Editor's Note: As a supporting project for the 2024 Paris Olympics, the Grand Paris Express project made an important contribution to the successful hosting of the Games. In particular, the Line 14 extension and associated stations of the Grand Paris Express were successfully completed one month before the opening of the Olympics, providing key support for transportation during the Games. To ensure the smooth progress of this critical project, the French construction team established several trial shafts specifically for validating various construction methods and techniques. In this issue, let us learn about the construction trials conducted for the Grand Paris Express project to verify the advantages and disadvantages of two ground improvement methods: jet grouting and artificial freezing.

Trial Overview

As a key project in France, the Grand Paris Express built a total of five trial shafts for validating various construction methods and techniques. The Aulnay-sous-Bois trial shaft was specifically used to compare and verify two ground improvement methods that are relatively mature in application in Paris: high-pressure jet grouting and artificial freezing. The project team hoped to use field trials to understand the structural support effect and waterproofing performance of these two methods in the Beauchamp sand layer in the Paris region.

The green circle marks the location of the Aulnay-sous-Bois trial shaft

The Beauchamp sand layer is a common stratum in the French plains, with a thickness of 10 to 15 m. It is a fine clayey sand containing a small amount of carbonate. This stratum is complex and variable, sometimes sandy and sometimes clayey to silty, with great differences among exploration samples. Due to the uncertainty of this stratum, there is considerable risk of water inflow and collapse when constructing cross-passages in the Beauchamp sand layer, so ground improvement measures during construction are particularly important. To determine which of the two methods, jet grouting or artificial freezing, is superior in the geological environment of Paris, the construction team decided to conduct field tests in the trial shaft.

Trial Shaft Introduction

The trial shaft consists of a diaphragm wall structure about 80 cm thick, with an inner diameter of about 7 m and a depth of about 25 m. At a depth of 21 m, two passages with a diameter of 3 m and a length of about 5 m were excavated in the Beauchamp sand layer, one using high-pressure jet grouting and the other using artificial freezing.

The left passage uses high-pressure jet grouting

The right passage uses the freezing method

Artificial freezing used a nitrogen-brine mixed freezing method. Fourteen freezing pipes spaced 1 m apart formed a 1.5 m thick frozen ring at -10°C, and monitoring was carried out by measuring temperatures at the surface and at the center of the frozen ring.

High-pressure jet grouting used 44 double-jet grout columns with a diameter of 1.5 m and a depth of 21 m. An observation column was also installed to monitor grouting quality, extract grouting samples, and adjust deviations in real time.

The construction team also established a finite element model based on laboratory and on-site measurement data from the trial shaft to study the thermal and mechanical parameters of the Beauchamp sand layer environment. The team used Plaxis 2D Thermal software and established a thermal model through the finite element method; when establishing the mechanical model, the special properties of the Beauchamp sand layer were incorporated into the calculation. Ultimately, the ground deformation calculated through back-analysis using the model and the stratum deformation values obtained from testing were almost consistent with the monitoring measurement results.

Trial Results

After actual testing, the project team summarized and analyzed the advantages and disadvantages of the two methods, jet grouting and artificial freezing: ■ The artificial freezing trial proved that artificial freezing is an effective ground improvement solution in this stratum. The freezing method is less affected by depth and has high safety, and it does not require surface drilling; after completion, the stratum can be restored to its original state, which better meets environmental protection requirements. However, because nitrogen is required and heat source interference must be excluded, the freezing method is more costly; tunnel excavation after freezing reinforcement is also slower, which can easily affect the construction schedule.

■ Jet Grouting The advantages of high-pressure jet grouting are that it generates less noise, does not require 24/7 operation, and tunnel excavation after reinforcement is faster than with the freezing method; however, high-pressure jet grouting also has some disadvantages. For example, in the Beauchamp sand layer, that is, at depths of 25 to 35 m, it is difficult to control drilling deviation, and the equipment required occupies a large area, causing many inconveniences for construction in urban Paris.

The passages excavated during the trial

After analysis, the construction team concluded that high-pressure jet grouting is more suitable than the freezing method for the project schedule and the conditions in Paris; of the original 12 cross-passages requiring treatment, 4 were planned to be reinforced using the freezing method, but based on the trial results, all were changed to high-pressure jet grouting reinforcement. Ultimately, the experience and data from the construction trials further helped the construction team improve efficiency and safety during actual construction.