MATEC Web of Conferences 162, 01005 (2018) https://doi.org/10.1051/matecconf/201816201005 BCEE3-2017 Geotechnical characteristics of some Iraqi gypseous soils Tom Schanz1 and Hussein H. Karim2,* 1 Ruhr Universität Bochum, Bochum- Germany 2 Building and Construction Engineering Department, University of Technology, Baghdad, Iraq Abstract. In Iraq, especially in the last three decades, extensive developments have been evidenced in the regions of gypseous soils due to the need of construction of many numbers of strategic projects. Failure of different structures constructed on gypseous soil in various regions in Iraq have been noticed. For this purpose, three areas in northern Iraq were selected (Samarra, Tikrit and Baiji) to study their geotechnical characteristics due to their high gypsum contents as well as many engineering problems are faced due to dissolution of gypsum. The experimental work involves testing of many properties such as: scanning electron microscopy (SEM), XRD, chemical, physical, compressibility, collapsibility, shear strength and suction. At low stress level, the test results revealed that, higher collapse potential (CP) is recorded for Tikrit soil. While at low stress level, higher CP is obtained for Baiji soil indicating the increase in CP with decreasing gypsum content. Furthermore, the CP significantly increases with increasing stress level and soaking period at a particular stress level. According to severity classification of the collapse potential, Baiji soil is considered as moderate trouble to slight, while Tikrit soil is considered as trouble to moderate. After soaking, both soils become trouble. As well as, the results showed a reduction in Tikrit soil shear parameters ( and ) after soaking period of 6 and 24 hrs as 12.2 to 9.2% in the internal friction angle and 91.5 to 94.2% in cohesion, respectively with respect to dry condition. Maximum total suction is measured for low consistency soils (liquid limit < 30%) represented by Tikrit soil. 1 Introduction resulting in a significant reduction in total volume of the soil mass. Noticeable collapse of soil demands three main Soils which collapse (undergo large consolidation) when conditions; an open (large void ratio), potentially unusual amounts of water are added to them exist in unstable, partly saturated structure; a high enough value many places around the world. There are many types of of an applied or existing stress; and a strong soil bonding problematic soils, one of these is the collapsible gypseous or cementing agent to stabilize intergranular contacts soil. Gypseous soil is found in arid and semi-arid regions when dry, while upon wetting will produce collapse. of the world where gypsum sediments and rocks are In general, the settlement of gypseous soils is mainly widespread such as in Iraq. There are different origins due to the dissolution of the cementing gypsum which and different definitions for gypseous soil, among which causes high softening of the soil and great loss in is the definition proposed by Barazanji (1973) [1], who strength. So sudden increase in compressibility occurs divided this soil into two main groups with respect to when these soils are fully or partially saturated gypsum content namely; soil with more than 50% known accompanied by structure collapse of the soil particularly as gypsiferous soil, and soil containing less than 50% in sandy gypseous soils. This can cause serious damage which in turn subdivided into five sub-groups. and even collapse of the structures founded on or in such Gypseous soils are considered as metastable or soils [7, 8]. collapsible soils defined as any unsaturated soil that goes Large areas of Iraq are covered with gypseous soils, through a radical rearrangement of particles and especially in the west, northwest and southwest regions susceptible to large abrupt volume changes and sudden and covers about 20-30% of Iraq’s total area which is collapses take place when such soils are wetted, soaked equivalent to about 7.3% of the total area of gypseous or leached with water. This can also happen when these soils in the world [9-12]. soils are inundated under loading [2, 3]. Gypsum mineral Most hazards which are related to construction on (CaSO4.2H2O) is hydrated calcium sulfate with these problematic soils occurred when such soils are intermediate solubility 2.6 g/l at 25 C) [3-5]. Gypsum subjected to water, causing softening of soils and presents in soil structure acts as a cementing material reducing the shear strength [13-16]. Gypseous soils are when the soil is dry. highly soluble materials in their nature, and the types of Dudley (1970) [6] pronounced the collapse problems associated with it. The failures include collapse phenomenon in terms of the bonding materials existing at and settlement, which can affect all construction the