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National University of SingaporeDepartment of Civil Engineering
CE 5112
Structural design and construction of
deep basements &cut & cover structures
Lecture 2
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Words of wisdom
The concept and execution of engineering must be based onINTEGRITY - integrity in applying the laws of nature, andintegrity in dealing with fellow engineers, clients,constructors and suppliers. Just as a structure will stand uponly with integrity, we need to establish a relationship basedon integrity in dealing with our fellow people.
INGENUITY is the very basis of engineering, meaning
creativity and excellence and is fundamentally part of progressive engineering. There will be times when unusual problems call for special solutions. When such a time comes,we should not shy away from the demand for ingenuity and
the change offered thereby. T.Y. LIN
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Practical Design Considerations
1) Introduction – sharing of structural engineer perspectives
2) General requirements – clients, builders & designers
3) Ground, soil profile & gases
4) Concept of effective stress vis-à-vis total stress5) Groundwater control
6) Movements caused by excavation activities
7) Methods of construction8) Types of earth retaining system
9) Influence of foundations type adopted
10) Site Investigation
11) Geotechnical & structural analysis, soil-structure interaction
12) Protective measures
13) Durability and waterproofing
14) Safety, legal and contractual issues & risk communications3
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Methods of construction
Deep excavations for underground structures requiresecure earth and groundwater retention in the
temporary/construction and permanent phases. There
are 4 main categories of techniques:1) Open unsupported excavation – slope stability, groundwater
control.
2) Steeper or vertical open excavation where the face of theexcavation is supported by nails, anchors, props or similar techniques and where conditions permit.
3) Bottom-up excavation with temporarily lateral strut support& wall.
4) Top-down (& up) excavation where the permanent walls &floors are used to laterally support the excavation in bothtemporary and permanent states.
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Methods of construction
There is also term used like semi-top-downconstruction which is done for reasons of
constructability and economy. Excavation can
use any combination of the 4 techniques:
1) Minimizing temporary works (e.g. king post/plunge columns only)
2) Maximum opening sizes in the permanent
works for ease of excavation – for spoilremoval and it is likely to be cheaper using bottom up construction where possible.
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Type of earth retaining structures
Forms of gabion retaining wallsThe permeability and flexibility of gabions make them suitable where the retained material is
likely to be saturated and where the bearing quality of the soil is poor. Wire mesh gabions are of
two forms: baskets, which are used for walls, and mattresses which are used for revetments and
the lining of river.
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Type of earth retaining structures
Forms of gabion retaining walls
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Type of earth retaining structures
Forms of reinforced concrete cribwork Crib walls is used for permanent and temporary retaining walls to embankments,
cuttings and bridge approaches. When used to support an existing slope it is advisable
to construct the wall to the maximum batter (1 horizontal in 4 vertical).
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Type of earth retaining structures
Forms of reinforced concrete cribwork
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Type of earth retaining structures
Flexible Wrap-around Facings, 45 to 70 with Vegetation
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Geogrids:
Bonded Geogrid
Naue/Fortrac/Paragrid etc
Extruded Punched Geogrid
Tensar/Tenax
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Geotextiles: Made from Filament or Tape
Non Woven Textile Needle
Punched or Thermally
BondedTerram/Polyfelt/Landolt etc
Woven TextileAutoway etc
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Type of earth retaining structures
Green Reinforced Earth Walls13
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Type of earth retaining structures
Green Reinforced Earth Walls14
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Mass Gravity Walls
Suitable for:
• Single tier walls up to 2m• Good ground conditions
• Low external loads
Keystone Retaining Walls
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Keystone Retaining Walls
Crash Barriers
The Keystone Advantage
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Reinforced Soil Walls
• Keystone Blocks + Soil
Reinforcement• Geogrid or Steel ladder
reinforcement
• Suitable for: – Walls up to 20m+
– Tiered walls
– Poor ground conditions
– High external loads
Keystone Retaining Walls
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Keystone Retaining Walls
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Type of earth retaining structures
Forms of mass or
RC concrete wallsMass concrete walls are suitable
for retained heights up to 3 m.
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Type of earth retaining structures
RC concrete wallsRC & reinforced masonry retaining
walls on spread foundations aregravity structures where overturning
stability is provided by the weight of
the wall together with the weight of
the retained material rests on the
base slab. The various structural
elements of the wall are designed to
resist bending.
Piles will be needed if bearing
capacity is inadequate.
