Field Measurements of the Behaviour of Lightly-loaded Piles in Swelling Clay

B. S.H. Hazzan, R.W. Sarsby

Dans les comptes rendus d’articles de la conférence: GEO2010 Calgary: 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost Conference

Session: M1-C

ABSTRACT: A full-scale test site has been established in northern Israel to study the behaviour of lightly-loaded piles installed in a swelling soil. Sixteen unloaded cast-in-situ piles, with uncoated and coated surfaces, were installed in an expansive clay soil. The piles were constructed to different depths ranging between 2.0m and 7.0m and their vertical movements were monitored over a period of 27 months. Another nine unloaded cast-in-situ bored piles, both coated and uncoated, were installed for carrying out pullout tests. Monitoring of the vertical displacement of the piles demonstrated a clear relation between rainfall and movement. Observations and full-scale static pull out tests indicate that separating the piles from the surrounding clay in the active swelling/shrinkage zone, using a twin-walled plastic sleeve, reduced the heave forces significantly. RESUME Un chantier expérimental a été établi au nord de l'Israël afin d'étudier le comportement des fondations profondes construites dans une terre qui se gonfle. Seize pieux, qui ne portaient aucune charge, étaient installés dans une couche de l'argile très expansive. Les surfaces de plusieurs de ces pieux étaient couvertes par une couche du bitume ou de la graisse. Les pieux étaient entre 2 et 7m de longueur et leurs déplacements verticaux étaient enregistrés pendant 27 mois. Les résultats montraient une relation très claire entre la précipitation et le déplacement vertical. Les observations des pieux et les essais du chargement soulevant indiquent que les couches du bitume ou de la graisse ont beaucoup réduites des forces soulevantes créés par la terre expansive. 1. INTRODUCTION In most parts of the world soils undergo seasonal wetting and drying periods and this results in cyclic swelling and shrinkage of the soil. This volume change behaviour is particularly marked for swelling soils and causes problems with foundations, pavements, roads and other engineering structures. Climate change causes cracks and damage in buildings due to movement of the foundations which are caused, generally, by the heave forces acting on the foundations. The problem will be worst for lightly-loaded piles below low-rise buildings. The magnitude of the volume changes and associated heave forces varies according to soil type, annual climate, the characteristics of the dry season, the seasonal temperature variation and other factors such as organic matter content, soluble salts and clay minerals. With lightly loaded piles founded in swelling clay subsoil anchoring of the piles deep in the ground is needed because the applied vertical load is smaller than the heave force. If it is impossible to drill into underlying rock, the length and diameter of the pile may be too small to provide sufficient resistance to the applied vertical load from the building. In such a case the usual solution is to increase the pile diameter, but by this way the heave forces on the pile, due to the climate change, will increase because of the increase in the contact area between the swelling soil and the pile shaft. On the other hand, if it is possible to drill into underlying rock, a pile might fail in tension due to upward heave forces. A potential solution to the foregoing problem is to provide a ‚slip layer' between the pile and the surrounding soil in the zone of soil expansion. In the last four decades significant effort has been devoted to designing piles in swelling soils which could accommodate the effects of climate change, but to date only scant effort has been made to the eliminate the heave force acting on the upper part of lightly-loaded piles (Hazzan, 2004). 2. THE TEST SITE The site is located in northern Israel where there are significant soil deposits with clay contents in excess of 35% and with 40 to 80% of the clay mineral, i.e.montmorillonite, being especially prone to high volume change due to moisture change. In Israel the dry season lasts for 6 months and the annual temperature variation is very large, i.e. from -40 C in Winter to 460 C in Summer. Total annual precipitation varies from about 450 to 1000 mm in the North of Israel at Karmiel (where the test site is located) to as little as 20 mm in the South, at Eilat. For the test site the total measured annual rainfall in the three consecutive winter seasons covering the pile monitoring period was: 437mm, 965mm and 507mm. The site is flat and covered with short, native grass. The soil profile was determined using a rotary drilling rig to carry out four exploratory borings with augers of 100mm diameter. The soil characteristics were defined from in-situ field tests (Standard Penetration Test, Vane Shear, Dynamic Penetration Test) and laboratory tests on seven undisturbed soil samples. The soil profile generally 29

RÉSUMÉ: measurements of the behaviour of lightly-loaded piles in swelling clay B Sh. Hazzan

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