Acta Polytechnica CTU Proceedings doi:10.14311/APP.2017.10.0052 Acta Polytechnica CTU Proceedings 10:52–55, 2017 © Czech Technical University in Prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app TESTING OF SOIL IMPROVEMENT BELOW A HALL FLOOR Monika Súľovská∗, Peter Turček, Zuzana Štefunková STU Bratislava, Faculty of Civil Engineering, Radlinského 11, Bratislava, Slovakia ∗ corresponding author: monika.sulovska@stuba.sk Abstract. Construction of a hall situated in North-West Slovakia was prepared in winter. After finishing an elementary ground adaptation in February there were extreme weather conditions: the precipitation exceeded by over 300 % the normal long term measured amount. This indisposition of climate led to a stopping of the construction work. Checks on the required quality of the base course using static load tests showed a failure in performance of compaction. The sampling and laboratory testing of the original and the improved soils are discussed in the paper. Keywords: Quality control, base course, laboratory analysis, consolidation test, cement content. 1. Introduction The required intensity of deadlines for finishing the construction regardless of limiting conditions (e.g. cli- matic, occurrence of soils with low bearing capacity) conduced on a good number of occasions to improve- ment efforts on the base course. These are usually similar technologies to those used in road construc- tion. Among the most common technologies there is included adding a certain percentage of lime or ce- ment to fine-grained soils6 (so-called soil stabilization). Mixing cement with soil and compaction reduces the compressibility of the treated soil. Subsequent quality control of any improvement is mostly done by static load test. The underlying layers of the future floors were simply improved within the rainy weather and were not protected. The adverse weather situation caused the suspension of work. Checking the quality of the underlying layers using static load tests after a week of completion of the stabilization showed a failure to satisfy the required improvement. It was necessary to immediately resolve the situation and help meet deadlines. Consequently soil was taken from the un- satisfactory area of the hall and then a check was made on the properties of both treated and untreated layers by laboratory tests. 2. Laboratory tests On the construction site of the hall there were removed undisturbed soil samples improved by cement, but also the original soil without improvement by cement. On the original soil samples there were firstly detected the basic descriptive soil characteristics. Based on STN 72 1001 [1] there was soil classified as firm clay with low plasticity (F6 - CL). The original moisture of the soil was more than 20 %. The moisture of the improved soil with cement did not differ from the moisture of the original soil. To determine the optimum moisture of soil for compaction there was used the Proctor test (see Fig. 1). The optimum moisture of the tested soil was determined wopt = 15.2 %. This corresponds to the maximum dry density of soil ρmax = 1.768 g/cm3. The collected samples had significantly higher mois- ture content than its optimal moisture. 2.1. Oedometric tests on undisturbed soil samples Compressibility testing on the oedometer was made to the original and also the improved soil. At improve- ment there was added into the soil 5 % of cement. The results of average deformation characteristics of original and improved soil are collected in Table 1. On the basis of laboratory results there can be de- termined for the improved soil the following increases in deformation characteristics: • at stresses in soil σ ≤ 50 kPa Eoed stab Eoed not = 32.25 5.10 = 6.32 (1) • at stresses in soil 50 ≤ σ ≤ 100 kPa Eoed stab Eoed not = 14.13 5.70 = 2.47 (2) • at stresses in soil 100 ≤ σ ≤ 200 kPa Eoed stab Eoed not = 15.19 9.62 = 1.58 (3) Based on the stress range of its own weight and the expected load from buildings, the improvement of soil after adding cement was 1.5 to 2 times. The recommended values in the deformation modulus of improved soil should not exceed the value Eoed = 14 MPa. 2.2. Compression tests of soil without improvement at compaction after the Proctor standard Artificially prepared samples of soil compacted after Proctor standard were tested in the oedometer. The 52 http://dx.doi.org/10.14311/APP.2017.10.0052 http://ojs.cvut.cz/ojs/index.php/app vol. 10/2017 Testing of Soil Improvement below a Hall Floor Figure 1. Determination of the maximum bulk density versus optimum moisture. Table 1. Deformation modulus of soils determined from compression tests. resulting average oedometer modulus corresponding to different moistures of tested soils are evaluated in Table 2. Effects of changes in soil moisture on the oedo- metric modulus are shown in Fig. 2. The plasticity index of soil was IP = 10 % (liquid limit wL = 30.5 % and plasticity limit wP = 20.5 %). Until the soil was in natural conditions and firm consistency, there were found very small values of deformation characteristics of tested soil to all extents of normal stresses. In- creasing the soil stiffness (higher values of Eoed) was possible by observing a decreasing of soil moisture below 17 %. The consistency of soil at such moisture was IC = 1.4. Decreasing the moisture below 17 % significantly increases the oedometric modulus of the soil. Analysis of Fig. 2 taking into account Fig. 1 can be stated as: • Exceeding the optimum moisture wopt occurred in samples loaded by normal stress of 50 kPa with rapid decreasing of Eoed. • With an increasing of moisture there was ob- served a significant decreasing of Eoed at lower normal stress (< 50 kPa) - more than 5-times. Exceeding the value of normal stress 50 kPa and with increasing the moisture there was found a decreasing of Eoed approximately 3.5 times. In the test range of normal stress Eoed was signif- icantly affected by moisture. In Fig. 3 there is shown the relationship between deformation characteristics and normal stresses of soil at various initial moistures of the soil. Also in this evaluation it was proved the moisture was around 17 % than the limit. At lower moisture there were detected significantly