Slope Stabilisation-Site investigation, Design and Application of Deep Soil Mixing

State Highways in the Northland region of New Zealand are susceptible to instability due to slope failure. One of the main reasons is the unique properties of the Northland Allochthon that is underlying the area. Typical geomorphology within the Northland Allochthon is characterised by numerous shallow -seated failures due to low residual strengths. In some locations the problems have been compounded by inappropriate drainage measures or additional surcharging by placing fill on a subsiding road formation.

Deep soil mixing has been used as one of the remedial solutions to this problem. This paper presents a discussion and description of the regional geology and unique geotechnical characteristic properties of the materials. A case study is presented to demonstrate typical methods of investigation, interpretation and design of the adopted remedial works.