J. Cent. South Univ. Technol. (2008) 15(s1): 386-390

DOI: 10.1007/s11771-008-385-0

Influencing factors of compressive strength of solidified inshore saline soil using SH lime-ash

QIN Yin-hui(覃银辉) ^{1, 2}, LIU Fu-hua(刘付华)³, ZHOU Qi(周 琦)⁴

(1. Institute of Rheological Mechanics & Material Engineering, Central South University of Forestry & Technology, Changsha 410004, China;

2. College of Civil Engineering and Mechanics, Central South University of Forestry & Technology, Changsha 410004 China;

3. Architectural Design Institute, Changsha, 410007, China;

4. Research Institute of Geotechnical Engineering, Hohai University, Nanjing 210098, China)

Abstract: Through unconfined compressive strength test, influencing factors on compressive strength of solidified inshore saline soil with SH lime-ash, ratio of lime-ash(1?K), quantity of lime-ash, age, degree of compression and salt content were studied. The results show that because inshore saline soil has special engineering characteristic, more influencing factors must be considered compared with ordinary soil for the perfect effect of solidifying.

Key words: solidified inshore saline soil with SH lime-ash; compressive strength; lime; ash; compaction degree; salt content

1 Introduction

Saline soil is defined as rock in which soluble salt content is higher than 0.3% and rock has the engineering characteristics of expansion, corrosion and so on^[1]. Saline soil in China is mainly in the arid northwest region of low-flat basin and plains, the north China plain, datong basin and Sonme low-lying land in the lake of Qinghai-Tibet plateau, the coastal region has a considerable area of the distribution of saline soil. Inshore saline soil arrives in coastal land for a short time, on the impact of soaking water and coastal retreat shifting, it has high ground water content and water salinity, through evaporation and capillary action, salt in water would concentrate on the surface or shallow horizon under the surface and has the undesired engineering characteristics such as depression, salt expansion, corrosion of saline soil are formed.

There are a lot of ways to make inshore saline soil be roadbed^[2-5], and inshore saline soil is solidified with lime-ash(lime, powder), which is the most effective method because of its economy and applicability. However, because the solidified inshore saline soil with lime-ash is a mixture of three materials, different contents of material would affect its unconfined compressive strength (hereinafter referred to the compressive strength) and the salt content would also affect the nature of inshore saline soil substantially. Therefore, compared with ordinary soil, the factors influncing the compressive strength of solidified soil with lime-ash are more and the study is also more complicated. Studying different factors which affect the compressive strength systemically can provide accurate parameters for construction and reduce cost.

2 Test materials and methods

The tested soil was taken from Shi-Huang highway along Cangzhou to Huanghua Port. It is representative soil sample along the highway, the basic physical and mechanical properties of soil are shown in Table 1. Dried the soil before test and crushed it by screen with 2 mm in diameter, the chemical constituents of the salt in soil are shown in Table 2.

Lime used was hydrated lime in Jinxian plant, its effective calcium and magnesium content are 56.2% which meets the standard of third-class lime. Fly ash was from power plant in Tianjin Junniang City, in it the content of SiO₂, Al₂O₃ and Fe₂O₃ is 76.2%, the loss on ignition is 15.6%, the surface area is 2 756.6 cm²/g, which meets regulatory requirements.

The sample preparation process met industry- standard “Testing methods of material stabilized with inorganic binders for highway engineering”^[6](JTJ 057—94) and “Technical specification for road bases and subbases with chemical stabilization”^[7](CJJ/T 80—98).

Table 1 Basic mechanical indexes of the soil

Table 2 Chemical contents of salt in the soil

3 Analysis for different factors of solidified inshore saline soil

3.1 SH content

When SH is added to the solidified soil with lime-ash, the first consideration should be the content of the issue. Although the raw materials of SH are industrial waste and SH has lower price, but that’s not to say the more the content, the better the solidified soil. The best percentage of SH is gotten by analyzing the proportion of lime-ash and the total content of it. In order to get the best percentage, 4 different proportions of solidified saline with 12% lime, 5 kinds of SH content percentage were chosen which starts from 0.6% with increasing rate of 0.6%^[3]. The test results are shown in Table 3.

