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Jetting in to the rescue


Jet grouting has proved successful in combatting seismic liquefaction of alluvial soils in Turkey, preventing damage to a new car plant during 1999's devastating earthquake. Dan Simpson reports.

Car manufacturer Ford Otosan's new plant on the south shore of the Sea of Marmara in north west Turkey was nearly finished when the region was hit by a devastating earthquake in August 1999.

Remarkably, despite the epicentre of the 7. 4 Richter scale event being just 1km from the site and extensive damage to the nearby town of Golcuk, only the plant's body shop was affected. This building was very close to the surface rupture line of the fault and its south west corner moved to the north west by about 450mm. Escape was down to the original design identifying the liquefaction potential of the underlying weak alluvium and including extensive ground improvement using jet grout columns and stone columns.

Work had began on the plant almost exactly one year earlier, in August 1998. The 1. 6M. m 2site includes a press shop, body shop, paint shop, assembly shop, administrative buildings and offices.

Future plans include construction of a jetty and a test road.

The site is underlain by recent alluvial deposits formed by irregular streams flowing from mountains to the south. Sedimentation took place in turbulent conditions producing a wide range of grain sizes, with clays and silts dominating in the upper 5m of the soil profile. Below is a 7m thick layer of medium dense silty sand, underlain by gravelly sand. Groundwater is typically 1m below ground.

Site investigations, comprising 27 boreholes to a maximum depth of 21m, revealed that some of the sand layers were susceptible to liquefaction. This is a common problem to the area, rendered one of the most seismically active in the world by North Anatolian fault zone stretching from East Anatolia into the Sea of Marmara. Seven major earthquakes hit the area in the 20th century.

The response was 800mm diameter jet grout columns and 600mm diameter stone columns transferring loads of between 20kPa to 50kPa from the basement slabs to denser strata, controlling settlement and limiting the risks of liquefaction.

Before jet grouting began, six 10m long trial columns were tested using unconfined compression tests to determine their strength. This allowed the operational parameters, such as grout composition, pressure, number and size of nozzles and lifting and rotation speeds, to be determined for the working columns.

Jet grouting was carried out on a 2m by 2. 5m grid and stone columns on a 2m by 2m grid. For the body shop, four jet grout columns between 10m and 12m long went in under each footing, carrying loads of between 1,000kN and 1,500kN. Four, 650mm to 800mm diameter bored piles between 15m to 18m long supported the columns in the paint and press shops In all, Turkish geotechnical contractor Kasktas' £15M contract comprised 460,000m of jet grouting, 50,000m of stone columns, 88,000m of bored piles, 12,000m 2of diaphragm wall and 5,000m 2of sheet piling.

Foundations were 90% complete when the earthquake struck on 17 August 1999. The entire site settled by between 1. 5m and 2m as the fault pulled apart, shifting 1. 6m vertically and 2m horizontally just metres from the site boundary. Some 30,000 people were killed and hundreds of thousands left homeless after the earthquake, with liquefaction and poor design cited as one of the main causes of destruction (Ground Engineering October 1999).

In the aftermath an extensive ground investigation was carried out to check soil conditions and examine any secondary fault surfaces. The liquefaction potential and dynamic properties of the silty sand layers were also determined. The menu included 225 CPTs, eight boreholes with sampling and SPTs, three auger borings and four groundwater and gas wells for sampling and monitoring.

The foundations were also analysed to assess the performance of the jet grout columns during the earthquake. A zone and a plate loading test on ground improved by grout columns and a plate loading test on ground improved by stone columns showed residual settlements to be negligible. Pull out tests were also carried out on the grout columns.

Nor did core samples from 30 jet grout columns show any cracking induced by the earthquake, and unconfined compression tests confirmed strengths of 6MPa to 28Mpa. The foundation system was safe and work could proceed.

As part of the post-earthquake quality control plan, compression tests on core samples from the head of one in 200 jet grout columns and continuous core samples from one in 1,000 jet grout columns investigated at depth the variations of strength characteristics and densities.

Despite the good performance of the foundations, geotechnical consultant Arup Muhendislik ve Musavirlik, which replaced original geotechnical consultant Enar Muhendislik Mimarlik Danismanlik in January 2000, made some design revisions, adding an extra 150,000m of jet grouting and 55,500m of bored cast insitu piles, installed by the end of April this year.

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