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Data Security

We are committed to ensuring that your information is secure. In order to prevent unauthorised access or disclosure we have put in place suitable physical, electronic and managerial procedures to safeguard and secure the information we collect.

All data is stored in secure electronic systems accessible only to Oasys staff with both valid network login credentials and specific authorisation to access the system.  Our systems further limit data access by role to ensure data is available only to those who have a specific need to see it.

If at any point you suspect or receive a suspicious communication from someone suggesting they work for Oasys or a website claiming to be affiliated with Oasys, please forward the communication to us or report the incident by email to oasys@arup.com or in writing to Oasys, 13 Fitzroy Street, London, UK, W1T 4BQ as soon as possible.

Data Security Notice Updated 27th February 2020

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Website Terms and Conditions

The contents of this web site are protected by copyright and other intellectual property rights under international conventions. No copying of any words, images, graphic representations or other information contained in this web site is permitted without the prior written permission of the webmaster for this site.

Oasys accepts no responsibility for the content of any external site that links to or from this site.

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Software Licensing Terms

Terms and Conditions of Purchase

The full conditions of purchase and maintenance for all Oasys software are set out in the Oasys Software Licence and Support Agreement. All prices are subject to TAX at the current rate.

Prices and specifications are subject to change without notice – please ask for a written quotation.

Although every care has been taken to ensure the accuracy of all information contained herein, the contents do not form or constitute a representation, warranty, or part of any contract.

Superseded Versions of Terms and Conditions

Oasys keeps copies of all superseded versions of its terms and conditions.

Maintenance & Support Services

Support and maintenance is included with all subscription licences for their full duration.

Annual maintenance contracts are available for software under a perpetual licence, prices are based on a percentage of the most recent list price.

This service includes:

  • telephone/fax/email/web based support
  • free software updates available via internet download
  • personalised output header for many products

Hong Kong Cut and Cover Tunnels

Software Used on this Project

Project Overview

The Kowloon Canton Railway Corporation (KCRC) constructed the the 7.4km Lok Ma Chau Spurline section of the rail crossing between Hong Kong SAR and Mainland China . The design and build contract, ‘LBD201 – Sheung Shui to Chau Tau Tunnels’ forms part of the Spurline and comprises the design and construction of 3.6km of 8.75m excavated diameter twin bored tunnels and approximately 1.4km of cut-and-cover approach tunnels and associated structures.

This case study focuses on the back analysis of the particularly challenging East Approach cut-and-cover tunnels section of the project, which has recently been completed. The results of the back analysis showed the ground stiffness to be consistent with other published data.

 

How Oasys proved invaluable

Ove Arup and Partners Hong Kong Ltd were commissioned by DJV to carry out the detailed design.

The construction and monitoring method used to build the 700m long section of cut-and-cover tunnels to the north of Sheung Shui station has been described in detail by Storry et al (2005). It involved excavations of up to 14m deep and 20m wide, within 1.5m of the live KCRC East Rail mainlines and within 1m of the Dongjiang Watermains, which supply Hong Kong’s potable water from China. Control of ground movements was critical to minimise disruption.

The East Approach cut-and-cover tunnels were constructed within restricted land boundaries and could not impact on the operation of the existing mainline. The final design required two singlebox cut-and-cover tunnels to be constructed side-by-side, both at different levels, ramping down into the main bored tunnels section.

The full length of the East Approach cut-and-cover tunnels was split into sections to facilitate construction. These were formed within temporary sheet-pile wall box excavations varying from 8m to 20m in width. The figure below shows one of the strutted sheet-pile excavations under construction sandwiched between the live railway and the watermains.

Ground Monitoring

The system of monitoring instrumentation was a key component in satisfying the railway operator and Water Supplies Department (WSD) of the robustness of the design and the suitability of the construction methods. The instrumentation included:

  • Ground deformation monitoring points on adjacent existing structures and utilities sensitive to movement;
  • Piezometers to monitor ground water levels around the excavations;
  • Inclinometers (installed within steel tubes that were welded and driven together with the sheet-piles) to monitor deflection of the temporary retaining walls; and
  • Real-time monitoring of the existing and diverted East Rail tracks.

Geotechnical Design

The design of the retaining walls and the strutting sequence was done using the OASYS Frew software, which outputs the lateral displacement of the pile face. To convert this into vertical and lateral movements behind the wall, the recommendations of CIRIA Report C580, on the soil deformation analysis and radius of influence were adopted.

The effect on the groundwater was estimated using seepage analysis. Assumptions on the ground movement profile were verified using two-dimensional finite element computations undertaken in OASYS Safe.

Ground movement predictions were carried out based on free field predictions of movement. Structural stiffness was not considered when assessing building, structure and utility movements, so these predictions were considered conservative.

Back Analysis – Comparison to Ground Monitoring Data

Inclinometer monitoring of the sheet-pile wall confirmed that the displacements of the retaining walls were well within those predicted by the design.

Back analysis was carried out based on the observed wall deflections to determine the actual ground stiffness.

The construction sequence at these sections can be summarised as follows:

  1. Install temporary sheet-pile walls and conduct pumping test to demonstrate water-tightness of the cofferdam;
  2. Excavate and install temporary top strut, S1;
  3. Excavate and install temporary lower strut, S2;
  4. Excavate to the final excavation level and install base slab and lower wall;
  5. Remove strut S2, and construct the remaining wall and roof slab of the permanent tunnel structure; and
  6. Remove strut S1.

A comparison of the predicated and actual wall movements are shown.

It can be noted that the actual wall deflections were less than those predicted during design. This is considered to be due to the adoption of conservative soil parameters, actual ground water levels about 0.5m below the design water levels and live loads from the trains not being permanently applied.

Back analysis was carried out by enhancing the soil stiffnesses to obtain a best fit of the wall deflection profile. Corresponding wall deflections are shown below for steps taken to ‘match’ the measured wall deflections, which can be summarised as follows:

Step 0: Use actual ground water levels and ignoring live loads from trains.
Step 1: Increase stiffness of CDV to 4N.
Step 2: Increase stiffness of coarse alluvium to 4N.
Step 3: Increase stiffness of fill to 1.5N.

A detailed discussion of the back analyses is available in the Pan et al paper.

In general, the back-analysed stiffness values compare favourably with case histories of other excavation projects in Hong Kong.

As for the effects of wall friction, this is dependent on the condition of the sheet-pile and the method of installation. The back analysis demonstrates that the wall friction could lie between 0.5 and1.0 Æ’. 

Construction

The LDB201 East Approach cut-and-cover tunnels have been successfully constructed within a narrow strip of land 1.5m from a live railway and 1m from one of Hong Kong’s primary water supplies.

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