contact points between soil particles. Collapse can including buildings [17-19], roads [20-22] and other result in a substantial rearrangement of the soil particles, * Corresponding author: [email protected] © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). MATEC Web of Conferences 162, 01005 (2018) https://doi.org/10.1051/matecconf/201816201005 BCEE3-2017 egineering structures It is worth to metion that i order is assigned to Tikrit soil sample (73.88%), an to stdy the intermediate for Samarra soil sample (65.73%) and the behaior of such soils, it is necessary to erform scannig lowest is assigned to Baiji soil sample (51.24%). electron icroscopy SEM) aalysis to correlate the morphological and icrostructral changes with ypsum crystals issoltion or softeing sig ifferet Mineral Type (%) Site manifications [23]. Baiji Tikrit Samarra It is necessary to study the properties of such soils ue Gypsum 51.24 73.88 65.73 to the daage cause y these soils to uilig on or i Calcite 10.81 9.43 12.21 it eeae through sandy gypseous soil causes serious Chlorite 7.61 1.77 3.18 daage to foundation uilt on such type of soil Muscovite 10.8 4.23 4.88 Gypsification offers a variety of serious eotechnical Plagioclase (Albite) 9.08 3.18 5.36 haards. uch a rolem causes serious anger in l- Quartz 10.47 7.51 8.64 osul am foundation largest am locate northern Iraq) which is uilt on hihly gypseous soil The seepae Table 1. Mieralogical composition for the teste soil samples. through the soil under foundation causes leachig and dissoltion to the ypsum which roht the dam to the dager imminent of collase which coul ill hundres Physical tests carrie out i this study icluding thousads of eople Atterberg’s limits, specific gravity (Gs), water content The preset study aims to investigate and assess the ad rai sie distriutions are presete i ale 2 with compressiility, collase potetial shear stregth ad their stadards. he Unifie oil Classification ystem suction of gypseous soils. hree sites locate northern (USC was use to classify the teste soils. ther Bahda-Iraq, hae been chose for their hih psum stadard echanical tests erformed i this stud iole contets. oe-dimesional compression, collase (sigle collase test, CT, ad ouble oeometer test, DOT), ad irect shear. smmary of soil’s reslts for some hysical 2 Methodology properties, rai sie distriution ad soil classification (USC are summarie i ale 3. 2.1 Soil’s location, sampling and properties Table 2. Tests with their standars Gypseous soil samples were tae from three sites, Item Property Standard orther ahad-Iraq namely aii Tirit ad amarra No. areas. These sites were chosen for two reasons first they 1 Grain size distributions ASTM D422 [24] are characterie y their high ypsum content while the Specific gravity BS 1377:6B [25] second is the foundation roblems that have occurre i 2 Atterberg’s limits (L.L BS 1377:2A [26]; area close to aii where Al-osul am site is locate and P.L) BS 1377:3 [27] 3 Water content ASTM D2216 [28] (iure ). 4 Direct shear tests ASTM D3080-98 (1998) [29] 5 One-dimensional ASTM D 2435-04 consolidation test (2004) [30] (Compressibility) 6 Collapsibility test ASTM D 2435, 2004 (Consolidation tests) [30] ; ASTM D5333 03, 2003 [31] ‐ 2.2 Compressibility, collapsibility and shear strength of gypseous soils Collapsible gypseous soils show severe geotechnical and structural engineering challenges around the world. Fig. 1. Study Area. Bonds between soil particles can be formed by capillary forces (i.e. suctions) and/or by cementing of fine Various tests have been carried out for such soil materials (i.e. clays or salts). When the applied vertical samples to determine their physical-chemical properties stresses by means of loading or wetting processes exceed and particle size distribution. X-Ray diffraction test yield strength of these bonding materials, various analyses were performed for the three soil sample kinds. amounts of collapse potential may abruptly occur [32- The results of the mineralogical composition for the 34]. Therefore, it is necessary to understand the process tested soil samples are shown in Table 1. Higher gypsum of collapse and whether the problems associated with content these soils are to be avoided or mitigated. So, the geotechnical properties of gypseous soils cover, 2 2 MATEC Web of Conferences 162, 01005 (2018) https://doi.org/10.1051/matecconf/201816201005 BCEE3-2017 compressibility, collapsibility, and shear strength Soil Water Gypsum Specific Atterberg′s Limits Grain Size Distribution % Classification Content Content Gravity (USCS) Type L.L P.L P.I Cu Cc Gravel Sand Fines (%) (W.C %) (G.C %) (Gs) (%) (%) (%) (%) (%) Silt Clay Baiji 1.25 51.2 2.53 37 19 18 3.4 1.1 6 78 16 - SC Tikrit 0 73.9 2.38 26 21 5 3.0 0.1 8 74 18 - SM-SC Samarra 6.5 65.7 2.48 32 25 7 1.6 0.3 22 61 17 - SM parameters (c and ) were studied.