Expansion of Dubai international airport's main terminal has seen the dynamic compaction of some 575,000m2.
Foundation contractor Bauer (UK) was involved in the two main phases of ground treatment between July 1996 and May 1998. Work was carried out to allow construction of new taxiways/ aprons associated with the expansion and building of a new runway north of the airport.
Working for client Dubai Department of Civil Aviation, consultant International Bechtel Incorporated and main contractor Al Naboodah Contracting assessed the various treatment options before deciding to use dynamic compaction, providing preliminary trials proved the technique's suitability.
Within the Dubai area of the Persian Gulf, wind erosion, capillary action and evaporation has led to the formation of extensive Sabkha deposits, notably at the head of the creeks. Sabkha generally consists of fine and fine to medium sand which may be loose to very loose, brown or grey, silty to very silty, with an increase in relative density and degree of cementation with depth. Zones of 'cohesive' silts and clays also occur within these layers, although they tend to be transitional in terms of soil classification, and generally of low to medium plasticity. There are also areas of shelly fine sand and aeolian (wind blown) deposits forming sand mounds or minor dunes over the low lying Sabkha.
Before dynamic compaction trials began, Al Naboodah placed a layer of approved dune sand typically between 0.75m and 1.2m thick. This 'cushion material' was intended to provide a working surface for the cranes, act as a source of material to place in the imprints formed during weight dropping, and to help maintain the working surface at a reasonable height above groundwater level. However, some dewatering was carried out where it was deemed necessary.
Dynamic compaction involves the controlled dropping of a heavy weight, or tamper, from a crawler crane on a predetermined grid pattern. The size and mass of tamper and, in turn, the crane size, is a function of the ground conditions, the required depth of treatment and performance requirements.
The objective of the treatment is to enforce settlement in the ground by reducing void spaces between constituent soil particles. This improves the geotechnical properties of the soil profile and controls settlement. At Dubai airport, improvement of the upper 5m of the soil profile was generally needed.
Normally, the heaviest drops - with maximum energy input - are carried out during the first tamping passes. The aim is to compact the lower or deeper sections of the improvement zone, with subsequent passes compacting progressively higher and shallower layers of the soil profile. Pauses between passes are allowed where necessary to allow adequate dissipation of excess pore water pressures. After each of the three to four tamping passes applied at Dubai, the resulting imprints were backfilled with the dune sand.
Cranes used in the dynamic compaction operation included Manitowoc 4100 machines and Lima 2400B machines, with between two and four cranes working at any one time dropping weights ranging from 10t to 15t, from heights of up to 15m.
Trials, with pre- and post-treatment cone penetration testing, were carried out on each main phase of work before the main treatment to establish optimum energy inputs and regimes for both taxiway and runway areas. Only after approval of trial results was the main dynamic compaction work permitted to progress.
Careful monitoring also forms an essential part of any dynamic compaction project, and typically involves measuring site levels before and after treatment passes, measuring imprint depths and recording details of any heave. Trial drops and/or heave tests are also undertaken to determine the optimum number of drops for each pass as work progresses across the site.
During trial drops, levels are typically taken on steel pins driven into the ground, radiating away from the dropping position and on top of the tamper, so that the volume of the depression and volume of heave generated around the resulting imprint may be calculated. Parameters such as drop height, number of drops and depth of imprint are recorded for each position on a daily report sheet, and on site drawings. Such monitoring is essential to ensure that the correct amount of energy is applied to the ground at the right time.
When dynamic compaction was finished, excess dune sand was removed by the main contractor and insitu density tests carried out prior to placement of granular material with rolling (and wetting as appropriate) to achieve the required levels and grades before con- struction of the taxiway and runway.
The Sabkha deposits were notable for their high energy absorption, with energies of up to 221tm/m2 applied, and enforced settlements typically between 300mm to 400mm depending on the depth of weak soil. Within the finer grained soil zones, evidence of 'thixotropic recovery' was evident with soil strength increasing with time as pore water pressures dissipated.
Specifications required standard penetration test N values of up to 20 depending on the phase and end use of the treated area. In the light of the fine grained nature of some of the Sabkha deposits, CPTs were used to assess treatment by correlating cone tip resistance values to equivalent SPT N values.
Dubai-based Technical Laboratory performed CPTs from a trailer-mounted 200kN hydraulically powered penetrometer with a mechanical Begemann cone. At first, pre- and post-treatment testing was carried out at a relatively high frequency, with less testing once confidence in the treatment grew and consistency of results had been established. Around four CPTs were carried out at each test site within the 500m2 site. The consultant also commissioned some of its own CPTs. Another requirement was to achieve appropriate insitu dry density values at specific levels.
Dynamic compaction has performed well at Dubai airport by improving the characteristics and geotechnical properties of Sabkha deposits before taxiway and runway construction. It has also proved to be an acceptable and quicker method than options such as extensive earthworks.