# Surface settlement induced by frictional force of epb shield tunneling in sandy gravels

## TL;DR

This paper introduces a new formula to predict surface settlement during EPB shield tunneling in sandy gravels, showing it is more accurate than existing methods.

## Contribution

A new surface settlement model incorporating reversed frictional forces during EPB tunneling stages is proposed and validated.

## Key findings

- The new formula provides more accurate surface settlement predictions than Peck’s formula in sandy gravels.
- Surface settlement cannot be controlled solely by increasing chamber pressure; a balance with thrust is needed.
- Field data from Chengdu Metro Line 7 validated the proposed formula’s effectiveness.

## Abstract

The excavation of Earth Pressure Balance (EPB) shield can be divided into two distinct stages, i.e. advancing and lining installation. The frictional force applied on surrounding soils reverses at these two stages, which is harmful to the settlement control. Based on Mindlin’s method, a new model of surface settlement is derived to involve the reversed friction. A closed form formula is then obtained for the major type of metro tunnels. Main operational parameters are also used as input of the formula. Numerous operational data and measured settlements are collected from EPB tunnels of Chengdu Metro, Line 7. The proposed formula is validated against these field data in sandy gravels. It is shown that the new formula gives reasonable prediction of surface settlement along the tunnel sections. The accuracy of new formula is significantly higher than that of Peck’s formula. This study provides a new vision in settlement control of EPB shield tunneling. The increase of chamber pressure will induce higher negative friction during the lining installation. Therefore, surface settlement of EPB tunneling cannot be controlled by just increasing chamber pressure. A balanced relationship between the chamber pressure and the thrust should be maintained instead.