix. Fluid for horizontal directional drilling that is suitable for use in salt-
water.
- ETAC. Two-component grout system with an adaptable hardening time for
efficient tunnel boring, even in impermeable soils.
The core of biological treatment is using specific bacteria and chemicals in the soil,
and enhancing the activities with appropriate treatment (nutrients), which results in
required changes, such as permeability (Biosealing: 10 to 50 fold reduction is
possible in sands) and strength (Biogrout: up to 5 to 10 times increment is possible
in sand), see Fig 13.14.
In organic clay and peat-containing soils, contaminated and uncontaminated
dredged sludge the improvement of soil properties via biological methods is under
investigation. This field of 'pore-space engineering' is a relatively new branch in
geotechnical engineering: it involves different disciplines and offers many
challenges.

30 30 30 30

Stress (MPa) Stress (MPa) Stress (MPa)

25 25 25 25

20 20 20 20

15 15 15 15

10 10 10 10

5 5 5 5

Strain Strain Strain

0 0 0 0

02468 0 02468 0 02468 0 02468 0

Figure 13.14 Biogrout: compression strength after successive bio-flushings

application 13.1
Consider a soft saturated clay layer with a height of h = 5 m. Vertical drains are
installed. The consolidation coefficient is determined in the laboratory: c v = 2x10 -6
m 2 /s. An additional permanent ground layer (weight = 18 kN/m 3 ) to be placed on

top involves an extra total vertical stress of kPa in the clay layer. To reduce the

consolidation period T hd = 2 h 2 /c v (see equation 6.8), in this case 2(5) 2 /2x10 -6 =
25x10 6 s = 9.7 months, a vacuum pressure of v kPa is applied. Which vacuum
pressure reduces the consolidation period by a factor 4, to about 2.5 months, and
what maximum height of the additional soil layer can be handled?
The total stress is counter balanced by with time decreasing pore pressure u

and increasing effective stress ' , and the corresponding settlement S increases

proportional to the effective stress. Hence, the progress of the degree of
consolidation U (see equation 6.6) is conformal to the pore pressure development,