Strength development of soft soils

Prediction and verification of undrained shear strength for loaded soft soils

Master thesis (2023)

Authors

M.M. Nooteboom Civil Engineering & Geosciences

Contributors

C. Zwanenburg Geo-engineering - CEG (supervisor 1)
S. Muraro Geo-engineering - CEG (supervisor 2)
C. Mai Van Hydraulic Structures and Flood Risk - CEG (supervisor 2)
J.G. Bol (supervisor 2)
Faculty
Civil Engineering & Geosciences
Copyright: © 2023 Tim Nooteboom
SHANSEP CPT-based correlations Undrained shear strength Soft soil
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Published Date

30-08-2023

Language

English

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Faculty

Civil Engineering & Geosciences

Abstract

The study delves into the crucial role of undrained shear strength in the stability of civil engineering projects involving soft soils. It explores the enhancement of this strength through (pre)loading in conjunction with Prefabricated Vertical Drains (PVDs). The Stress History and Normalized Soil Engineering Properties (SHANSEP) framework is employed to forecast changes in strength across various phases - before, during, and after loading. Validation is conducted using Cone Penetration Tests with pore pressure measurements (CPTu), and the Nkt strength factor is employed to correlate CPTu data with undrained shear strength.

The research combines laboratory experiments and field data to predict undrained shear strength via SHANSEP, which is subsequently validated through CPTu measurements and monitoring. Two distinct datasets are examined in the study. The first dataset involves dike projects at 'de Markermeerdijken' in the Netherlands, where PVD-assisted surcharges of 5 meters are implemented. SHANSEP is utilized to predict strength levels before surcharge, post-surcharge removal (after >90% consolidation), and post-removal phases. These predictions are cross-validated using CPTu data and laboratory tests. The second dataset focuses on a reclamation project involving Marine Soft Clay strength in the Philippines. Strength correlations with CPTu tests during consolidation are established, and comparisons are made with data from piezometers and SHANSEP predictions.

The analysis uncovers that SHANSEP's precision is contingent on several factors:

1. The uncertainty surrounding Pre Overburden pressure (POP), leading to inaccuracies in initial strength predictions.
2. The effectiveness of preloading being compromised by limited load area relative to depth.
3. Submergence of the surcharge beneath the phreatic surface, resulting in diminished preload and gained strength.
4. Incorporating creep into predictions necessitates a reduction in SHANSEP's S factor.
5. Accurate predictions rely on high-quality laboratory tests.

SHANSEP demonstrates its ability to effectively forecast undrained shear strength in soft soils, with its predicted surcharge strength increments aligning with CPTu values. However, challenges are identified in predicting and verifying strength during consolidation due to the dominating influence of excess pore pressure.

The study's key findings recommend the inclusion of adjustments for preload submersion, consideration of load distribution, and the evaluation of partially drained CPTu data using parameters such as Bq and qt to enhance the verification of SHANSEP-predicted strengths. The study emphasizes caution against relying solely on CPTu or piezometer data for ensuring reliability and accuracy.