Prefabricated vertical drains (PVDs) in soft Bangkok clay: a case study of the new Bangkok International Airport project

Abstract

This paper presents the performance of a full-scale test embankment constructed on soft Bangkok clay with prefabricated vertical drains (PVDs) at the site of the new Bangkok International Airport in Thailand. The embankment was square in plan with a maximum height of 4.2 m, 3H:1V side slopes, and base dimensions of 40 m by 40 m. The piezometric level with depth is characterized by negative drawdown starting at around 8-10 m depth caused by excessive withdrawal of groundwater. Instrumentation was provided to monitor both horizontal and vertical movements of the test embankment. The measured increases in undrained shear strengths with depth are in agreement with the values calculated from the SHANSEP technique. The secondary compression ratio, Cα, was 0.018, or within the normal values for marine clays. The coefficient of horizontal consolidation measured in the field, Ch(field), was higher for soil at 4 and 10 m depths than for the weakest soil at 6 m depth. The back-calculated Ch(field)values range from 3 to 8 m2/year, and the ratio of Ch(field)to Ch(lab)ranges from 4 to 5, where Ch(lab)is the coefficient of horizontal consolidation measured in the laboratory. The degree of consolidation estimated from the pore-pressure dissipation measurements agreed with those obtained from settlement measurements. The water-content reductions from field measurements were also in good agreement with the values computed from the consolidation settlements. The full-scale study confirmed that the magnitudes of consolidation settlements increased with the corresponding decrease of PVD spacing at a particular time period. Lastly, the results of the full-scale study have proven the effectiveness of PVDs for the improvement of soft Bangkok clay.Key words: soft clay, consolidation, prefabricated vertical drain, preloading, test embankment.