Geotechnical Investigation into the Causes of the Failure of Portions of Pavement Along Army Engineer – Mista – Ali Road, Jos Plateau, North Central

Abstract

The recurrent failure of pavement sections along the Army Engineer–Mista Ali Road was 
investigated through a detailed geotechnical assessment aimed at identifying the underlying 
causes of distress. A total of fifteen (15) representative soil samples were collected and 
analyzed, comprising nine (9) samples from visibly failed pavement sections and six (6) samples 
from stable, unfailed sections. Laboratory testing included grain size distribution (sieve 
analysis), Atterberg limits, standard Proctor compaction, and shear strength tests to evaluate 
the engineering behavior of the subgrade materials. Results from the Atterberg limits tests 
reveal significant differences in plasticity characteristics between the failed and unfailed 
sections. The plasticity index (PI) of soils from the failed sections ranges from 21.5% to 
29.4%, indicating high plasticity and substantial swelling potential typical of clay-rich 
materials. In contrast, soils from the unfailed sections exhibit PI values between 6.2% and 
12.7%, reflecting low plasticity and reduced swelling potential, characteristic of sandy or 
gravelly soils. Grain size analysis further supports this distinction. The failed sections contain 
52–73% fines passing through the No. 200 sieve, signifying a predominance of clay and silt 
fractions. Conversely, only 9–25% fines were recorded in the unfailed sections, indicating 
granular soils dominated by sand and gravel. Compaction characteristics show that soils from 
the failed sections possess lower Maximum Dry Density (MDD) values (1.421–1.688 g/cm³) 
and higher Optimum Moisture Content (OMC) values (21.3–27.4%), compared to MDD values 
of 1.844–1.948 g/cm³ and OMC values of 10.7–14.8% in the unfailed sections. These lower 
densities and higher moisture demand reflect weak bearing capacity and poor load-support 
capability. Shear strength parameters reveal angles of internal friction ranging from 12.4° to 
29.3° and cohesion values between 2.2 and 14.3 kN/m², confirming the cohesive nature of the 
soils. Such soils are susceptible to moisture-induced softening and structural instability under 
traffic loading. The study concludes that the high clay and silt content of the subgrade soils 
is the principal cause of pavement failure. Therefore, effective stabilization and proper 
compaction are strongly recommended during reconstruction and rehabilitation to enhance 
subgrade performance and long-term pavement durability.