降低强膨胀土膨胀率提高抗剪强度的复合改良剂筛选

Complex ameliorants screening for reducing swelling ratio and improving shear strength of strong expansive soil

为减小膨胀土对土木工程设施和农业生态环境的严重危害,该文进行复合改良膨胀土方法的室内试验。利用离子交换、高分子成膜作用,使用不同配比的石灰(CaO)、氯化钾(KCl)、聚乙烯醇(polyvinyl alcohol,PVA)、聚丙烯纤维组合对膨胀性土壤进行复合改良。通过54组自由膨胀率试验筛选出自由膨胀率低于40%且性价比高的改良组合(改良剂以质量分数计,下同):0.8%PVA+5%KCl、0.8%PVA+4%CaO。最后根据2种组合设计5组(天然膨胀土、添加0.6%聚丙烯纤维、0.8%PVA+5%KCl、0.8%PVA+5%KCl+0.6%聚丙烯纤维、0.8%PVA+4%CaO)三轴压缩剪切试验,测定抗剪强度并比较改良效果。自由膨胀率试验、三轴压缩剪切试验结果表明复合改良方法效果显著。经过0.8%PVA+5%KCl的混合溶液改性,并添加0.6%聚丙烯纤维改良后的土样自由膨胀率由130%降至27%,三轴抗剪强度提高15%;经过0.8%PVA+4%CaO的混合溶液改性的土样自由膨胀率由130%降至33%,三轴抗剪强度提高110%。研究表明:PVA的成膜效应能强化KCl、CaO对膨胀土水化膨胀的抑制作用;KCl、PVA对聚丙烯纤维提高抗剪强度有抑制作用;聚丙烯纤维能有效减少膨胀土裂隙的萌生,提高抗剪强度。该文提出的强膨胀土复合改良方法的有效性得到了室内试验的验证,可以为今后工程建设中膨胀土的改良、水土流失防治等提供参考。

More than 5 billion dollars economic loss is caused by expansive soil each year around the world. Soil chemical amelioration is better than the other methods, but better effect and higher safety cannot be achieved by a single ameliorant. In order to weaken serious damages to civil engineering facilities and agricultural ecological environment induced by expansive soils, composed ameliorants to improve the expansive soil were tested in laboratory tests in this paper. The low free swelling ratio（no more than 40%） and the high shear strength were required for the soil amendment. Expansive soil used for testing was collected from Handan in Hebei province in China. It was a strong expansive soil with 130% free swelling ratio（more than 90%） and contained large hydrophilic minerals. Based on the physical-chemical effect of ion exchange（to decrease thickness of hydration shell）, polymer film-forming effect（to prevent hydrone from permeating clay particle） and so on, the combinations of different ratios of CaO（1%-8%）, KCl（3%-10%）, PVA（0.3%-1.0%）, polypropylene fiber（0.2%, 0.4%, 0.6%, 0.8%, 1.0%） were used to treat expansive soils. Based on the construction safety and experiment error, among 54 groups of free expansion ratio tests（using for measuring free swelling ratio）, 5% KCl＋0.8% PVA and 4% CaO＋0.8% PVA had better improvement effect, and they were chosen for testing shear strength. A total of 5 groups were designed in triaxial compression tests： no amendment, 0.6% polypropylene fiber, 5% KCl＋0.8% PVA, 5% KCl＋0.8% PVA＋0.6% polypropylene fiber, 4% CaO＋0.8% PVA. They were tested for studying improvement effect by measuring axial force, axial strain and shear strength. The results showed that the improvement effects of composite ameliorants were remarkable. The free swelling ratio was reduced to 27% from 130%, and the shear strength was increased 15% by using the mixed solution of 0.8% PVA＋ 5% KCl＋0.6% polypropylene fiber. The free swelling ratio was reduced to 33% from 130%, and shear strength was increased 110% by using the mixed solution of 0.8% PVA＋4% CaO. In addition, the research showed that the reduction of expansion rate due to adding KCl or CaO was reinforced by the film-effect of PVA and the increase of shear strength due to adding polypropylene fiber was weekend by KCl and PVA. Crack initiation of expansive soil was effectively reduced by adding polypropylene fiber and shear strength was improved too. The effectiveness of composed improvement methods of strong expansive soil was validated by the laboratory tests. The promotion and inhibition mechanism needs further research.