Như đã đề cập trong chương trước, trong một thử nghiệm một nén chiều trên đất sét, dưới một tải trọng không đổi, biến dạng thường xuất hiện để tiếp tục thực tế mãi mãi, ngay cả khi áp lực lỗ rỗng từ lâu đã được giảm xuống bằng không, xem hình 18,1. Các loại tương tự của hành vi được tìm thấy trong các vật liệu khác, như nhựa, bê tông, Etcetera. Hiện tượng này thường được ký hiệu là leo. Đối với nhiều vật liệu, hành vi này có thể được mô hình hợp lý cũng bởi lý. | Chapter 19 SECULAR EFFECT As mentioned in the previous chapter in a one dimensional compression test on clay under a constant load the deformation usually appears to continue practically forever even if the pore pressures have long been reduced to zero see Figure . Similar types of behavior are found in other materials such as plastics concrete etcetera. The phenomenon is usually denoted as creep. For many materials this behavior can be modelled reasonably well by the theories of visco-elasticity or visco-plasticity. In such models the creep is represented by a viscous element in which part of the stress is related to the rate of deformation of the material. Although the behavior of soils may contain such a viscous component the creep behavior of soils is usually modelled by a special type of model that has been based upon the observations in laboratory testing and in field observations. Keverling Buisman In 1936 Keverling Buisman of the Delft University found that the deformations of clay in a consolidation test did not approach a constant final value but that the deformations continued very long. On a semi-logarithmic scale the deformations can be approximated very well by a straight line see Figure . This suggests that the relation between strain and stress increment after very long values of time can be written as Ep s log . t0 Here p is the primary strain and s is the secular strain or the secondary strain. The quantity t0 is a reference time usually chosen to be 1 day. Keverling Buisman denoted the continuing deformations after the dissipation of the pore pressures as the secular effect with reference to the Latin word seculum for century . In most international literature it is denoted as secondary consolidation the primary consolidation being Terzaghi s pore pressure dissipation process. The primary strain p is the deformation due to the consolidation of the soil. This is being retarded by the outflow of groundwater from the soil as .