Table 3 Compressive strength variation along with content of SH  MPa

From Table 3 it can be seen that in a certain ratio of lime-ash, the unconfined compressive strength of mixture increases with the increase of SH content either in water or without water. When the SH content is up to 1.8%, the soaking strength for 7 d can get 0.618 MPa which meets the strength requirement of grassroot of second-class highway and below it. When the SH content 3.0%, the soaking strength for 7 d reaches 0.770 MPa, which is near the minimum strength requirement of grassroot of expressway. SH is water-soluble polymer which contains a lot of polar hydroxy corporations in its elements, such groups can cause physical-chemical reaction between soil and lime-ash and form a long chain of viscoelastic filamentous film. At the same time, such physical-chemical reaction is not reversible, thus the soul particles bond into a whole firmly, then the soaking strength of solidified saline soil increases greatly.

When the proportion between lime and ash is 1?1 and 1?2, a small number of SH content can effectively improve the strength of solidified soil. When the SH content is less 1.8%, both of their strength are higher than that of the solidified soil with the proportion of 1?3. With the increase of SH content, the compressive strength of soil with the proportion of 1?3 is over than those of the first two. The case is particular when the ratio is 1?4, the sample strength is significantly smaller than those with the former proportions. It can be concluded that fly ash is the main mixture bonded with SH as a whole, the main determing factor affecting SH content is the content of fly ash.

3.2 Age

The compressive strength changes with age of soil, the strength results of solidified soil with lime-ash and solidified soil with SH lime-ash are shown in Fig.2 and Table 4. The proportions of solidified soil with lime-ash are as follows: the mass ratio of lime, ash and soil is 12?24?64, while for the solidified soil with SH lime-ash, the mass ratio of lime, ash, soil and SH is 12?24?64?3.

The test results and Fig.1 show that the compressive strength for 3 d of soil is similar to that of solidified soil, even greater, but the strength for 7 d and 28 d decreases. The reason is that the salt is mainly choride in inshore soil, and for chloride absorbed moisture seriously in the humid environment, for example, calcium choride crystals can absorb 4-5 times of their own water, then the weight of specimen increases and thus leads to lower strength.

Table 4 Influence of age on compressive strength of different soils

Fig.1 Influence of age on unconfined compressive strength

Compared with the soil, although the salt in solidified soil with lime-ash absorbs moisture, the crystallization of compounds produced by the reaction of volcanic ash will harden gradually in the air at the same time and the latter plays significantly role, so the strength of solidified soil increases with age. Compared with the solidified soil, the early strength is improved markedly for solidified saline soil with SH, but the strength differs little for 28 d and 60 d. The reasons are as follows:

1) SH forms viscoelastic long chain of filamentous film only in dry environment, but in standard curing condition, the humidity is greater than 90%, SH is difficult to dry, so the its strength is not improved clearly.

2) The volcanic reation between lime and fly ash is full and complete when the age is long. It is stable crystal production such as CaO?SiO₂?nH₂O and CaO?Al₂O₃?nH₂O, the main strength is provided by CaO?Fe₂O₃?nH₂O, so their strength is similar.

3.3 Compaction degree

Mixture produced dissolves crystalline compounds through chemical reaction. They bonded solid articles of lime-ash mixture forms larger aggregate structures which filled in the pore structure to make mixture has structural strength and stability. If compaction degree is insufficient, the pore of mixture will be too large, then the aggregate structure will evacuate leading to unfull physical- chemical reaction of mixture. At last, the structural strength and stability of solidified soil are reduced significantly. In order to understand the reason that compaction degree influence compressive strength, specimen with 4 kinds of compaction degree, i.e., 100%, 97%, 94%, 91% were made. Then the compressive strength for 14 d was tested. The results are shown in Table 3; the relationship between compaction degree and compressive strength is shown in Fig.2.

