Estimation of Degree of Consolidation for Vacuum Preloading Projects J. Chu 1 and S. W. Yan 2 Abstract: The degree of consolidation is usually used as one of the criteria for assessing the effectiveness of soil improvement work using the fill surcharge or vacuum preloading method. It is also often used as a design specification in a soil improvement contract. Degree of consolidation is normally calculated using settlement data. However, as the effect of vacuum preloading is controlled largely by pore water pressure changes, it is necessary to analyze the pore water pressure variations and to assess the degree of consolidation using pore water pressures. In this paper, the problems involved in the estimation of degree of consolidation using settlement data are discussed. A method to estimate the average degree of consolidation using pore water pressure data is suggested. Two case studies are presented to examine the characteristics of the pore water pressure variation of soil under vacuum loading. The degree of consolidation achieved in each of the two cases is assessed using pore water pressure data and compared with that estimated using settlement data. Factors affecting the degree of consolidation assessment are discussed. DOI: 10.1061/ASCE1532-364120055:2158 CE Database subject headings: Soil consolidation; Pore water pressure; Soil improvement; Preloading. Introduction One of the commonly used soft soil improvement methods is vacuum preloading. This method has been successfully used in a number of countries for land reclamation and soil improvement work Holtz 1975; Choa 1990; Jacob et al. 1994; Bergado et al. 1998, 2002; Chu et al. 2000. Sand drains and recently prefabri- cated vertical drains PVDs have often been used to distribute vacuum load and discharge pore water. The principles and mecha- nism of vacuum preloading have been discussed in the literature, e.g., Kjellman 1952 and Holtz 1975. A vacuum load of 80 kPa or greater can be maintained as long as it is required. Compared with the fill surcharge method for an equivalent load, the vacuum preloading method is cheaper and faster Chu et al. 2000. A major difference between fill surcharge and vacuum pre- loading is in the pore water pressure change. Under fill surcharge, the excess pore water pressure will first build up from its initial normally hydrostatic state by the same amount as the surcharge, and then dissipate gradually, as shown in Fig. 1a. On the other hand, under vacuum pressure, the pore water pressure in the soil will reduce from its initial normally hydrostatic state by the same amount as the applied vacuum pressure, as shown in Fig. 1b. As the pore water pressure can reduce to a negative value, the pore water pressure changes due to the vacuum load are more complicated. This is particularly the case when a combined fill surcharge and vacuum load are applied. Therefore, for vacuum preloading projects, the pore water pressure variation during con- solidation should always be monitored. In addition to pore water pressure, the ground settlement is also usually monitored and used to calculate the degree of con- solidation DOC. DOC is an important parameter in evaluating the effectiveness of soil improvement. It is also often used as a design specification in a soil improvement contract. DOC is nor- mally calculated as the ratio of the current settlement to the ulti- mate settlement. However, for a soil improvement project, the ultimate settlement is unknown and has to be predicted. Several methods are available for estimating the ultimate settlement. Among them, Asaoka’s 1978 and hyperbolic Sridharan and Rao 1981 methods are commonly used. In Asaoka’s method, a series of settlement data S 1 ,..., S i-1 , S i , S i+1 ,... S N  which are ob- served at constant time intervals are plotted in a S n versus S n-1 plot n =1,..., N. The ultimate settlement, S ult , is taken as the intersecting point of the line with the 45° line Asaoka 1978, as illustrated in Fig. 2. However, S ult obtained from Asaoka’s method is affected by the time interval used. Matyas and Rothenburg 1996, Bo et al. 1999, and Goi 2004 have shown that the longer the time interval, the smaller the S ult predicted. In the hy- perbolic method, settlement data are plotted as time/settlement versus time curve Sridharan and Rao 1981. The S ult is estimated as the inverse of the linear slope of the plot. However, S ult ob- tained from this method is affected by the DOC achieved when the last set of data was taken. The higher the DOC that the soil has attained, the smaller the S ult obtained as observed by Matyas and Rothenburg 1996, Bo et al. 1999, and Goi 2004. Goi 2004 also shows that the plot is not strictly a straight line and thus, the value of the linear slope can be different if the slope is taken from different sections along the curve. A smaller S ult is obtained when the slope is taken from the end of the curve. The uncertainties involved in the ultimate settlement calculation will affect the estimation of the DOC. As a result, different DOCs are obtained using different methods. As an alternative, pore water pressure data can be used to 1 School of Civil and Environmental Engineering, Nanyang Technological Univ., 50 Nanyang Ave., Singapore 639798. E-mail: cjchu@ntu.edu.sg. 2 Geotechnical Research Institute, Tianjin Univ., Tianjin, China. Note. Discussion open until November 1, 2005. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on March 26, 2004; approved on October 25, 2004. This paper is part of the International Journal of Geomechanics, Vol. 5, No. 2, June 1, 2005. ©ASCE, ISSN 1532-3641/2005/2-158–165/$25.00. 158 / INTERNATIONAL JOURNAL OF GEOMECHANICS © ASCE / JUNE 2005