Technical Note Some basic geotechnical properties of expansive soil modified using pyroclastic dust Ezekwesili Ene ⁎, Celestine Okagbue Department of Geology, University of Nigeria, Nsukka, Nigeria abstract article info Article history: Received 25 January 2009 Received in revised form 21 March 2009 Accepted 29 March 2009 Available online 5 April 2009 Keywords: Geotechnical properties Pyroclastic dust Stabilization Expansive soil This paper reports an investigation of the influence of pyroclastic rock dust on the geotechnical properties of expansive soil. The plasticity, linear shrinkage, compaction, California bearing ratio (CBR) and shear strength characteristics of the soil when mixed with varying proportions of pyroclastic rock dust were investigated. The results show significant reduction in plasticity and linear shrinkage of expansive soil with increasing amount of pyroclastic rock dust. The maximum dry density, optimum water content, shear strength and CBR all increased with increasing pyroclastic rock dust content. Optimum CBR values were obtained with the addition of up to 8% pyroclastic rock dust. The behavior of the geotechnical properties of the expansive soil when blended with pyroclastic rock dust indicates that the rock dust is a good modifier for this problem soil. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Expansive soils denote clayey soils that not only possess the tendency to swell or increase in volume but also to shrink or decrease in volume when the prevailing moisture condition is allowed to fluctuate. Such fluctuation or alteration of moisture content of these soils can emanate from rains, floods, leakage of sewer lines, or from the reduction of surface evaporation due to placement of geotechnical structures. The response of expansive soils in the form of swelling and shrinkage due to changes in water content is frequently expressed superficially as heaving and settlement of lightly loaded geotechnical structures such as pavements, railways, roadways, foundations and channel or reservoir linings. (Okagbue, 1990; Cokea,1999). Even when mitigating measures such as drain systems have been provided to prevent these soils from reacting to changes in their moisture condition, the soils still exhibit inherent low shear strength and undergo large secondary compression. In spite of these obvious known problems, many geotechnical and geoenvironmental engineering structures are founded on them. Perhaps the main reasons for their inevitable use are their prevalence. To combat these challenges, some geotechnical engineers decide to design to accommodate the potential problem, some opt to excavate and replace the entire soil while some opt to improve the geotechnical properties of the soil. These options are ultimately constrained by project cost. The third option informed the present study for the construction of a 20 km highway underlain by expansive soil in Southeastern Nigeria. Improvement of geotechnical properties of poor construction materials such as expansive soils has been achieved in many places using soil additives. Among these additives are lime, cement, rice husk ash, asphalt, limestone ash etc. (Indraratna et al., 1995; Cokea, 1999; Muntohar, 1999; Muntohar and Hantoro, 2000; Okagbue and Yakubu, 2000). This paper presents the influence of pyroclastic rock dust on some basic geotechnical properties of expansive clay soil. The properties investigated include Atterberg limits, compaction, California bearing ratio, and strength. The behavior of these properties with varying proportion of the rock dust was used to assess the effectiveness or otherwise of pyroclastic rock dust in improving the geotechnical properties of expansive soils. 2. Research materials The materials used for this investigation were pyroclastic rock dust and expansive clay soil. The pyroclastic rock dust was sourced from quarry sites located at Ezza and Abakaliki area of southeastern Nigeria. The rock dust is an industrial by-product of quarries exploiting the light to dark grey lapilli, tufts and lapillistones of basaltic composition (Hoque, 1984). The rocks consist of a compact chaotic mixture of Engineering Geology 107 (2009) 61–65 ⁎ Corresponding author. E-mail addresses: ezekwesiliene@yahoo.com (E. Ene), celokogbue@yahoo.com (C. Okagbue). Table 1 Chemical (oxide) composition of the pyroclastic rock dust. Oxides Amount (%) a Obiora, 1994 SiO 2 38.00 42.45 Al 2 O 3 16.20 14.52 Fe 2 O 3 3.05 2.51 TiO 2 1.25 – CaO 9.44 8.04 MgO 3.88 6.76 K 2 O 0.32 1.60 Na 2 O 4.12 5.41 FeO – 10.10 P 2 O 5 – 0.20 Loss of ignition 11.85 – a Present work. 0013-7952/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2009.03.007 Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo