Foundation settlement is a verification criterion in foundation design, particularly important for construction projects, high-grade highways, railways, airports, and other engineering works, sometimes even determining their success or failure. Engineering practice shows that most engineering accidents are caused by excessive or uneven foundation deformation. High-speed railways and underground projects require deformation accuracy at the centimeter or even millimeter level. The success of high-speed railways depends on settlement, and these modern projects impose higher requirements on foundation settlement calculation. In recent years, breakthroughs have been made in the study of soil strength and stability, but little progress has been made in foundation deformation research. The calculation results obtained from various settlement calculation theories always differ greatly from measured values, and there is still no satisfactory calculation method for foundation settlement (Chen Xiangfu [1], 2004).

Taking sand foundation as an example, to verify the accuracy of various settlement prediction methods for sand foundations, Briaud and Gibbens [2] (1994) conducted detailed geological surveys and in-situ tests on a homogeneous sand foundation site (18 m long × 12 m wide) on the campus of Texas A&M University in the United States, and performed loading tests on 5 spread footings of different sizes. They then organized a famous international workshop on foundation settlement problems, requiring participants to predict, based on the provided footing dimensions, geotechnical tests, and various in-situ test results: 1) the loads Q25 and Q150 corresponding to settlements of 25 mm and 150 mm for each footing (load duration 30 min); 2) the settlement increment Δs of Q25 occurring within 1 to 30 min; 3) the settlement s2014 of a 3 m footing under Q25 after 20 years (in 2014). Thirty-one geotechnical experts and scholars from around the world submitted their prediction reports. Briaud and Gibbens analyzed and summarized these reports, and the conclusion was disappointing: no one could achieve predictions with errors less than 20% for all footings. The ratio of predicted to measured Q25 ranged from 0.07 to 1.73, with an average underestimation of 27%, while the ratio for Q150 ranged from 0.12 to 3.34. The 31 reports employed as many as 22 analysis methods, among which the methods proposed by Schmertmann (1970, 1978), Burland and Burbidge (1985), and the finite element method were most frequently used. Lee & Salgado (2002) [3], Sargand et al. (2003) [4], and Brian Anderson et al. (2007) [5] also conducted similar verification analyses based on footing loading test results.

The verification results indicate: Various sand foundation settlement prediction methods have obvious deficiencies: the allowable bearing capacity of the foundation is often underestimated, while settlement is often overestimated.

Analysis of numerous engineering accidents shows that the vast majority are caused by excessive or uneven foundation deformation. The principle of deformation-controlled design for foundations has long been recognized in China's engineering community. The Chinese Code for Design of Building Foundations (GB50007-2002) explicitly stipulates that buildings of design grades A and B shall be designed based on foundation deformation. However, it is extremely rare for predictions obtained by various foundation settlement analysis methods to agree with measurements. For example, the foundation code method (modified formula in the code) widely used in China often yields settlement calculations several times the measured settlement, and its accuracy and reliability are far from meeting the requirements of modern engineering for deformation-based design.

1 Review of Foundation Settlement Analysis Methods

Calculation methods for foundation deformation include the elastic theory method, the layer-wise summation method, the stress history method, the Skempton-Bjerrum method, and the stress path method [6]. The foundation settlement calculation methods used in domestic roadbeds and other projects mainly fall into three categories [7]:

Classical Layer-wise Summation Method

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