Vol. 20 No. 2 Journal of Wuhan University of Technology - Mater. Sci. Ed. Jun. 2005

Engineering Properties of Expansive Soil

DAI Shaobin SONG Minghai HUANG Jun (Institute of Architecture Design and Research, Wuhan University of Technology, Wuhan 430070, China)

Abstract: The cornpanents of expansive soil were analyzed with EDAX, and it is shown that the main

contents of expansive soil in the northern Hubei have some significant effects on engineering properties of expansive soil. Furthermore, the soil modified by lime has an obvious increase of C a 2 § and an improvement of connections

between granules so as to reduce the expansibility and contractility of soil. And it also has a better effect on the modified expansive soil than the one modified by pulverized fuel ash.

Key words: expansive soil; engineering property; mineral composition; chemical component

1 Introduction

The engineering properties of expansive soil are con- trolled by especial mineral components and chemical con- stitutions. A study on its mineral and chemical compo- nents was conducted in order to know intrinsic factors that control engineering properties, to discuss mechanics on expansion, to ameliorate and strengthen its properties, and to search new techniques and new methods for study on it. This paper made use of some ways, such as EDAX and so on, to study the mineral and chemical constitutions of un-modified and modified expansive soil respectively. The experimental results can significantly be used to eval- uate characters of shale in expansive soil, and to disclose the characters and intensity of the un-modified and the modified expansive soil form fSl .

2 Experimental

The Phoenix EDAX, called energy spectrometer, produced by American General Electric Appliances Com- pany, worked as a part of JSM-5601 scanning electron microscope, was used to analyze elements qualitatively and quantitatively. To study the mechanism of the reaction among expansive clay granules, lime and pulverized fuel ash, some analyses on the original expansive soil and the expansive soil modified by lime and pulverized fuel ash were carried out with SEM, and the connection points of granule were analyzed with EDAX at the same time. This paper measured the component of chemical elements at ar- bitrary points with EDAX. During analysis, an electron beam probes the studied points to get a series of X-ray spectra. EDAX was used to analyze the influence of addi- tion with lime and pulverized fuel ash on the stability of expansive soil. 9wt% pulverized lime and 50wt% pulver- ized fuel ash were mixed into expansive soil samples for playing down the expansion and shrinkage. Chemical con- stitutions and contents are shown in Table 1.

( Received: Sep. 25,2004; Accepted: Jan. 12,2005) DAI S h a o b i n ( ~ ) : Assoc. Prof. ; E-mail: dsb123 @ mail. whnt. edu. cn * Funded by the Natural Science Foundation of Hubei Province (2000J141)

3 Results and Discussion

Among the chemical constitutions of colloidal particle in expansive soil, the molecule ratio of Si to A1 is 3.81, which indicates that mineral components are mainly illite. So the result is anastomotic with that of authentication. In the chemical constitutions of expansive soil, general cont- ents of some more active metals, such as alkali metals K, Na, Ca, Mg, and alkali soil metals are high, which indi- cates that the degree of weathering and leaching in expan- sive soil is limited and chemical weathering is limited too. The change that illite turns into roseite or montmorillonite will result in an improvement of hydrophilicity, the engi- neering properties of the expansive soil will be worsened.

From Table 1, in the expansive soil, which is modi- fied by lime and pulverized fuel ash, the contents of MgO and CaO increase and that of Na20 and K20 decrease. This kind of treatment plays a significant role in explain- ing the mechanics of the expansive soil modified by lime and pulverized fuel ash. Because of the main ingredient SiO 2 in pulverized fuel ash, it results in an increase of the content of SiO: in the modified expansive soil. Fig. 1 shows the analysis of EDAX of identification points, which are displayed in SEM image, here, expansive soil was not be modified; Fig. 2 shows tile analysis of expan- sive soil mixed with 9 % lime; Fig. 3 shows the analysis of expansive soil mixed with 50% pulverized fuel ash. Com- pared these figures, the obvious increase of Ca ions in soil sample's granules agglutinate can increase connections between granules and play down the expansibility and contractility of soil after the expansive soil is dealt with time and pulverized fuel ash. Comparing the expansive soil modified by lime with the one modified by pulverized fuel ash, we can find that Ca ions increase obviously in the former and it has a much better property.

