This is a presentation given in November 2011, for the Geotechnical Design of Crossrail Project; Farringdon Station and Pudding Mill Lane Portal.

1. State of the Art and
Practice
Geotechnical Design
Farringdon Station
&
Pudding Mill Lane Portal
Dr Mazin Alhamrany, BSc MSc PhD CEng MICE MISSMGE
Design Manager Geotechnical Engineer

2. Experience Summary
I have over 25 years of experience in leading geotechnical design teams in the UK, Europe
and the Middle East. I lead a geotechnical design team within the London Office of URS
Scott Wilson responsible for the delivery of technical solutions to a wide variety of clients.
I am also leading and developing the Numerical Modelling capabilities for the wider
GeoServices Group at URS Scott Wilson.
My current position is Design Manager Geotechnical Engineer for the Crossrail Project –
Farringdon Station and Pudding Mill Lane Portal. My main target is always to deliver
projects on time and on budget at technical standards that mostly exceed client’s
expectations.
My position as a Design Manager Geotechnical Engineer for these two main elements of
the Crossrail project, in addition to leading and developing numerical modelling capability
within the GeoServices Group at URS Scott Wilson, allow me to continue updating my
knowledge with the latest developments in the fields of geotechnical engineering,
management, Commercial, Health and Safety and sustainability.
Note: this presentation has been given in November 2011

3. Farrington Station

4. Farringdon Station is one of the major central stations to be built and involves the
construction of two 400m long platform tunnels and associated cross passages with a
ticket hall at either end. The West Ticket Hall (WTH), which will house both Thameslink and
Crossrail Services, includes a Circular Shaft of 15m diameter, a Rectangular Shaft of 24m
by 28m, an Escalator Shaft 9m x 25m and individual 1.2m and 2.1m diameter piles to
support the ticket hall and over-site development. The East Ticket Hall (ETH) includes a
Trapezoidal Shaft of approximately 32m by 28m and a further Double Basement with
individual 1.2m diameter piles supporting the ticket hall and another over-site development
as well as works that link with London Underground Barbican Station. The excavation
depth of these shafts is 25 to 30m.
The works include the challenge of installing the 2.1m diameter piles of the oversite
development at the WTH among the tight constraints set by the new tunnels. The piles
have also been designed to carry the negative skin friction and the bending moments
induced from the construction of the future Crossrail tunnels and the adjacent Shafts.
The estimated construction cost of the station is in the region of £350m, and should take
70 months to complete from award of contract.

5. Farrington Station: Street Level Plan

7. West Ticket Hall East Ticket Hall
Farringdon Station - Long Section

10. West Ticket hall - 3D Image

11. Some Geotechnical Design
Aspects
Design of Circular Shaft; West Ticket Hall:
The design of a circular shaft needs considerable engineering judgment. The stiffness of
the walls plays a significant role in the design analysis. The analyses of a very stiff wall
will result in very high hoop forces and relatively low deflections, bending moments and
shear forces and vice versa. The stiffness of the shaft’s wall needs therefore to be
accurately assessed.
The wall of the shaft at Farringdon station will be formed by hard/firm secant Piles of
1200mm/900mm diameter, respectively, at spacing of 1750mm c/c using bentonite support.
The toes of the hard piles will be at 73mATD and the firm piles at 80mATD. The excavation
level is 81.36mATD. The stiffness of the firm piles can significantly vary based on the
concrete mix adopted. Vertical tolerance of piles during construction is another factor that
will significantly effect wall stiffness.
The decision was therefore taken to estimate the most likely value of the wall stiffness (by
carrying out some FE analyses) and then to carry out sensitivity analyses in order to
establish a robust design covering any potential uncertainty regarding this aspect. The
sensitivity analyses included two extreme cases; case 1 Analysis for the maximum
possible stiffness of the wall. This will provide the maximum hoop force on the wall, and
case 2 Analysis for the minimum possible stiffness of the wall. This will provide the
maximum bending moments and shear forces on the wall. The results of the former have
been used for the design of the ring beams and lining walls while the results of the latter
have been used for the design of the secant pile wall.