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Type of earth retaining structures
RC counterfort &
buttressed wallsCantilever wall up to 8 m height
is generally economic; for greater
heights a counterfort wall is more
appropriate.
Buttressed reinforced concreteretaining walls are seldom used.
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Retaining structures on Soft Ground
Reducing lateral force on retaining wall using EPS – Engineered foam. This
application saves construction time and overall project cost
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Excavation methods and support systems
Open cutFor large excavations. It is fast, cheap and gives full accessible site.
Practicable in relatively good stable soil with a large open field site. If
permeability and water table are high dewatering may be necessary
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Slope cutting enhanced with ground anchor
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Excavation methods and support systems
Temporary support
against central dumping or
by fully braced trenchSuitable for large excavations in plan
rather than in depth. Evades ground
water problems If sheet piling/wall
can effect seal in underlying stratum.Slow and radically constrains
program and access. Wall has to be
self-supporting to withstand soil
pressures before the rest of basementarea can be excavated.
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Excavation methods and support systems
Long flying shores across
excavations
Suitable for narrower excavations.
Impedes construction.More difficult incorporation of
monitoring Jacks.
Fully braced temporarysupport
Suitable for very deep excavations -
traditional. With incorporation of jacks
for pre-loading to minimize wallmovement.
Slow and costly particularly when width
of excavation increases.
Constrains construction works because
of access difficulties.
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Excavation methods and support systems
Fully braced temporary
supportSuitable for very deep excavations -
traditional. With incorporation of
jacks for pre-loading to minimize
wall movement.
Slow and costly particularly when
width of excavation increases.
Constrains construction works
because of access difficulties.
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OPEN-CUT & BOTTOM-UP CONSTRUCTION METHODEarth is excavated to required depth with retaining walls & struts. Upon the
completion of excavation, the base slab of the underground structure is cast at the bottom-most level, followed by side walls. Casting of concrete progresses upwards,
level by level till the roof of the structure is completed. Ground is then backfilled
and reinstated.
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Excavation methods and support systems
Concurrent upward and
downward construction
Good for deep excavations. Affordsspeedier construction on super-
structure.
Excavation and removal of spoil
form enclosed area relativelydifficult.
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Excavation methods and support systems
Floors cast on ground
with excavation
continuing belowGood method for deep excavations.
Temporary strutting & beams
eliminated.
Excavation under slabs and removalof spoil relatively difficult.
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RC Compression Member
in Bending
The Reinforced Concrete Council
offers the following Excel
Spreadsheet files for Design toBS8110
RCC-2000
SPREADSHEETS FOR
CONCRETE DESIGN TOBS8110 and EC2
mirrored in:
http://www.civl.port.ac.uk/rcc2000/
Balanced failure point ≈
0.20Fcu
Ac
= 0.2x.035x300² = 630 kN
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Ground anchorage nomenclature
Soil Nailing
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Soil Nail Failure Modes
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Reinforced Earth
Reinforced Earth walls are gravity structures consisting of alternating layers of granular backfill and reinforcing strips
with a modular precast concrete facing. They are used
extensively in transportation and other civil engineering
applications. Because of its high load-carrying capacity,
Reinforced Earth is ideal for very high or heavy-loaded
retaining walls.
The inherent flexibility of the composite material makes it possible to build on compressible foundation soils or unstable
slopes. These performance advantages combined with low
materials volume and a rapid, predictable and easy construction
process make Reinforced Earth an extremely cost-effectivesolution over conventional retaining structures.
