Cases and solutions Environmental Geology 37 (4) April 1999 7 Q Springer-Verlag 317 Received: 26 March 1997 7 Accepted: 23 March 1998 A. S. Al-Homoud 1 (Y) Civil Engineering Department, Jordan University of Science and Technology, P. O. Box 3030, Irbid, Jordan tel: 962-2-295111 ext 2126 Fax: 962–2-295123 Present address: 1 Dar Al-Handasah (Shair and Partners), P.O. Box 28873, Dubai, United Arab Emirater Tel: 971-6-286129 Fax: 971-4-210976 G. Prior Dames and Moore International, Jedda, Kingdom of Saudi Arabia A. Awad Civil Engineering Department, Tishrin University, Lattikia, Syria Modelling the effect of rainfall on instabilities of slopes along highways A. S. Al-Homoud 7 G. Prior 7 A. Awad Abstract During and after the very wet 1991/1992 winter experienced in Jordan, significant instabili- ties occurred in several sections of the new Ir- bid–Amman Highway in Jordan. A potential impact of surface and subsurface flow on the failed areas was noted. This paper is concerned with a hydro- logical study of the Irbid–Amman Highway, includ- ing watershed characteristics, hydrological data analysis, and hydraulic analysis of remedial works to failed areas and other parts of the highway drai- nage to minimize the impact of surface runoff and the effects of subsurface water with respect to the stability of the upslope and to limit the porewater pressure within the road formation. The study shows that the culvert capacity and the discharge velocity of the cross-highway drainage are deficient. Recommendations (e.g. gabion cascades) are there- fore suggested to reduce drainage to the cross high- way drainage structures. Grouting is suggested to stabilize the embankments above the inlets to cul- verts. Remedial drainage works are suggested for each major failed area. These include: culverts, ta- ble drain, drainage blanket beneath road formation, cut-off drains above cuttings, subsurface drains into the cut in the vicinity of springs, curtain drain beneath the lined table drain, and others. Key words Slope instabilities 7 Highways 7 Rainfall characteristics 7 Surface and subsurface flow 7Watershed characteristics 7 Hydrological and hydraulic analysis 7 Remedial drainage works Introduction In recent years, development requirements in Jordan dic- tated the construction of new roads, dams, and buildings; and, due to the nature of terrain and population distribu- tion, a significant number of projects are taking place in mountainous to hilly areas. As a result, natural slopes were manipulated by excavations or placement of fill to meet engineering requirements (Al-Homoud and others 1994a,b, 1995a,b). This expansion was unfortunately ac- companied in many instances by slope failures, causing either severe property damage or serious delays to con- struction schedules, or both, not to mention the addition- al incurred costs. Landslides were reported in many areas in Jordan, such as Amman, Kerak, Tafila, Ajlun, the Ir- bid–Amman Highway and Na’ur–Dead Sea road. Several investigations undertaken by local and interna- tional firms over the past three decades attempted to shed some light on the causes and mechanisms involved in certain cases, and suggested remedial measures for them. Some of these measures were not implemented due to the lack of advanced geotechnology in Jordan. Others were implemented, several of which were, nevertheless, unsuccessful, reflecting a misunderstanding of the prob- lem and the factors involved. The Ministry of Public Works and Housing (MPWH) in Jordan is constructing three sections of dual carriage highway north of Amman between the old Jerash Road junction and the Irbid Highway (Fig. 1). During construc- tion, several slope failures have occurred on the following two sections of the highway: section II (km 24c037 to km 41c974); and section III (km 41c974 to km 57c343). There is a potential impact of surface and sub- surface flow on the failed areas. Hydrological studies are necessary as part of the road de- velopment project to determine the parameters for use in hydraulic design of the highway drainage structures.