The mechanism of the modified expansive soil with lime lies in the great increase of Ca 2 + , M~ + after lime is added. As we all known, slaking of lime with water and taking swap action with basic ions, step-by-step, Na + and K + with C 2+ and Mg 2+ in the expansive soil reduce plasticity index of expansive soil evidently. Because the decrease of plasticity, sometimes it results in the decrease

110 Journal of Wuhan University of Technology - Mater. Sci. Ed. Jun. 2005

of the expansive potential energy d soil and swell-shrink characteristics of soil. In addition, the pH value of expan- sive soil goes up after the expansive soil is modified by lime, which increases the swap between ions. The mech- anism of modified expansive soil with pulverized fuel ash lies in main contents ( Si 2 O, A12 03, Fe2 O3 ) of pulverized

Table I The chemical components and content of the un-modified expansive soil and the modified expansive soil/%

Chemical components Na20 MgO M2 03 Si03 I(20 CaO TiO2 MnO Fe2 03

fuel ash granules just like hollow spheres. These granules flocculate dispersive clay granules under the condition of that divalent and trivalent cations (such as Si 4§ , A13§ , Fe 3+ and so on) ionize, which may reduce the specific surface, the hydrophilicity of granules of expansive soil, and the dilatancy of expansive soil additionally.

SiO2/A12 03

Original expansive soil 2.65 3.71 22.39 50.20 2.49 1.19 1.25 4.60 11.52 3.81

Expansive soil + 9%lime 1.06 4.12 11.73 45.66 1.67 31.35 0.94 - 3.46 -

Expansive soil + 50% 1.58 5.30 15.53 58.88 1.74 12.04 1 .(30 - 3.93 -

pulverized fuel ash

Si

0

Mg

Fe Na K Ca Mn Fe

0.9 1,5 2.1 2.7 3.3 3.9 4.5 5.1 5.7 6,3 keV

Fig.l The analysis ofun-modified expansive soil with EDAX

0

Mg

C Na I, r~ .....

0.8 1.5 2.2 2.9 3.6 4.3 5.0 5.7 6.4 7.! key

Fig.2 The analysis of expansive soil modified by 9% lime with EDAX

C a

Mo Ti 1[ Fe

. . . . mill ,,

The granulous substance of the expansive soil con- sists of these concretions. Some of them stratify and turn into framestone so that it plays a significant role on the stabilization of fill slope. The result of analyzing the cont- ent of these concretions in the expansive soil in northern Hubei with EDAX indicates that this kind of concretions mainly consists of iron-manganese concretion, including a little of calcareous ones. These concretions come into be- ing because of geochemical evolution when expansive rock turns into soil, which relates nearly to terrain, climate, groundwater regime and so on.

The formation of iron-manganese concretion in the expansive soil results from that iron-manganese under- neath! the interface of oxidation-reduction is deacidized into Fe 2+ and Mn 2+ because there exists different oxida- tion-reduction potential at the interface. And because the low compounds of two elements have a larger dissolvability and a higher concentration in the water than the high compounds of them, they diffuse gradually towards the water upwards, and are oxidized into compounds of Fe 3§ and Mn 3§ . Then the compound deposits come into being iron-manganese concretion. This kind of iron-manganese concretion varies in shape and size largely. Its diameter is about 20-150mm, and the smallest about 2-10mm. Some of them partly stratify and deposit or look like lens, some- times 5% to 15% of iron-manganese concretions distrib- ute assembly near crevice or bedding plane. Because the content is less than 20% and they do not function as a frame in the expansive soil, it helps soil increase the in- tensity rarely I61 .

4 Conclusions

a) The main chemical contents of the expansive soil, in Jingmen of northern HuBei, are A1203 , Si02, and Fea 03 (84.11% in all), which shows that there are rela-

Si

Mg C a

0.8 1.5 2.2 2.9 3.6 4.3 5.0 5.7 6.4 7.1 keV

Fig.3 The analysis of expansive soil modified by 50~ pulverized fuel ash with EDAX

tively richer silica minerals than other coarse granule, more aluminosilicate clay minerals than other little clay granules, and the higher content of active alkali metals (K, Na, Ca, Mg, etc). It is possible that illites turn in- to roseite or montmorillonite and increase the hydrophilici- ty of the expansive soil, furthermore, deteriorating engi- neering properties of the expansive soil.

b) The main concretions of the expansive soil in Jingmen of northern HuBei, are iron-manganese concre- tions and a little of calcareous concretion.

c) After the expansive soil is modified by lime and pulverized fuel ash, its agglutinate has an obviously in- crease of Ca 2 + , makes an improvement of connections be- tween granules and reduces the expansibility and contrac- tility of soil. The soil modified by lime has a much more obvious improvement of Ca 2§ , a better effect on the modi- fied expansive soil than the one modified by pulverized fu- el ash.

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