Comparisons of different procedures of pre-compaction stress determination on weakly structured soils

• Published in: Journal of terramechanics Vol. 44; no. 1; pp. 53 - 63
• Main Authors: Mosaddeghi, M.R., Koolen, A.J., Hemmat, A., Hajabbasi, M.A., Lerink, P.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 2007
• Subjects: behavior; compaction; Confined compaction; displacement; distortion; Kneading compaction; Pre-compaction stress; samples; Semi-confined compaction; Sharpness of pre-compaction region; Unstable structure; Kneading compaction; Semi-confined compaction; Sharpness of pre-compaction region; Confined compaction; Unstable structure; Pre-compaction stress
• ISSN: 0022-4898; 1879-1204
• DOI: 10.1016/j.jterra.2006.01.008

Compaction is an important component of soil degradation. In this regard, the pre-compaction stress (
σ
pc) concept is considered useful in mechanized agriculture nowadays. When the external forces exceed the internal strength (
σ
pc) of soil, soil structure and soil physical quality will deteriorate. This concept was introduced at first for confined consolidation of non-structured, homogenized and saturated subsoils in civil engineering, though it is also suitable for agricultural conditions where the topsoil and subsoil are considered and both are often structured, heterogeneous and unsaturated. The best method for predicting
σ
pc is by the plate sinkage test (PST) in the field, but it is expensive and time-consuming. This study was conducted to find an alternative laboratory method besides the confined compaction test (CCT) for predicting
σ
pc. The CCT may not be a good method, especially at higher water contents, and for soils with low organic matter content, because of low sharpness of the critical region on the stress–strain curves. The study was performed on five soil types with a range of soil textures and organic matter content from central Iran using three loading types and three pF (i.e. Log [soil matric suction in cm]) values of 2.3, 2.7 and 2.9 with two replicates. Loading types consisted of CCT, the semi-confined compaction test (SCCT) and the kneading compaction test (KCT) at three maximum (or pre-compaction) stresses of 200, 400 and 600
kPa. The experiment was a completely randomized factorial design. The aim was to determine how accurately each loading type test could predict
σ
pc of soil pre-compacted by one of the other methods. The applied combinations of CCT–SCCT, SCCT–CCT and KCT–CCT mean that the soil was pre-compacted by the first loading type and evaluated by the second one. The results showed that
σ
pc and the sharpness of the
σ
pc region were significantly affected by loading types as well as the soil conditions. The sharpest
σ
pc region was observed in SCCT and the least sharp in CCT with the overall order being CCT–SCCT
>
SCCT–CCT
>
KCT–CCT. The sharpness of the
σ
pc region was reduced for the soil samples with higher water content and coarser texture. Regardless of the soil and loading conditions, the prediction by SCCT was consistently more accurate than by CCT. The prediction of
σ
pc by SCCT was more precise in comparison with CCT especially at higher stresses and soil water contents. However, the prediction of
σ
pc by SCCT was very accurate at pF values of 2.7 and 2.9, and with low
σ
pc values, when compared with the actual values of the
σ
pc. For the clay soil at a pF value of 2.3, no pre-compaction region (i.e. zero
σ
pc) could be determined by CCT at a maximum axial stress of 600
kPa. This was because of the incompressibility of soil water at this near-saturated soil condition at high stress. However, the sharpness of the critical region in SCCT was high enough to predict
σ
pc satisfactorily. There was no significant difference between the combinations of SCCT–CCT and KCT–CCT in predicting
σ
pc. The SCCT is a compromise method that lies between CCT and PST. SCCT has the advantage of using a limited and definite soil volume that can be modeled as a soil element. Marginal effects of disturbance caused by coring/sampling as well as pre-test sample preparation seem to have minor effects on the stress–strain curve determined by SCCT in comparison with CCT. Moreover, the soil volume needed for this test is the same as for CCT.