Lime-cement columns have been used in Sweden to improve poor soil conditions since the 1970’s. The method is inexpensive and flexible, but is difficult to test since the columns are manufactured in-situ. Many test-methods have been developed for testing the column strength during the years.
Most of them need to be evaluated using an empirical correction-factor known as the cone factor. The column penetration test, KPS, is the most commonly used method in Sweden, it is considered to be reliable since a large part of the column cross-section is tested. The problem is that the probe easily deviates out of the column to the softer surrounding soil.
Today a pre-drilled guiding-hole, a soil-rock sounding, helps the probe to stay vertical. Although the soil-rock sounding is commonly not used for evaluation of column strength, the penetration resistance is recorded. A visual comparison between the plotted penetration resistances from the two methods shows similarities in both hard and soft areas of the columns.
The relation can be measured using statistics, such as the correlation coefficient. A strong correlation was also found, suggesting that a similar equation used to evaluate the undrained shear strength from the column penetration tests can be applied with the data from the soilrock soundings. The statically pushed column penetration test probe and the rotated soil-rock sounding bit bore are likely to cause different failure modes in the column. This means that different empirical cone factors are needed when the undrained shear strength is evaluated.
By evaluating the ratio between the cone factors of the column penetration test and the data from the soil-rock soundings from three sites, E-road E18 north of Stockholm, E-road E45 outside Gothenburg and at a construction site at Lidingö, the following aspects of the ratio was investigated: if the ratio was site-specific; the sensitivity to the binder content; the sleeve friction and; the sensitivity to rotational speed and rate of penetration. Average columns formed from the penetration resistance at depth from each site were used during the evaluations.
The Swedish geotechnical society has standardised two methodologies that can be used for pre-drilling. The soil-rock sounding methodology which has no fixed rate of penetration or rotational speed, and the total sounding methodology, based on the Norwegian total sounding methodology which has fixed rate of penetration and rotational speed. The latter is to prefer when comparing results between sites.
To remove the sleeve friction, the data from the soil-rock soundings needed to be de-trended. The amount of de-trending needed to find a constant cone factor varied at the sites between 0.5 kN/m and 1.0 kN/m. This however caused high interference, partly from scaling the variation. The cone factor for the total sounding methodology was found to be between 0.30-0.45 times the cone factor for the column penetration test.
Author: Fransson, Johan