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Reinforced Earth http://www.nehemiah.com.my/main1.htm
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Reinforced Earth
Polymer Straps Relatively
Inextensible
Steel Strips or Grids
Inextensible
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Reinforced Earth
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Reinforced Earth
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Design CriteriaExternal Stability
1. Sliding along base of reinforced soil block
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Design Criteria
External Stability
1. Sliding along base of reinforced soil block
2. Bearing capacity
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Design Criteria
External Stability
1. Sliding along base of reinforced soil block
2. Bearing capacity
3. Overturning
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Design Criteria
External Stability
1. Sliding along base of reinforced soil block
2. Bearing capacity
3. Overturning
4. Overall stability
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Design Criteria
Internal Stability
1. Tensile failure of reinforcement
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Design Criteria
Internal Stability
1. Tensile failure of reinforcement
2. Reinforcement pullout
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Ground anchorage nomenclature
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Ground anchorage nomenclature
Typical temporary anchorage in soil during stressing
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Ground anchorage nomenclature
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Ground anchorage nomenclature
Typical anchorage in soil with fixed anchor protection- restressing
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Removable Ground Anchorage
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Removable Ground Anchorage
Normal multistrand anchorage & Single bored multiple anchorage
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Ground anchorage nomenclature
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Ground anchorage nomenclature
Removable Anchorages
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Multi-Anchor System
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Withdrawn Prestressed Strands of Ground Anchors
Corrosion should be monitored near anchorage zone
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Corrosion should be monitored near anchorage zone
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Excavation – Sheetpile,
Soldier Pile & Spray
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Soldier Pile & Spray
Concrete (Gunite)
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Ground anchorage nomenclature
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Ground anchorage nomenclature
Typical rock bolt fully bonded over free tendon length
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Ground anchorage nomenclature
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Ground anchorage nomenclature
Typical anchorage in rock debonded over free tendonlength with fixed anchor protection - restressing
Typical unprotected bar anchorage
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Typical frictional strength in rock
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Typical frictional strength in rock
1. Only primary grout applied in GA installation, grouting
pressure 0.3‐0.5Mpa, limit of unit friction range from
0.2‐0.3MPa. C856 – Labrador Park Station with
moderately strong to strong sandstone. Ultimate unitfriction adopted 250kPa (bored/micro‐pile).
2. Primary grout with post‐grouting, grouting pressure 0.3‐
0.5Mpa for primary and 3‐4Mpa for post grouting, unit
friction ranged from 4‐6N (0.4
‐0.6MPa) for design. C856
– West Coast Station with weathered siltstone & SPT “N”
= 40‐70. Ult. unit friction adopted is 4N with limit at
250kPa
In S’pore, high pressure grouting > 10Mpa is uncommon.
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Typical frictional strength in rock
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yp g
DTL C911: Soil condition is moderately strong to strong, moderatelyweathered to slightly weathered and fractured to coarse grained Granite.
Only primary grout is applied in the GA installation, unit friction of
250kPa adopted in initial design and higher value may be used subject to
trial anchor test, due to uncertainty and variation of ground conditions.
Ground anchor nominal diameter = 200mm
Non‐shrink grout = 0.4% cement weight
28 days grout strength = 40 N/mm², stressing = 24.5 Mpa
Factor of safety = 1.6 (Structure) & 2.5 (Geotechnical)
Unit skin friction adopted = 2N < 200 kPa GVI & N > 8 (Soil)
= 440 kPa for GIII, GII & GI.
Rock bolt 400 kPa for GIII and 800 kPa for GII with 150mm shotcrete
Factor of safety = 2.0 (Structure) & 3.0 (Geotechnical)
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Typical Bond Stress Value for Selected Rock
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It is not generally recommended that design bond stress exceed
1.379MPa even in the most competent rocks
Rock Type (Sound)
Granite & Basalt 1.724 to 3.103
Limestone (competent) 2.068 to 2.758Dolomitic Limestone 1.379 to 2.068
Soft Limestone 1.034 to 1.517
Slates & Hard Shales 0.827 to 1.379
Soft Shales 0.207 to 0.827
Sandstone 0.827 to 1.034
Chalk 0.207 to 1.034
Marl (stiff fissured) 0.172 to 0.248
Ultimate Bond Stress plus δskin (Mpa)
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Ground anchorage nomenclature
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Externally supported Retaining System
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Ground anchorage nomenclature
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Short-term design stress-strain curve for normal andlow relaxation products – Prestressing strand & bar
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Ground anchorage nomenclature
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Typical sizes & characteristic of prestressing tendon
General allowable anchor load:
Service load ≤ 0.6 f pu
Proof load ≤ 0.8 f pu
Proof load factor is 1.25 (temp) & 1.5
(perm)
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Ground anchorage nomenclature
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Table 2. Minimum safety factors recommended for design of individual anchorages
Anchorage category
Minimum safety factor
Proof
load
factor Tendon
Ground/
grout
interface
Grout/ tendon or
grout/
encapsulation
interface
Temporary anchorages
where
a service
life
is
less
than
six
months and failure would have no serious consequences and
would not endanger public safety, e.g. short term pile test
loading using anchorages as a reaction system.
1.40 2.0 2.0 1.10
Temporary anchorages with a service life of say up to two
years where, although the consequences of failure are quite
serious,
there
is
no
danger
to
'public
safety
without
adequate
warning e.g. retaining wall tie∙back.
