Gang Zheng

Performance of Cement-Fly Ash-Gravel Pile-Supported High-Speed Railway Embankments over Soft Marine Clay

The Beijing-Tainjin high-speed railway were constructed between two major cities in China. Since part of the railway is over soft marine clay, appropriate ground improvement, cement-fly ash-gravel piles, was selected to improve the soft marine clay to meet technical requirements for the high-speed railway. This article presents the field measurements in the railway embankment project including the load distribution between soils and piles, excess pore pressure, and settlement and lateral displacement. The test results show that the stress concentration to the piles reduced the excess pore pressure effectively. The proportion of the load carried by soils was small, and thus the settlement was significantly reduced. The compression of the rigid piles contributed to less than 21% of the total settlement. The majority of the settlement was contributed by the penetration of the piles and the compression of the soft soil below the pile tips. The average compression thickness below the pile tips was 22 m, which is equivalent to one times the width of the treated area. The measured differential settlement of the embankment was approximately 3 mm.

Improvement of very soft ground by a high-efficiency vacuum preloading method: A case study

ABSTRACT This paper describes a full-scale test on a very soft clay ground around 70,000 m2, which is conducted in Huizhou of Guangdong Province, China, to present a new method of vacuum preloading method. A novel moisture separator was developed, which can automatically regulate the vacuum pressure variation by changing the volume of the gas inside it. A large quantity of water drained by the proposed moisture separators can be directly used as a surcharge loading, which would shorten the ground improvement time and save costs as well. Three levels of silt-prevention prefabricated vertical drains were used in the treating process to accelerate the consolidation. In addition, the vacuum preloading method also included an effective radial drainage device which would strengthen the dredged soft clay fill in a deep layer. In the in situ test, tens of piezometers and settlement plates were installed to measure the variations of excess pore water pressures and settlement of two stages of observation points at different positions in the ground. The results show that the largest average consolidation settlement was 314.1 cm and made a saving of more than 66% in power consumption compared with traditional method. It demonstrates that this adopted method is an efficient, cost-effective, and environmentally friendly method for improving sites with low bearing capacity and high compressibility soils.

Stability Failure Modes of Rigid Column-Supported Embankments

Rigid columns such as such as vibro vibro-concrete columns and reinforced concrete columns are widely introduced to reinforce soft subgrade under embankment. Centrifuge tests and numerical analysis were performed to study the stability failure mechanisms of the embankments on soft soil reinforced with a single-row or group rigid columns. The numerical analysis and centrifuge tests both indicated that the failure modes of rigid columns varied with column bending stiffness and strength, slenderness ratio, and toe embedment and subsoil conditions. The possible failure modes of rigid columns leading the stability failure of embankment include the following types: (1) the primary bending failure of the columns occurred near the interface of soft and hard strata, followed by the secondary bending failure within the upper part of the columns; (2) the primary bending failure of the columns was followed by a collapse of the upper portion of the columns toward the embankment toe; (3) soil flew around columns.

Consolidation of Soft Foundations Treated with Composite Columns

Foundations constructed on soft soil often require columns to increase bearing capacity and reduce total and differential settlements. Columns can be flexible (e.g., stone columns and sand compaction columns), semi-rigid (e.g., deep mixed (DM) columns and jet-grouted columns), and rigid (e.g., vibro-concrete columns). Research has demonstrated that columns can also accelerate the consolidation of soft foundations by column drainage and stress transfer. In this paper, a three-dimensional coupled numerical analysis was adopted to study the consolidation behavior of soft foundations improved with composite columns, which consist of two types of columns. For example, a DM column or jet-grouted column can be installed in the middle of the stone column or sand column. In this composite column, the stone column or sand column provides drainage while the DM column or jet-grouted column provides stiffness. Columns and soil in this study were modeled as elastic materials and one quarter of the unit cell was used due to the symmetry. To demonstrate the benefit of the composite columns in accelerating consolidation, two soft foundations improved with stone columns and DM columns were analyzed for comparison purposes. The numerical results show that both column drainage and stress transfer contributed to the acceleration of the consolidation of the soft foundations improved with composite columns. In addition, a parametric study was conducted to evaluate the influence of the stiffness of DM columns on the rate of the consolidation of the soft foundation improved with composite columns.

Consolidation of Column-reinforced Soft Foundations under Embankments

Flexible (stone columns, sand compaction piles, etc.), semi-rigid [deep mixed (DM) columns, jet-grouted columns, etc.], and rigid (vibro-concrete columns, etc.) columns have been increasingly used to support embankments over soft soils by increasing bearing capacity and stability and reduce total and differential settlements. Field and theoretical studies have demonstrated that columns, no matter whether they have high or low drainage characteristics, can accelerate the rate of consolidation of the composite foundation formed by soft soil and columns. In addition to the permeability of the column, the stiffness of the column contributes to the increased rate of consolidation. This research letter summarizes a three-dimensional numerical study to investigate the consolidation of stone column, DM column, and composite column-reinforced soft foundations. The effects of the modulus, permeability, plastic behavior, and partial penetration of columns on the rate of consolidation were studied.