Application	PLAXIS 2D
Version	PLAXIS 2D 2023.1 PLAXIS 2D 2023.2
Original Author	Hung Ho Manh - Technical Support Group
Date created	10 October 2023
Date modified	10 October 2023

Introduction

Excavations (open cut or tunnelling) generally cause ground movement, which could result in damage to adjacent buildings. Therefore, a damage assessment of buildings nearby induced by excavations is needed, especially for shallow foundations. In common practice, the design of an excavation needs to ensure that the surrounding ground movement and wall displacement are within acceptable limits.

Classification of damage (Burland et al., 1997) associated with limiting tensile strain (Boscardin and Cording, 1989) of buildings includes six categories in Table 1:

Table 1: Damage categories (Mair, et al., 1996)

Categories of damage	Normal degree of severity	Limiting tensile strain (elim,%)
0	Negligible	0 - 0.05
1	Very slight	0.05 - 0.075
2	Slight	0.075 - 0.15
3	Moderate	0.15 - 0.3
4 to 5	Severe to Very severe	>0.3

The tensile strain in a building can be caused by the building bending or shearing deformation and horizontal strain (ε_h). These strains are calculated based on building deflection (Δ/L) in which the building is considered as an idealized beam with span L and height H, and shown in the following equations (Burland and Wroth, 1974): 

where:

ε_{b max}/ ε_{d max}: tensile strain caused by bending/ shearing, H: height of the building, L: length of the building, E and G are Young’s modulus and shear modulus of the building, I: second moment of area the equivalent beam (i.e. H³/12 in sagging zone and H³/3 in hogging zone), h: furthest distance from the neutral axis to edge of the beam (i.e. H/2 in sagging zone and H in hogging zone).

The maximum deflection (Δ) and the deflection ratio (Δ/L) can be determined based on building deformation shown in Fig. 1 for hogging zone (Δ_h/L_h) and sagging zone (Δ_s/L_s). 

Fig. 1 Building deformation (Mair, et al., 1996)

The average horizontal strain (ε_h) is added to the bending or diagonal strain to give the total strain of bending (ε_bt) and shearing (ε_dt) shown as follows (Burland, 1995):

Where: v is the Poisson's ratio.
By applying the values of tensile strain limit (ε_lim) in each damage category, a relationship between deflection ratio (D/L) and horizontal strain (ε_h) for appropriate values of h, L/H, E/G, v and I of a building is established. The damage criteria diagram corresponding to L/H=1 and E/G=2.6 in the hogging zone is shown in Fig. 2 (Burland, 1995).

Fig. 2. Damage category chart (H/L=1) in hogging mode (Burland, 1995)

Mair et al. (1996) proposed three stages of damage assessment of an adjacent building due to excavations, namely preliminary, second stage and detailed assessments. The first stage is evaluated based on ground surface settlement without building stiffness included (green field site). If the settlement and slope are less than 10 mm and 1/500, respectively, the building is considered to have negligible risk of damage, and no further assessment is necessary. If one of these ground deformations is beyond the limit, the second stage (using green field site) and third stage (building stiffness included) are further evaluated based on deflection ratio (Δ/L) and maximum tensile strain (ε_bt or ε_dt). The third stage is carried out if the building in the second stage is classified as being at risk of moderate damage or greater.

The deflection ratio and tensile strain in a numerical model are conveniently extracted if automation scripting is used. A Python script built up to extract these outputs in PLAXIS to evaluate the damage risk of an adjacent building due to an excavation is briefly introduced in the following section.

Damage assessment automation

Once a model has been run (Fig. 3), the Python script for damage assessment can be used. This script is designed to work with PLAXIS Output only. It extracts the footing deformation, including settlement, deflection, and horizontal strain. Two criteria used for the damage assessment comprising of footing deflection ratio and horizontal strain are plotted in a diagram of damage levels (Fig. 4), which is determined based on the building parameters, i.e., E/G and L/H, and a hogging or sagging zone.

Fig. 3. Excavation model

Fig. 4. Damage criteria and footing deformation   

It is noted that there are two points of footing deformation (hogging and sagging) in the plot. Depending on the plot type (hogging or sagging), the corresponding point is used for the building risk assessment. Furthermore, it is generally used for shallow foundations.

Main goals

Extracting footing deformation in PLAXIS 2D and plotting it in a damage chart to evaluate the damage risk of a building.

• Extract footing deformation.
• Show footing settlement.
• Show footing deformation in a chart of damage criteria.

Requirements

You will need the following:

• PLAXIS 2D 2023.1 or later
• Activate Geotechnical SELECT Entitlement (GSE) and Start server for Configure remote scripting server.
• Create a 2D model and ensure the phase used to evaluate damage risk has been run and is ready for extracting outputs.
• Opening PLAXIS 2D Output
• Python files are available in the Downloads section of this article, and it includes:
  • extract_output_burland_chart.py script to extract and plot outputs

Usage instructions for Python script

To use this Python script:

• Download the package from Downloads below
• Extract the zipped package and copy the extract_output_burland_chart.py file to:
  [PLAXIS 2D installation directory]\pytools\ output
  By default: C:\Program Files\Seequent\PLAXIS 2D 2023.1 or PLAXIS 2D 2023.2 \pytools\output
• In PLAXIS 2D Output, run the script:
  Expert > Run Python tool > extract_output_burland_chart

Version

The script is tested with PLAXIS 2D 2023.1, PLAXIS 2D 2023.2 and Python 3.8.

Disclaimer

Copyright (c) Plaxis bv. All rights reserved. Unless explicitly acquired and licensed from Licensor under another license, the contents of this file are subject to the Plaxis Public License ("PPL") Version 1.0, or subsequent versions as allowed by the PPL, and You may not copy or use this file in either source code or executable form, except in compliance with the terms and conditions of the PPL.

All software distributed under the PPL is provided strictly on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESS OR IMPLIED, AND LICENSOR HEREBY DISCLAIMS ALL SUCH WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT, OR NON-INFRINGEMENT. See the PPL for specific language governing rights and limitations under the PPL.

References

1. Burland J.B., Wroth C.P. 1974, Settlement of buildings and associated damage, Cambridge Pentech Press, Cambridge, UK.
2. Burland, J.B. Broms, B. and De Mello, V.F.B. (1977), Behaviour of foundations and structures, SOA Report, Session 2, Proc. 9th Int. Conf. SMFE, Tokyo, 2;495-546
3. Burland, J.B. (1995), Assessment of risk of damage to buildings due to tunneling and excavations. Invited special lecture to IS-Tokyo’95: 1st Int. Conf. on Earthquake Geotechnical Engieering, Tokyo.
4. Boscardin, M.D. and Cording, E.G. (1989), Building response to excavation-induced settlement, Jnl Geo Engrg. ASCE, 115 ;1;1-21.
5. Mair R.J., Taylor R.N., Burland J.B. 1996, Prediction of ground movements and assessment of risk of building damage due to bored tunnelling, Int. Symp. On Geotechnical Aspects of Underground Construction in Soft Ground, London.