Using old ideas for new methods of compaction

New compaction equipment makes Palm Jebel Ali Debut

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By  Colin Foreman Published  October 2, 2004

Using old ideas for new methods of compaction|~|Landpac body.jpg|~||~|Old ideas often hold the answer to many of the technical problems that we face. Construction is no different as commercial pressures drive the industry to constantly search for more effective methods of getting the job done. This is true of almost all disciplines, including ground compaction. For hundreds of years ground has been compacted by using the ancient principle of raising a mass and allowing it to fall onto the ground. This has now been developed further and forms the basis of a continuous dynamic compaction process known as impact compaction. Impact compaction uses the lifting and falling of a non-circular rotating mass to transfer energy into the soil and compact it. The rotation of the mass to its highest point creates a build up of effective potential energy, which, as the mass rotates further, is converted into falling kinetic energy before it is transferred into the soil upon impact with the surface at the lowest point of rotation. The relatively high levels of energy that impact compactors create as they rotate, generates large compactive forces that allow for the deep compaction of soils and layerworks. Impact compactors also operate at relative high speeds (between 8 and 10 km/h), which enables high production levels. The Dutco Balfour Beatty Group has recently entered into an association with Landpac, a South African Company, to make this technology available in the United Arab Emirates. Equipment has already been delivered and is currently available for work in Dubai and the surrounding region. Impact compaction can be used for a wide range of different applications. For deep in-situ sub-grade compaction impact compaction allows for deep, in-situ compaction of sub-grade material up to depths of 2 m and beyond, depending on the type of compactor used, the soil type and the moisture regime. This negates the need for costly excavation works, the replacement and compaction of material in thin layers, which is typically required when using conventional compaction equipment. Impact compaction can achieve the same results with a single process of deep compaction from the surface. Impact compaction equipment is also able to perform thick-lift compaction of fill layers. In contrast to the relatively thin lift compaction capabilities of conventional plant, impact compaction is capable of achieving thick-lift, often single layered, compaction of fills, in layers as much as 600 – 1500 mm. This capability allows relatively high production rates to be achieved, resulting in improved utilisation of earthmoving equipment. Landpac impact compactors are also particularly well suited to compacting the “collapsible” soils that are found in many regions of the world. These soils contain particles, which are lightly cemented together, for example, by clay, mineral or salt bonds, in a highly voided structure and are susceptible to give way once built upon when there is an increase in the moisture content of the soil or the surface loading. Because of the energy delivered with each blow, the required moisture content is reduced to well below that of the modified AASHTO optimum (it is typically 3-5% lower). Because of this, impact compactors are very effective in dry regions where water for construction purposes is scarce and expensive. The high-energy blows delivered are also capable of significantly increasing the pore pressure within saturated soils. This increase in pore pressure can lead to an increased rate of dissipated water provided that suitable drainage paths are available. Regularly repeated applications of impact compactors can be used in this application to accelerate primary consolidation of saturated soils compared with the conventional technique of surcharging an area with a large static load. In this way consolidation periods can be drastically reduced. The ability to compact to greater depths makes impact compaction well suited for effectively compacting rock fill. This is achieved by subjecting the rock fill to a substantial dynamic compaction force which is able to rearrange the rock fragments into a dense, interlocking stable mass well able to resist lesser service loads without deformation. The ability to pre-stress soils from the surface to a stress level that exceeds the levels that the materials are likely to be subject to in service, makes impact compactors ideally suited for use as a “proof” roller. Proof rolling is especially useful for detecting any weak areas associated with low density and/or excessive moisture conditions, which can be improved by soil replacement and/or remedial reworking. Impact compaction forms a cost-effective solution for a variety of specialist applications, particularly where a significant decrease in permeability through a reduction of air voids and a reduction in bulk volume are required at depth. Possible applications include: Cement kiln dust; coal; landfill (refuse) and the rehabilitation of brownfield sites. In addition to impact compaction, Landpac has recently developed and patented the innovative Continuous Impact Response (CIR) measurement system. This system makes it possible to use impact compactors as a proof roller to measure the soil’s response to the dynamic loading of the impact compactor. The CIR system is capable of measuring and recording the soil response to every impact of the roller, resulting in a direct measurement of soil stiffness (which is a function of the soil density). The CIR system employs an accelerometer measuring and recording the peak deceleration with each impact. These deceleration values relate to the “stiffness” of the soil and are easily correlated to conventional soil density and strength test values. The CIR employs a GPS (Global Positioning System) receiver to locate the position of the impact roller. Each of the peak decelerations is then recorded relative to its position on site as determined by the GPS receiver. The two pieces of information are then combined to map the soil response over the entire site giving continuous quality monitoring of compaction. The machine demonstrated its capabilities in front of an audience of engineers and consultants on the Palm Jebel Ali last month. After several passes with the roller the CIR system sucessfully showed how it can be used to eliminate differential settlement, which can cause serious problems for developers. ||**||

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