ايجاد معامل رد فعل التربه لانواع مختلفه من الارض الطبيعيه باستخدام جهاز فحص تحميل الصفيحه الديناميكي خفيف الوزن

-Assessment of Modulus of Subgrade Reaction of Different Soils Using Light Weight Deflectometer

    The subgrade is a soil layer on which a pavement and unbound soil layers are placed. Thus, it works as a foundation to support the pavement’s structure. The important geotechnical design parameter that describes the relationship between stress and the associated settlement of subgrades is the modulus of subgrade reaction (Ks). The modulus of subgrade reaction is an essential parameter in the design and analysis of rigid pavements. It has a significant effect on the required thickness of pavements surface, and gives an estimation of the supporting of the layers under pavement surface. The Ks is typically obtained from the plate load test, PLT. The PLT test has a several limitations; difficult to carry out, costly, and time-consuming. To overcome some of these limitations, it is requisite to find an alternative testing technique which can rapidly and simply predict this parameter.

    This study aims to evaluate the effectiveness of using a light weight deflectometer (LWD) in predicting subgrade reaction modulus of subgrade soils. LWD is a portable non-destructive testing device also known as the dynamic plate load test that is utilized to measure the properties of soil layers under the effect of dynamic loads. To achieve the aim of this study, a series of tests were carried out on three types of subgrade soils. The subgrade soils were collected from different site projects in Karbala city (Al-Meelad, Al-Faris, and Al-Rofae), the soils collected and tested into two phase; [1] in the laboratory to identify the type and classification of soils, and to identify its basic properties. [2] in a laboratory setup model which is designed and manufactured to simulate the field conditions. The collected soils were prepared and compacted in the laboratory setup, three degree of compaction were achieved and tested the soil under each degree of compaction using the dynamic light weight deflectometer (LWD) in conjunction with the static plate load test (PLT).

     On the other hand, the experimental results were statistically modeled to predict the subgrade reaction. The statistical analysis was carried into to phase; granular subgrade soils, and fine subgrade soil.

For granular subgrade soils, three groups of regression models were developed based on independent variables; [1] LWD measurements data, [2] basic soil properties, [3] both the LWD measurements and basic soil properties. In this phase the higher value of R² was 0.93 for subgrade reaction modulus – LWD degree of compatibility model (KS –Dc) model.

     For fine subgrade soil, two groups of regression models were conducted based on independent variables; [1] LWD measurements data and [2] basic soil properties. The results showed that there is a good correlation between Ks and LWD measured data (Ed, δd, and Dc). Also, a strongly correlation was identified between the dry density of the subgrade soil and Ks. The results also showed acceptable relation between Ks and water content.

Additionally, the results of experimental work were verified using the finite element software PLAXIS 3D. Two types of models were used to simulate the soil behavior. The linear elastic model was used to represent the behavior of subgrades under the effect of LWD dynamic loads. And the Mohr-Coulomb was used to represent the behavior of subgrade under the effect of static PLT load.

    The FE results were compared with those obtained from experimental work, and the T-Tests were carried out to examine the variance between the results.

    In the term of LWD surface deflection the mean deference between predicted and measured value ranged from 0.003 to 0.242 mm. And in the term of PLT surface deflection the mean deference ranged from 0.105 to 0.15 mm, and for the subgrade reaction modulus the mean deference ranged from 3.088, and 25.125 kPa/mm.

    Finally, the results of this study showed the efficiency and possibility of using the LWD device rapidly and easily to predict the subgrade reaction modulus of pavements materials.