Steven Roberts, principal geotechnical engineer and director of Kirk Roberts Consulting Engineers, enlightens us about the most commonly used ground improvement techniques adopted in post-earthquake Christchurch
Just over six years ago Christchurch woke up to an event that reminded everyone in New Zealand how vulnerable we are to nature’s unpredictability. The September 2010 and February 2011 events changed the city’s residential landscape irrevocably.
After the dust settled, the Institute of Professional Engineers of New Zealand (IPENZ), in collaboration with the Ministry of Business, Innovation and Employment (MBIE), carried out extensive ground improvement test trials in Christchurch to determine the most appropriate techniques to protect lightweight dwellings from irreparable damage on earthquake-prone soils.
The most commonly used ground improvement techniques in Christchurch are: Reinforced Gravel Rafts, Stone Columns, Driven Timber Poles and Reinforced Soil-cement Rafts.
What’s best for my site?
As a geotechnical engineer, the question I hear most often is: ‘what type of foundation can I use on my site, and what will it cost me?’
To highlight which ground improvement option is likely to be the best for your site, the cost estimate table below has been reproduced from EQC’s Residential Ground Improvement Report, released October 2015. It should be noted that the construction cost estimates apply to a 146m2 house and were calculated between April and October 2015, and are subject to current material and labour costs.
Reinforced Gravel Rafts (1)
This involves the construction of a compacted ‘raft’ of engineered gravel. The raft is mechanically stabilised by introducing layers of geogrid mesh. The geogrid is designed to limit ground surface damage due to undulations and uneven ground surface settlement caused by liquefaction in a future earthquake.
Stone Columns (main image)
Within the treated area, vertical columns of stone are inserted into the ground at least four metres below the surface, in either a triangular or square grid arrangement. Stone column treatment to a greater depth may be required depending on the size and weight of the building and the site-specific ground conditions. The treated ground is made stronger and more resistant to liquefaction by densifying the soil between the stone columns and/or reinforcing the soil to create a stiff composite soil mass.
In addition, stone columns provide a series of drainage paths that may reduce the build up of pore water pressure in the soil mass during strong ground shaking, and thereby suppress the onset of liquefaction.
The number of stone columns required for adequate improvement is determined by the specific design from a geotechnical engineer. The technical term, Area Replacement Ratio (ARR), is the number of stone columns required for ground improvement, and simply expresses the accumulated surface area of the individual stone columns as a ratio of the total treated ground surface area.
Driven Timber Poles (2)
Timber poles are driven into the ground, in a triangular arrangement, using either a pile-driver or a vibrating plate mounted on a track excavator. In established residential areas, the vibrating plate method is preferred, due to reduced noise and vibrations during installation. Driven timber pole ground improvement helps to limit the effects of liquefaction by densifying and reinforcing the soil between the timber poles.
Reinforced Soil-cement Rafts (3)
This involves the construction of a stabilised crust layer up to 1.2m thick made of cement-stabilised soil laid over reinforcing mesh called geogrid. The stabilised crust layer is designed to be stiff enough to limit ground surface damage caused by liquefaction in a future earthquake.
The ground improvements discussed provide different levels of ground performance based on the site-specific ground conditions determined through a geotechnical investigation. In some cases, the ground and weather conditions can influence which ground improvement option to use. For example, for a site with a high water table, construction of a reinforced gravel raft may not be the best solution, as excavation below the water table would require dewatering the site.
From my experience, stone columns provide the best value in regard to predicted foundation performance for the weaker sites located in Technical Category 3 (TC3) areas of Christchurch; while in the case of Technical Category 2 (TC2) areas, the reinforced gravel raft remains the preferred ground improvement option.
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