12. Many other aspects have also been thoroughly investigated such as:
• Connectivity of base slab to the secant pile wall and the potential maximum
bending moments on the base slab considering load distribution on wall and base
slab.
• Assessment of heave forces and upward water pressure.
• Bearing Capacity of the secant piles wall and the shaft as a whole.
• Impact of tunnel construction on the bearing capacity of the shaft’s wall.
• Impact of the construction of the circular shaft itself on the existing adjacent
structures and bearing capacity of oversite development piles.

13. West Ticket
hall

14.  Impact of construction of the shafts and the tunnels on the
piles supporting the oversite development
The OSD piles will be installed prior to the excavation of the Rectangular, Escalator and
Circular shafts at the WTH of Farringdon Station. The OSD piles are deemed to be affected
by the excavation of the shafts. The horizontal soil stresses along the piles will reduce due
to adjacent excavations (as the coefficient of lateral pressure of soil will reduce from Ko-
value to Ka-value).
The station’s oversite development piles (OSD) at the West Ticket Hall (WTH) have been
designed in terms of effective stress allowing for the different shafts and tunnel
excavations that are to take place and for long term groundwater conditions in accordance
with the requirements of Civil Engineering Design Standard / Underground Box Structures
and Deep Foundations - Crossrail Project (CEDS Part3).
Changes in soil effective stresses in the vicinity of the piles due to soil disturbance caused
by the construction of the shafts and the tunnels have been thoroughly investigated and
considered for the design of the bearing capacity of the piles. The changes in stresses
have been determined from the SLS finite element analyses undertaken for the various
shafts and basement structures. In addition checks were undertaken to ensure that pile
capacities in terms of undrained shear strength within cohesive strata were not exceeded.
As a result of the tunnel and cross passage constructions ground settlements will be
induced around the piles and cause additional pile loadings due to the resulting negative
skin friction. This has been considered in detail.

15. Impact of Tunnel Construction
on OSD Piles

16. Geological Section

17. 50
60
70
80
90
100
110
0 50 100 150 200 250 300 350 400
Piezometric Pressure (kPa)
Level(mATD)
Pressure Min
Pressure Avg
Pressure Max
Long Term ULS/SLS
Construction ULS
Short Term/Construction SLS
Pre 2000 Data
77.46mATD WTH Rectangular
Shaft
60
70
80
90
100
110
F26R
Made Ground
River Terrace
Gravel
London Clay A2
Upper Mottled
Beds
sand channel
Laminated Beds
Lower Shelly Beds
Lower Mottled
Beds
Upnor Formation
Thanet Sand
Bullhead Bed
Chalk
F27 Data
Pore water pressure profile

18. Add value and Make design Simple

19. West Secant Piled Wall
East Secant Piled Wall
Finite Element Analysis
(2D Model)

20. Finite Element Analysis
3D Model

21. PCC Segmental
Pilot Tunnel
Depressurisation Zone notionally
3m outside excavation line
Tunnel Construction
Vertical Radial Depressurisation
wells installed every 3 m
5.1 m O.D TBM Pilot Tunnel

22. Ground Settlement Contours

23. Planning and Management
• Agree precise specification for the project – Terms of
Reference.
• Plan the project: Time, Team, Activities, Resources,
Financial using project management tools
(Brainstorming, Fishbone, Gantt Chart, Critical Path
Analysis and Flow Diagrams).
• Communicate the project to the team.
• Manage and motivate: Inform, Encourage, enable the
project team.
• Check, Measure, Monitor, Review project progress –
adjust project plans and inform project tea and others.

