Improvement of Different Grounds
Based on a grain size analysis a soil can be judged for its compactability:
Soils in zones A and B can be compacted by the deep vibratory compaction method Vibro Compaction (also called “Vibroflotation”), while soils of zonesC and D cannot be compacted by vibration alone.
Soils in zone C are often found on sites where liquefaction due to earthquakes is of concern. These soils can be compacted during the installation of Stone Columns.
Soils in zone D are not compactable by vibration, but can be substantially reinforced, stiffened and drained by installing Stone Columns.
Requirements for the soil to achieve good compaction by vibration :
- The soil must be permeable enough to allow rapid drainage of the pore water during the compaction process. The permeability is high enough for all granular soils with less than 10 % fines smaller than sieve #200 (0.074 mm) AND less than 2 % clay.
- The friction angle of the soil must be high enough to permit the passage of the compacting shear waves. This requirement is usually satisfied if the soil is well graded.
- The sand or gravel should not be easily crushable (carbonate content in form of shells) or contain very platy mica minerals that would increase soil compressibility.
Vibro Compaction creates the most balanced relative density profile of all compaction methods.
Digging out a stone column and inspecting the as built diameter is an inexpensive exercise that generates trust in the construction process.
Settlement as a result of compaction of sand.
Soil Classification by Cone Penetration Testing (CPT)
Soil classification by grain size analysis is more precise than any indirect way of soil classification.
However, as CPT sounding becomes more and more popular, this test is used to predict not only the in situ relative density but also the soil type in which the sounding was made.
CPT Soil Classification Diagram after Robertson
Experience of our company with over 500 million m3 of sand and gravel compacted over the past 15 years suggests that three limit lines can be drawn that together define the zone of compactable granular soil indicated by thegreen box, added to the graph by Robertson et al., 1986 :
Zone Soil Behaviour
|1 – sensitive fine grained
2 – organic material
3 – clay
4 – silty clay to clay
5 – clayey silt to silty clay
6 – sandy silt to clayey silt
7 – silty sand to sandy silt
8 – sand to silty sand
9 – sand
10 – gravelly sand to sand 11 – very stiff fine grained*
12 – sand to clayey sand** overconsolidated or cemented
|Soils above this line are already dense and need no further compaction.|
|Granular soils, including very loose fills, are hardly ever found to have an initial density of less than the value corresponding to the red line.|
|Experience shows that soils with a friction ratio Rf of above 0.8 % are problematic for compaction. This is believed to be mainly due to high fines content but there may also be other less well understood soil characteristics that coincide with a friction ratio above 0.8 %.|
ARABIAN BORING GENERAL CONTRACTING
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Foundation Engineering on EPC basis (Design and Build) including:
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