1.60 2.5* 2.5* 1.25
Permanent anchorages and temporary anchorages where
corrosion risk is high and/or the consequences of failure are
serious, e.g. main cables of a suspension bridge or as a
reaction for lifting heavy structural members .
2.00 3.0t 3.0* 1.50
* Minimum value of 2.0 may be used if full scale field tests are available.
†
May
need
to
be
raised
to
4.0
to
limit
ground
creep.
NOTE 1. In current practice the safety factor of an anchorage is the ratio of the ultimate load to design load. Table 2 above defines minimum safety factors at all
the major component interfaces of an anchorage system.
NOTE 2. Minimum safety factors for the ground/grout interface generally lie between 2.5 and 4.0. However, it is permissible to vary these, should full scale field
tests (trial anchorage tests) provide sufficient additional information to permit a reduction.
NOTE 3. The safety factors applied to the ground/grout interface are invariably higher compared with the tendon values, the additional magnitude representing a
margin of uncertainty.
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Trial of Ground anchorages
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Recommended load increments and minimum periods of observation for proving
tests on anchorages where the ground conditions are not known, or prior experienceof anchoring does not exist
It is recommended that load-displacement results should be plotted as the test
proceeds. In this way it should be possible at an early stage to observe trends & in
particular, the yield of the fixed anchor as failure approaches.
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Trial of Ground anchoragesR d d l d i d i i i d f b i f i
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Recommended load increments and minimum periods of observation for proving
tests on anchorages where previous anchorage knowledge is availableNote: As an alternative use next figure where Tw is known.
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Trial of Ground anchoragesR d d l d i t d i i i d f b ti f it
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Recommended load increments and minimum periods of observation for on-site
suitability tests
Temporary anchorage Permanent anchorage
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Acceptance criteria for disp. of tendon @ anchor head
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For residual load-time behavior
For displacement-time behavior @ residual load
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Apparent free tendon length
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Apparent free tendon length may be calculated from the load elastic displacement
curve over the range 80% to 5% using the manufacturer’s value of elastic modulus
and allowing for the effects of temperature, bedding of the anchor head and other
extraneous movements.
Where working load, Tw, is known, the analysis should be carried out on the load-
displacement curve over the range 125%Tw
to 10%Tw
for temporary & 150%Tw
to
10%Tw for permanent anchorages respectively.
The analysis should be based on the destressing stage of the results of the 2nd or
subsequent unloading cycles. Any difference between the calculated apparent free
length and the free length intended in the design should be stated. For simplicity in
practice the equation to calculate the apparent free tendon length is:
where
At is the cross section area of the tendon: E s
is the manufacturer’s elastic modulus for the tendon unit;
e is the elastic displacement of the tendon, where e is equated to the
displacement monitored at peak cycle load minus the displacement at datum
load, after allowing for structural movement.
T is the peak cycle load minus datum load.
T
E Ae st
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Acceptance criteria
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Apparent free tendon length limits. The apparent free tendon
length calculated should be not less than 90% of the free
length intended in the design nor more than the intended free
length plus 50% of tendon bond length intended in the designor 110% of the intended free tendon length. The latter upper
limit takes account of relatively short encapsulated tendon
bond lengths and fully decoupled tendons with an end plate or
nut.
Where the observed free tendon length falls outside the limits,
a further 2 load cycles up to the proof load should be carried
out in order to gauge reproducibility of the load-displacementdata. If the anchorage behaves consistently in an elastic
manner, the anchorage need not be abandoned.
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On-site acceptance criteriaG ll h d h ld b bj t d t t t t i
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Generally every anchorage used should be subjected to an acceptance test in
accordance with BS8081 clause 11.4.2 to 11.4.7, except rock bolts where 1% to 5%of the anchorages may be loaded to the proof load (but see also 11.1 and 11.3.2).
checked for fixed anchor displacement (see 11.4.11) and then locked off at 110% Tw.
Load-displacement data should be plotted continuously over the range 10% Tw to
125% Tw for temporary anchorages and 10% Tw to 150% Tw for permanent ones, using load increments of not more than 50% Tw with displacements carefully
monitored. During unloading, displacements at not less than two load decrements,
in addition to datum, should be measured preferably occurring at one third points
with respect to proof loads.
Each stage loading in the 1st cycle should be held only for the time necessary to
record the displacement. Each stage loading in the 2nd cycle should be held for at
least 1 min and the displacement recorded at the beginning and end of each period.
For proof loads, this period is extended to at least 15 mm, with an intermediate
displacement reading at 5 min.