Fig.2 Relationship between compressive strength and compaction degree

According to Fig.2, the compressive strength of solidified soil decreases significantly with the decrease of compaction degree. It decreases from 1.03 MPa with 100% compaction degree to 0.379 MPa with 91% compaction degree, the reducing rate is up to 34.1%. Thus, although the fly ash in lime-ash has the characteristics of light mass, high strength, less compression which is favorable for improving the stability of foundation or embankment. However, when lime-ash is used to solidify inshore saline soil, because the salt in soil will dissolve in water and cloride will absorb moisture, the compression deformation will increase. Therefore, to insure that the solidified soil has sufficient strength and good road performance, its compaction degree must be controlled strictly. The relationship between solidified soil with SH lime-ash and compaction degree is similar to that of solidified soil and compaction degree. Compressive strength decreases from 1.205 MPa with 100% compaction degree to 0.807 MPa with 90% compaction degree, the decreasing rate is up to 33.2%. In addition, there is good linear relationship between solidified soil with lime-ash or solidified soil with SH lime-ash and compaction degree, a straight line is available to fit, for solidified soil with lime-ash, y=  0.036 7x-2.621 2，R²=0.980 5, for solidified soil with SH lime-ash, y=0.046 6x-3.419 8, R²=0.962 5.

Table 5  Influence of compaction degree on unconfined compressive strength

3.4 Salinity

According to “Specifications for Design of Highway Subgrades”(TJT 013—86)^[8]: saline soil is divided into four types according to salinity, i.e., weak saline soil, middle saline soil, saline soil and super saline soil. In order to study how salinity influence compressive strength, one certain salinity was selected in four types of saline soil to test respectively, the curing age of specimen is 7 d. The test results are shown in Table 6 and Figs.3-4.

It can be seen from Figs.3-4 that the compressive strength of soil reduces dramatically with the increase of salinity, the compressive strength of solidified saline soil also decreases with the increase of salinity from 1.057 MPa with 0 salinity to 0.455 MPa with 7% salinity, the decreasing rate is up to 58%. But when the salinity is over than 7%, the strength is basically stable and it declines less obviously, when the saline is 12%, the strength is 0.352 MPa. The mass increases with the increase of salinity, when the salinity is more than 7%, mass is stable.

Comparing the soaking compressive strength curve of solidified saline soil with the stated strength curve, and referring to “Technical Specifications for Construction of Highway Roadbases”, it can concluded that if inshore saline soil is improved with lime-ash, the maximum allowed salt content should not exceed 4% for grassroot of second-class highway and 3% for grassroot of first-class highway and expressway.

Table 6 Test results of influence of salinity on compressive strength and mass

Fig.3 Influence of salinity on compressive strength

Fig.4 Influence of salinity on sample mass

4 Conclusions

1) Lime ash ratio (1?K) and the content of lime are the most significant factors influencing the compressive strength of inshore saline soil. Both lime ash ratio and content of lime K have a critical point. Finding the critical point of K and lime content are the key issues to determine the proportion of lime, ash and soil of inshore saline soil with SH lime-ash.

2) The compressive strength of inshore saline soil decreases with age, while that of solidified inshore saline soil with SH lime-ash increases with age.

3) The compressive strength of inshore saline soil with SH lime-ash decreases with the decrease of compaction degree, and when the compaction degree decreases by 3%, the compressive strength decreases by 11%, the rate is high, thus the compaction degree of solidified soil with SH lime-ash should be controlled strictly.

4) The salt content affects the compressive strength significantly. When the salinity increases from 0 to 7%, the compressive strength decreases by 58.0%, if solidified inshore saline soil is used as grass-roots level of highway, the maximum allowed salt content for second-class highway should not exceed 4% while for first-class it is 3%.

References

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[3] ZHOU Qi, HAN Wen-feng. Experiment research of saline soil for roadbed filling in highway[C]// WANG Jian-ting. The First Science Forum Volume of Graduate Student in Tianjin Institute of Urban Construction. Tianjin: Tingjin University Press, 2004: 49-53. (in Chinese)

[4] WANG Xiao-sheng, ZHANG Hong-qing, XUE Ming, FANG Jian-hong. Road disease and treatment in saline soil area[J]. Journal of Tongji University, 2003, 31(10): 1178-1182. (in Chinese)

[5] LU Zhao-jun. Study on the redbed engineering properties of saline soil[M]. Beijing: China Architecture & Building Press, 1997. (in Chinese)

[6] JTJ 057—94, PRC industrial standard. Testing methods of material stabilized with inorganic binders for highway engineering[S].

[7] CJJ/T 80—98, Technical specification for road bases and subbases with chemical stabilization[S].

[8] JTJ 034—2000, PRC industrial standard: Technical code for construction of highway roadbed[S].

(Edited by CHEN Can-hua)

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: QIN Yin-hui; Tel: 13637401079; E-mail: qinyinhui2003@126.com