different deformation parameters, especially at normal stress to 50 kPa. 2.3. Improving the soil characteristics by adding cement At natural moisture of soil 20 % of collected samples had values of deformation modulus of the original soil which were very low. The addition of cement decreased soil moisture, whilst increasing the defor- mation parameters was not sufficient. Very interesting results were obtained from eval- uation of the effect of the 5 % addition of cement (see Fig. 4). At lower humidity than 20.5 % (on a stiff consistency) there was observed a significant ef- fect of the cement. At the same time, the effect of improvement increases with lower levels of normal stress. Adding 5 % of cement at optimal soil mois- ture brought a 2-3 times increase of Eoed compared to unimproved soil. Exceeding soil moisture w = 20.5 % the soil fell into firm consistency and the impact effect of the cement was shown to be very low to negligible. Cement added to the soil with natural moisture, got between 19 to 21 %, extracting a portion of water by hydration. This will inevitably reduce soil moisture itself. It can be assumed that the excess of the initial 53 M. Súľovská, P. Turček, Z. Štefunková Acta Polytechnica CTU Proceedings Table 2. Average oedometric modulus of tested soils at various moistures. Figure 2. Relationship between moisture of tested soil and oedometric modulus. moisture content in excess of the wopt + 5 % there is to be expected a significant decrease in the effect of stabilization. 3. The quality assessment and the cement content in the soil Cement and its contents were independently tested in the stabilized soil. Laboratory tests of the physical- mechanical properties of cement were carried out and supplemented by X-ray and DTA analysis. Stabilized soil was investigated by DTA and also X-ray analysis. Tested cement was used to stabilize the soil at the construction site. The studied properties and the results derived are as follows: • After 28 days, the compressive strength reached 48.3 MPa. This value is in accordance with the requirements of the class of cement 42.5 MPa according to EN 197-1. • Bulk density with a timed deposit of test samples slightly increased. After 28 days, the bulk den- sity reached a value around 2190 kg/m3, which corresponded with standard values. • By testing the cement slurry, it was found that the cement needs to reach a slurry of normal density of 32 % water. • The initial set of cement slurry observed after 195 minutes, in which it fulfilled the limit set by the standard. Final set occurred after 250 min- utes. • Soundness of the testing cement went to 6.0 mm, which met the standard set limit of 10 mm. • Bulk density of fresh cement mortar was 2160 kg/m3. All testing samples had low scatter from the average. • X-ray analysis showed the incidence of cement clinker minerals as Alite - C3S (Tricalcium sili- cate), Belite - C2S (Dicalcium silicate), Brown- millerite - C4AF (Tetracalciumaluminoferrite), C3A (Tricalcium aluminate) and gypsum setting regulator (CaSO4 · 2H2O). Furthermore, there oc- curred Silica (SiO2), Calcite (CaCO3) and Port- landite Ca(OH)2. According to this composi- tion it can be assumed that this was Portland- composite cement CEM II. Tests of the stabilized soil by method X-ray and DTA showed that samples taken from the hall after the defective area of the static load tests contained less than 2 % of cement. By applying the same method to artificially prepared samples there was found good correspondence with the percentage of added cement. 4. Conclusion Based on the compressibility tests on the stabilized soil it is clear that the high moisture of the soil af- ter the rainy season had not reached sufficiently an increasing of the soil parameters. They showed two reasons for failure to satisfy the criteria required for static load testing. The first reason was lower cement content as determined by the project. Instead of 5 % of the cement which was in samples taken from the 54 vol. 10/2017 Testing of Soil Improvement below a Hall Floor Figure 3. Dependence of normal stress and moisture on deformation characteristics of soil. Figure 4. Influence of addition 5 % cement into the soil on Eoed at optimum moisture. subgrade in the construction site there was found only a cement content in the range of 1 to 2 %. Signif- icant precipitation caused the natural soil moisture to exceed 20 %. Laboratory experiments have shown that at such a level of moisture in the not improved original soil the deformation properties of soil are very low. Mixed cement decreased soil moisture, but it reflected only a small increase of Eoed. Deformation parameters of soil by adding cement may bring an increase 2-3 times. This effect is linked to the initial soil moisture before stabilization; value wopt should not be exceeded by 5 %. In exceeding this moisture there is not expected a major stabilization effect. 5. Acknowledgement The paper is a part of results from a research project supported by the Slovak agency VEGA No. 1/0882/16 “Influence of boundary conditions to limit states of geotechnical structures”. [2] [3] [4] [5] [1] References [1] STN 72 1001: Classification of soil and rock in engineering geology and geotechnics (in Slovak), 2010. [2] P. Turček, M. Súľovská, Z. Štefunková. Evaluation of soil layers improving below the production hall (in Slovak). SvF STU Bratislava, 2016. [3] P. Turček, M. Súľovská, Z. Štefunková. Experimental research of deformation properties of fine-grained soil situated in hall subgrade (in Slovak). SvF STU Bratislava, 2016. [4] P. Turček, J. Frankovská, Súľovská. Geotechnical design according to Eurocodes (in Slovak). Engineering Advisory Centre of Slovak Chamber of Civil Engineers, 2010. [5] EN 197-1: Cement. Part 1: Composition, specifications, and conformity criteria for common cements, 2012. 55 Acta Polytechnica CTU Proceedings 10:53–56, 2017 1 Introduction 2 Laboratory tests 2.1 Oedometric tests on undisturbed soil samples 2.2 Compression tests of soil without improvement at compaction after the Proctor standard 2.3 Improving the soil characteristics by adding cement 3 The quality assessment and the cement content in the soil 4 Conclusion 5 Acknowledgement References