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Abstract For the design of the foundation level and stability assessment of concrete dams three criteria with respect to the rock-mass are important to establish as soon as possible for a safe and economic dam design; the rock shear strength parameters, concrete-rock shear strength parameters and modulus of deformation. In this respect timely in-situ testing carried out according to international standard is considered best practice. The Lai Chau dam is situated in northern Vietnam and is the upper most dam in a cascade of hydropower dams along the Song Da. The site geology is of a composite granitoid intrusive body of grano-diorite, quartz-diorite and diorite, which has been syn-tectonically intruded and extensively tectonised post intrusion. In the technical design stage dam foundation levels are initially defined based on weathering. Vietnamese weathering classification systems describe 5 weathered weathering Vietnamese weathering classification systems describes 5 weathered grades from IA1 (completely weathered rock) through to IIB (fresh rock). For the Lai Chau Project two exploratory adits; HN-1 (length: 260 m) and HN-2 (length: 200 m) were carried out at the proposed dam axis in the left and right abutments respectively. The rock mass of the weathering grade IIA (slightly weathered) was considered the most appropriate foundation material. However, tests were also carried out in rock masses of weathering grade IB (moderately weathered) and IIB (fresh). A comprehensive in-situ testing program (rock-rock shear and concrete-rock shear tests and modulus tests) was performed in the rock-mass on the various weathering grades to establish the parameters associated with the identified weathering grades. On the basis of these tests an optimized foundation design was possible with foundation in IIA in the river section and in IB on the upper parts of the abutments. As a result excavation and RCC volume could at design stage be minimized. 1. Introduction The dam for the Lai Chau HPP (under construction) is the most upstream of a cascade system on the Song Da within Vietnam (see Fig. 1 for location). Downstream the Son La dam (2012) and the Hoa Binh dam (1994) have already been constructed. The completion of the Lai Chau HPP will result in a total installed power capacity of 5'520 MW on the Song Da mainstream. For the Lai Chau dam an RCC (roller compacted concrete) gravity type dam was chosen as the most economic. The maximum dam height is131 m. The original dam design called for a maximum height of 137 m. A higher quality rock mass in the river section allowed for excavation to be stopped 6 m higher. The installed power capacity is 1'200 MW with 3 Francis turbines. Construction of the Lai Chau dam started in 2011 and is due to be completed in 2015.