Hyperbolic method for evaluation of settlement of ground pretreated by drains and surcharge

Abstract

The common method of determining degree of soil improvement in clay deposits treated with vertical drains and surcharge is by means of the settlement record, where the degree of settlement is a reliable measure of the degree of consolidation achieved. Thus, reliable estimation of the ultimate settlement is critical to the estimate of degree of soil improvement before surcharge removal is permissible. The rectangular hyperbola fitting method (Tv/U vs Tv) is extended to the case of drains and surcharge by considering the hyperbolic plots for combined vertical and radial flow consolidation in a soil cylinder with varying thickness of clays and drain spacing ratio for typical soil properties of marine clays with cv of 1 to 5 m2/yr, and ch/cv ratio of 3. The results showed that the hyperbolic plots have an initial linear portion between U50% and U90% and linear regression gives the values of these slopes with regression coefficients of at least 0.99. For the lines radiating from the origin to U50% point the slope is (1/0.5 = 2.0), and to the U90% point the slope is (1/0.9 = 1.11). Thus the ratio of the slopes of these radiating lines to the slope of the initial linear portion of the hyperbolic plots enable the identification of the U50% and U90% for any settlement record using drains and surcharge. With these points, the ultimate settlement can be estimated as twice the settlement of the 50% point, or 1.11 the settlement of the 90% point. The method is examined through a case history, involving sand drains at Ska-Edeby, Sweden, where long term settlements and pore pressures data of 14 years are available. For the long term loading tests at Ska-Edeby, the prediction of ultimate settlement by the proposed method agrees very well with actual records. Thus, the method can serve as an additional simple means, apart from compressibility calculations, of monitoring the progress of consolidation for ground treatment with drains and surcharge.