Significant cost implications in using Janbu's simplified or Morgenstern-Price slice methods for soil nail design of cut slopes

Abstract

Since 1985, soil nailing has been a method of choice for slope stabilisation in the landslip preventive measures project in Hong Kong. Different slice methods have been used in the design of soil nails for stabilising substandard cut slopes, with Janbu's simplified and Morgenstern-Price methods being the most popular ones in Hong Kong. This paper investigates the effects of the two different slice methods on the soil nail design. A number of actual cut slopes involving soil nail design are studied in the paper. For each case, the two different slice methods are used to calculate the factors of safety before and after the stabilisation works. Soil nails are designed according to the current design practice. The corresponding costs of the proposed soil nailing works are then estimated. The two slice methods are compared on the basis of both slope safety and construction cost. It is found that the construction cost of soil nailing stabilisation works adopting Janbu's simplified method could be 10% to 90% higher than that adopting the Morgenstern-Price method under the same safety standards. Technical reasons for such substantial differences are also given in the paper. It is believed that the findings presented in the paper could be used to reduce soil nail construction costs without sacrifice of the current slope safety standards.