On completion of the 2nd load cycle, reload in one operation to 110 % T and lock-
off. Reread the load immediately after lock-off to establish the initial residual load.
This moment represents zero time of monitoring load/displacement-time behaviour
(see 11.4.6 and 11.4.7).67
R d d l d i t d i i i d f
On-site acceptance criteria
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Recommended load increments and minimum periods of
observation for on-site acceptance tests
(a) Temporary anchorage (b) Permanent anchorage
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Acceptance criteriaResidual load time data: Using monitoring equipment with a relative accuracy of
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Residual load-time data: Using monitoring equipment with a relative accuracy of
0.5 %, the residual load may be monitored at 5, 15 & 50 minutes.Pass: If the rate of load loss reduces to 1 % or less per time interval for these
specific observation periods.
If the rate of load loss exceeds 1 %, further readings may be taken at observation
periods up to 10 days. If, after 10 days, the anchorage fails to hold its load, the
anchorage should be deemed to have failed. After an investigation as to the cause of failure and dependent upon the circumstances, the anchorage should be:
(a) abandoned and replaced; or
(b) reduced in capacity; or
(c) subjected to a remedial restressing programme.
Where prestress gains are recorded after 1 day, monitoring should continue to
ensure stabilization of prestress within a load increment of 10%Tw. Should the gain
exceed 10%Tw, a careful diagnosis is required to ascertain the cause and it will be
prudent to monitor the overall structure/ground/anchorage system. If, for
example, overloading progressively increases due to insufficient anchorage capacityin design or failure of a slope, then additional support is required to stabilize the
overall anchorage system. Destressing to working load should be carried out as
prestress values approach proof loads, e.g. 120%Tw and 140%Tw in the case of
temporary and permanent anchorages, respectively, accepting that movement may
continue until additional support is provided.69
Excavation methods and support systems
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Empirical method for approximate location of fixedanchor zone in soils
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Excavation methods and support systems
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Empirical method for approximate location of fixedanchor zone in soils
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Excavation methods and support systems
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Typical anchorage geometry using wedge method ofanalysis (BS8081)
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Excavation methods and support systems
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Stability analysis for determining the free and totalanchorage length.
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Excavation methods and support systems
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Tied-back wall in rocks &
method of failure control
Rock bolting at toe of wall
Pre-boring to excavation base level
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Excavation methods and support systems
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Principal failure modes in
rock cuts and slopes
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Excavation methods and OVERALL STABILITY
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When the difference in ground level is small, it may besufficient to ensure that the top frame is set sufficiently
deep so that the excess active pressure from the high
side can be resisted by developing passive resistancefrom the soil at the same level on the low side.
Overall stability with difference in ground level76
Excavation methods and OVERALL STABILITY
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Excess active pressure may be transferred to a lower level on the opposite side of the cofferdam by means of
raking struts. Alternatively, the unbalanced active
pressure can be resisted by ground anchors installedfrom the top frame level into the soil on the high side.
Overall stability, raking struts or tie rods77
Excavation methods and OVERALL STABILITY
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Possibility of overall circular slip failure must be checked. Thetoes of the sheet piles must intercept the critical slip circle,
which means that the part of the circle in front of the line of the
piles becomes ineffective and the shear strength it would have
contributed must be replaced by passive resistance from the piles. A check on the slip circle passing under the toes of the
piles should be carried out to ensure an adequate factor of
safety.
Overall stability, circular slip instability78
Excavation methods and OVERALL STABILITY
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Overall stability, circular slip instability caused by Surcharge Overloading of Embankment
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Excavation methods and OVERALL STABILITY
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Overall stability, circular slip instability caused by Surcharge Overloading of Embankment
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Effects of wall and prop stiffness
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This figure illustrates the effects of wall stiffness on earth pressure & movements for a singly-propped wall with an
infinitely stiff prop – soil-structure interaction
Top-downconstruction
81
Effects of wall and prop stiffness
Chart for estimating maximum lateral wall movements and ground surface
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settlements for support systems in clays (from Clough and O’Rourke, 1990).
4
Increasing System Stiffnessw avg
EI
h
M a x . w
a l l d
e f l e c t i o n ∆ H / E x c a v a t i o n D
e p t h H
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Effects of wall and prop stiffness
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Stiffness of a support system have a significant effect on themagnitudes of the pressures to be resisted & the groundmovements.
