Baghdad Subgrade Resilient Modulus and liquefaction Evaluation for Pavement Design using Load Cyclic Triaxial Strength

Abstract

Pavements fail for different reasons; poor design, poor materials and poor construction methods are the most common. The pavement foundation (subgrade) represents one of the key elements in the pavement design. The American Association of State Highway and Transportation officials (AASHTO) published the AASHTO Guide for Design of Pavement Structures (AASHTO, 1986) in which the use of Resilient Modulus (M_r) was adopted as the principal soil property contributing to the design of flexible pavements. It can consider that resilient modulus (M_r) is a key value in pavement design.

The present study uses the standard laboratory test for load cyclic Triaxial strength to evaluate the resilient modulus and liquefaction condition of some Baghdad soils ,as well as using the neural network approach to develop a model that can be used to predict resilient modulus values for Baghdad soils . The model uses the results of routine laboratory tests like specific gravity, water content, Atterberg limits, soil classification and unconfined compressive strength to predict M_r.

It is well-known that the Performance of resilient modulus tests are difficult, expensive and time consuming and hence there has been an interest in adopting the Ohio State University  mathematical model (OSU Model) introduced by Kim 2004 and confirmed by Rodgers 2006 that satisfactorily predicts resilient modulus values without the necessity of a laboratory test. It is very important for a mathematical model to accommodate new data as it becomes available.

It is concluded that soil brought from Baghdad City exhibited the resilient modulus (M_r) of pavement subgrade soils which has been adopted by the American Association of State Highway and Transportation Officials (AASHTO) for the purpose of designing flexible roadway pavement systems, values ranging from 40 MPa to about 100MPa. Based on ASTM subgrade resilient modulus criterion, the A-7-5 and A-6 untreated subgrade soil would be classified as fair to poor (unacceptable as a competent subgrade).

To prove the capability of the network, M_r predicted values for Baghdad soil were compared with its corresponding M_r measured. It is concluded that Baghdad soils need to be provided with new network and model with some modification needed to be done on the OSU models to provide a good estimation of M_r for the Baghdad soils.

The results of cyclic load test carried out in laboratory to conduct Liquefaction indicate that for a given initial water content and specific dry density with initial effective stress, it is concluded that generally all samples didn’t exhibit significant gain in liquefaction condition and didn’t show conflict values due to the reduction in the rate of pore water pressure generation and shear strain of all samples subjected to cyclic loading. they shows withstanding against liquefaction by reaching high value of Normalized principal Stress when reaching to critical built up of Pore water pressure which lead to the fact that a liquefied condition could not possibly develop in those soils.

Keywords: Resilient Modulus, C.B.R, Subgrade Compaction, Pavement Design