24. Safety in Design
• Compliance with CDM 2007 Regulations
- First introduced in 1994 to improve safety standards on construction sites and in maintenance of
structures by placing duties on The Client, The Designer and The Principal Contractor.
- These have now been superseded and included the requirements of the Construction (Health,
Safety and Welfare) Regulations 1996.
The object of the new CDM 2007 Regulations is to reduce the risk of harm to those that have to build,
use and maintain structures.
Designer Duties:
• Check that the Client is aware of his own duties.
• Cooperate with others and coordinate work so as to ensure the health and safety of construction
workers or others who may be effected.
• Avoid foreseeable risks. Eliminate hazards (so far as is reasonably practicable) and reduce risks
associated with those hazards which remain during design.
• Provide information about remaining risks.
• Report obvious risks.
Workplace (Health, Safety and Welfare) Regulations 1992
• Health: Ventilation, Temperature in indoor workplaces, Work in hot or cold environments, Lighting,
Cleanliness and waste material, Room dimensions and space, Workstations and seating.
• Safety: Maintenance, Floors and traffic routes, Transparent windows, doors and gates, Escalator and
moving walkways.
• Welfare: Sanitary conveniences and washing facilities, Drinking water, Facilities for changing,
Facilities for rest and eat meals,

25. Risk Assessment
The potential risks that may be encountered in design, construction
and operational construction need to be specified during design
process.
Quantification of Risk
– Hazards (any thing that has the potential to cause harm)
– Impact rating ( indication of how serious the harm could be on
Health and Safety, Cost, Time, and operational risk)
– Likelihood rating (likelihood of occurrence – high or low)
– Risk score (product of the Likelihood and Impact scores.
An assessment of the effectiveness of the proposed mitigation
measures is made and the residual risk is then assessed and
rated.

26. Sustainable Design
• Value for money – quality and durability
• Minimization of disturbance to natural terrain
• Energy efficient design
• Reduction and utilization of waste material
• Protection of wildlife habitats
• Blend woks with surroundings

27. Pudding Mill Lane Portal

28. Pudding Mill lane Portal will act as the transition ramp between underground
tunnels and above ground works for Crossrail trains to the east of London. It includes a
Tunnel Bore Machine (TBM) Chamber, a Cut-and-Cover Tunnel, a Covered Ramp
structure, and further bridges, retaining walls and a piled embankment support structure.
The works will also include the demolition of the existing Docklands Light Railway (DLR)
Pudding Mill Station and a realignment of the DLR, incorporating a new DLR station,
viaduct, reinforced soil embankment and retaining walls. The estimated construction
cost is £100m.

29. Site Overview
BORED TUNNELSBORED TUNNELS
TBM SHAFTTBM SHAFT
CUT AND COVER BOXCUT AND COVER BOX
DLR STATIONDLR STATION
COVERED RAMPCOVERED RAMP
PROPOSED DLR ALIGNMENTPROPOSED DLR ALIGNMENT
GREAT EASTERN MAINLINEGREAT EASTERN MAINLINE
CROSSRAIL ALIGNMENTCROSSRAIL ALIGNMENT
UP ELECTRIC LOOPUP ELECTRIC LOOP
RIVER LEARIVER LEA
GREEN WAY/
NORTHERN OUTFALL SEWER
GREEN WAY/
NORTHERN OUTFALL SEWER
CITY MILL RIVERCITY MILL RIVER

30. Key Elements of Work
EIPBored
tunnels
NR Gantry works
400kV diversion
11kV diversion Barbers Road
diversion
New DLR
station
New DLR
viaduct
City Mill River
bridges
Northern Outfall Sewer
bridges (DLR tracks)
Retaining
wall
Marshgate
Lane bridges
Retaining
Walls
Retaining wall
Northern Outfall Sewer
bridges (Crossrail/ Up
Electric Lines)
River Lea bed
protection works
Cut & cover
box tunnel
4-span
underpass

31. Tunnel Long Sections
Cut & cover tunnelEvacuation and
Intervention Point
Bored
tunnels
NOS bridgesMarshgate Lane bridges
City Mill River bridge
Covered ramp 4-span
underpass
Extent of 1:27 Gradient

32. Internal Interfaces

33. Questions