Ali Porbaha

CMC Foundation System for Embankment Support -- A Case History

A wide range of deep foundation systems has recently been developed for construction of embankments on soft soils. Controlled Modulus Columns (CMC) is one technique of ground modification, originally developed in France, for support of light structures such as highway and railway embankments. This paper presents the case history of a column supported embankment project in which 2193 CMC columns were installed up to a depth of 12.5 m to minimize settlement of an embankment. Main challenges were soft ground condition of the site, and the need for an accelerated construction technology for timely delivery of the project. Evaluation of various alternatives, design considerations, construction related issues and monitoring system are discussed in detail.

Two and Three-Dimensional Modeling of DM Columns under Embankments

Deep mixed (DM) columns have been commonly used to increase bearing capacity of soft soil and reduce total and differential settlements of embankments. In most analyses (especially numerical analyses) of embankments over DM columns, two-dimensional models are commonly used for simplicity. However, it is not clear how well the two-dimensional (2D) models represent the behavior of embankments over DM columns that are typically installed in a three-dimensional (3D) pattern, such as a square or triangular pattern. In this study, 2D and 3D numerical analyses were conducted to show the comparisons of their results including maximum settlements and vertical stresses. In the 2D analysis, individual DM columns and the soil between these columns are converted to several DM walls parallel to the centerline of the embankment. The DM walls are assumed to have equivalent moduli or strength averaged from the individual DM columns and the soil between these columns based on their areas and moduli or strengths. The comparisons show that the 2D model calculates slightly (conservatively) larger maximum settlements at the base of the embankment than the 3D model if the DM walls/columns are within the elastic limit. However, the 2D and 3D models produce very different results of vertical stresses on the top of DM walls/columns.

Design and Monitoring of an Embankment on Controlled Modulus Columns

The controlled modulus columns (CMC) foundation is one technique of ground modification for support of light structures such as highway and railway embankments. In this technique, a specially designed auger, powered by equipment with large torque capacity and high static down thrust, displaces the soil laterally without vibration. During the auger extraction process, a column is developed by pressure grouting to achieve a predetermined stiffness ratio with the surrounding soil. Because of the combined effect of densification and reinforcement, the properties of soft soil are improved because of composite action. The design and monitoring of a column-supported embankment project are presented; 2,193 CMC columns were installed up to a depth of 12.5 m to control settlement. The main challenges were construction on soft ground and the need for an accelerated construction technology for timely delivery of the project. Various alternatives, stability analysis, design considerations, construction challenges, quality control, and monitoring system were evaluated.

NUMERICAL ANALYSIS OF EMBANKMENT STABILITY OVER DEEP MIXED FOUNDATIONS

When embankments are constructed over soft foundations, stability often becomes one of the controlling factors in design. Deep mixing methods have been commonly used as an alternative to improve soft soil to enhance the stability. Bishops modified method is a commonly adopted approach for analyzing slop stability including embankments over deep mixed (DM) foundations. Bishops modified method assumed failure along a circular slip surface and the soils along this slip surface provide shear resistance. However, experimental and numerical studies have showed that deep mixed columns under a combination of vertical and horizontal forces could fail due to shearing or bending. The bending failure cannot be analyzed by Bishops method. A finite difference method was used in this study to evaluate the factors influencing the stability of embankments over deep mixed foundations, which include strength, spacing, and size of DM columns, undrained shear strength and thickness of soft soil, quality and height of embankment fill, and traffic loading. Mohr-Coulomb failure criterion was used for embankment fill, foundation soil, and deep mixed columns. In addition, the tensile strength of deep mixed columns was assumed to be 10% of their undrained shear strength. A row of deep mixed columns was modeled as a wall in 2-D for simplicity of analysis. Deep mixed columns were embedded into a firm soil to create a possibility of bending failure. The numerical analysis indicated that the factors of safety of the embankments over DM foundations depend on these influence factors and the failure mode changes from deep-seated failure to slope failure within the embankment fill.

International Perspectives on Quality Assessment of Deep Mixing

A wide range of in situ techniques has been developed in the last two decades in Europe and Asia to assess the quality of deep mixed soil columns. These methods use either conventional or specially designed tools to penetrate into the hard columns (such as pushing, pulling, rotary, driven, or displacement methods); or use geophysical concepts (such as seismic, resistivity, and echo-pulse methods). The main objective of this paper is to present the results of an international survey conducted for the quality assessment of deep mixed columns. The objectives were (a) to identify the most commonly accepted field methods of quality assessment practiced in different parts of the world, and (b) to understand the operational problems occurred when the method was applied for deep mixing projects. The respondents of the survey had diverse backgrounds, which included engineers, academicians, researchers, and contractors. The summary of the international survey provided valuable insights for the selection of in situ techniques for the quality assessment of deep mixing projects.

In Situ Test Protocols for Quality Assessments of Deep Mixing Columns

Deep mixing (DM) technology involves the mixing of soils extending to large depths with cement, or lime, or other types of stabilizers for raising the strength of soft and compressible soils, reduction of settlements of roads, embankments, and enhancing the stability of excavation support and slopes. Quality Assurance (QA) of deep mixing methods used in the construction projects must be addressed to evaluate the effectiveness of DM treatment methods adapted in the field. If not addressed, columns with deficient strength and stiffness properties will induce damage to infrastructure built on them. Several in situ testing protocols for QA studies were developed as a part of a research study conducted for the National Deep Mixing Program. This protocol development was based on extensive literature reviews, documented case study information and surveys with practitioners associated with the in situ methods. This paper provides comprehensive details on three testing protocols of standard penetration test, cone penetration test and pressuremeter test. The protocols covered both step by step testing methodologies on soil columns, and interpretation methods for strength and stiffness properties. A case study example utilizing SPT protocol is presented to illustrate the steps in QA studies.

EMBANKMENT CONSTRUCTION ON MARSHLAND USING VACUUM CONSOLIDATION TECHNOLOGY

Vacuum consolidation is a soil improvement technique that accelerates construction on soft ground using the action of the atmospheric pressure in combination with a ground pore-pressure relief system. As an alternative to the conventional preloading (physical surge), vacuum assisted consolidation can be used to consolidate soft alluvial soils, to improve bearing capacities prior to construction, and to reduce post-construction settlements. This paper presents case studies of roadway embankments of 3.3 to 7.9 m high on three marshland sites using vacuum consolidation technology. Lack of shear strength of the soil at the project site did not allow construction of embankments without soil improvement. Although the use of conventional wick drains and subsequent application of surcharge was sufficient to solve the consolidation problem in most areas, in some specific areas a different approach was needed because of acute slope instability combined with strict construction schedule requirements. Vacuum consolidation was applied in three sites with recent alluvial clayey soil deposits. In addition, the sub-soil was fully instrumented with a number of pore pressure, multi-point settlement gauges and inclinometers for real-time monitoring of the embankment during construction at the marshland zones. The details of design and site monitoring program along with post-construction performance for three sites are presented here.