Compaction from surface

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Description

Compaction of the natural material may be carried out from the surface applying one of the following principles:

1. Compaction due to high static pressures from heavy equipment;

2. Compaction due to vibratory equipment;

3. Compaction due to heavy impact, using either eccentric rollers or dynamic compaction;

4. Compaction due to pressure waves induced by blasts.

All systems aim to cause the soil grains to rearrange into a denser microstructure (fabric); they are effective only on  unsaturated materials, with relatively low water content or, to a lesser extent, in free draining materials where pore water can readily escape (Forssblad, 1981)

Impact compaction, dynamic compaction and blasting are not considered further, since they are not applicable to landslide stabilization because the very high levels of energy involved could itself trigger movement and because of the intrinsic difficulties of applying these techniques on sloping ground. In fact, even vibratory compaction needs to be applied with caution in certain conditions. There are reports of landslides in quick clay triggered by construction-induced vibrations. Vibration from compaction may also cause nuisance and in extreme cases damage outside the zone of application, to a distance of several tens of metres.

High pressure compaction

Surface compaction is generally achieved by driving heavy equipments repeatedly on the soil. Different types of equipment have been developed for this purpose.

The simplest equipment consists of a heavy duty machines or towed units with regular tires, referred to as “pneumatic rollers” (Figure 1a); these compactors may commonly be as heavy as 500 to 2000 kN. As these compactors run slowly on the ground, the top soil gets mechanically compacted by the temporary increased vertical stresses.

Other equipment, referred to as “sheep’s foot roller” (Figure 1c), has been designed to penetrate into the shallow soil to get better compaction; the penetrating parts result in a smaller contact surface to the soil and thus in higher pressures applied; pressures as high as 4.2 MPa may be achieved by the heaviest equipments in common use. The penetrating method is applicable only in presence of fine grained materials, resulting ineffective in coarse grained materials. In clay, these rollers prevent the formation of pre-sheared surfaces sub-parallel to the compaction surface, which can be highly deleterious to stability.

Vibratory compaction

Vibratory compactors are available with vibrating drums (Figure 1d), pneumatic tires or plates (Figure 1b). These compactors use high frequency, low amplitude vertical oscillations in addition to high vertical stresses due to their high weight. In this way the material is shaken and brougth into a more dense state.

As for the non-vibratory equipments, the smoot surface equipments are best suited to compact coarse grained materials; padded or “lagged” equipments, like a vibratory “sheep’s foot roller”, are best suited for fine grained materials.

 

Even in optimal conditions, with these methods the maximum thickness of improvement is less than 2 m and more often less than 0.5 to 1.0 m, hence the applicability of these methods to slope stabilization work is limited.

The high weigth of the equipments is also a limitation. Heavy rollers (static and vibratory) are designed to operate on quasi-level ground; they become relatively ineffective and  difficult to operate on sloping ground. On relatively short slopes they can operate along the line of maximum slope assisted by a winch securely anchored at the top of the slope, but this severely limits their operation and may have safety implications.

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Design methods

Compaction of the top 0.5 to 1.0 m of soil should be sufficient to produce density states characterized by strong interlocking of grains, making the material highly dilatants and thus resistant to shear stresses and the erosive effects of wind, rain and runoff. Compaction can be specified in terms of “method”, detailing the type of equipment and the compaction procedure to be adopted, or in terms of “performance”, specifying the density to be achieved. This is typically specified in terms of the dry density to be achieved in relation to standard (for fine grained soils) or modified (for granular soils) Proctor compaction tests. For granular soils it is also common to refer to relative density instead (Parsons, 1987).

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

References

• Forssblad L. (1981). “Vibratory soil and rock fill compaction”. Dynapack Maskin, Sweden
• Parsons A.W. (1987). “Shallow compaction”. In Ground Engineer’s Reference Book, F.G. Bell ed., Butterworth and Co., London.

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