As the wall stiffness reduces, movements increase & earth
pressures redistribute. Redistribution, which reduces earth pressure behind the central portion of the wall & increases it atthe top of the wall behind the prop.
Earth pressure redistribution in turn leads to a substantial
reduction in wall bending moments but with increased wallmovements.
The effects of the wall & prop stiffness on bending moments &movements also depend very much on the propping &
excavation sequences. For a typical multi-propping wall, oncethe wall is stiff enough the soil will tend to move by a similar amount regardless of how stiff the wall itself becomes. Further stiffening the wall will increase the bending moments rather
than reduce movements.83
Wall types for temporary & permanent soil support in
basement construction
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There is a range of wall types to
fulfill either temporary or both
temporary and permanent soil
support. Their availability variesgeographically according to
market demand, predominating
subsoil conditions and specialist
local labor resources.
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Road Settlement as results of Ground Loss
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Types of walls - Sheet piles
Th i h i f h il f b d &
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The economic choice of sheet piles for basement and cut-&-cover construction depends primarily on soil conditions, depth of excavation and any restrictions on noise and vibration. Recentchanges to available sections by steel producers have increasedthe flexural strength of sheet piles, and developments in pileinstallation methods (using hydraulic clamps and ramequipment) have reduced installation noise and vibrationcompared with conventional driven operations. These changes,together with improved methods of sealing pile clutches, have
led to the greater use of sheet piles with high standards of water resistance even in water-bearing ground.
The use of sheet piles coupled with structural steel sections (e.g.H soldier piles) produces walls of considerable flexural strengthand finds particular application in excavation works where high bending capacity is needed.
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Types of walls - Sheet piles - Installation
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Sheet pile presses
Vibratory pile driversRapid blow hammersHydraulic hammersDrop hammers
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Types of walls - Sheet piles type
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http://www.arcelor.com/sheetpiling/index.cfm?fuseaction=Products.U
http://www.skylinesteel.com/products/wall_systems/default.aspx
http://www.hlcorp.com.sg/hlatest/hlanew/operation_steel_upile.htm
U & Z sections Straight Web sections
Combination HZ ...-12/AZ18 & ...-24/AZ18
Combination C1 PAZ sections PAL and PAU sections
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Sheet piles - Nippon Steel
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90
Sheet piles - Nippon SteelComposite & Combined Properties
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Sheet piles - Nippon Steel
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Types of walls – King post or soldier piles
Walls for temporary soil support during construction using soldiers, or king
posts of (H) steel sections with horizontal timbers spanning between them
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posts, of (H) steel sections with horizontal timbers spanning between them(or reinforced concrete skin walls spanning between king posts) are usedextensively in non-water-bearing ground. The soldier piles may cantilever for shallow excavations or may be propped with rakers, bracing or groundanchors for deeper excavations. The wall is often used as a permanent back
shutter to the permanent reinforced concrete basement wall.
Soldier Piles with Horizontal or Vertical Sheeting (lagging)
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Types of walls – King post or soldier piles
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Various methods of locating the sheeting or lagging
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Excavation - Soldier pile with sheetpile lagging
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Soldier Pile with Sheet Pile & Guniting
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Treatment of Diaphragm Wall Joint at Slab Connection
96
Types of walls – Contiguous bored piles
Closely spaced bored in-situ concrete piles, installed by auger or Continuous
flight auger provide an economical wall for excavations of moderate depth in
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flight auger provide an economical wall for excavations of moderate depth insubsoils that are easily drilled and where free groundwater is limited.
Availability of powerful rotary machines has promoted the use of this low-cost system at greater depth, with minimum installation noise and vibration.
Where groundwater is likely to seep into the gaps (100-200mm) between piles, it may be necessary to plug them with in-situ grouting behind and between the piles.
Contiguous bored piling must be lined or faced with a reinforced concretewall if there is risk of water ingress or loss of loose soil through the gaps
between piles. Independent blockwork walls with a drained cavity may also be used.
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Types of walls – Secant pile
Formed by installing bored piles on a hit-and-miss basis at pilecentres slightly less than pile diameter. The initial female piles
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centres slightly less than pile diameter. The initial female pilesmay be concreted with normal mix concrete (hard-hard secantwall) or with a weaker grade concrete allowing the male piles tocut the secant area into the female pile cross-section with less
effort (hard-soft secant wall).
Secant pile walls are preferred in granular water-bearing soils,where contiguous piles are unlikely to be satisfactory.Constructing guide walls for secant pile installation involvesadditional time and expense.
98
Types of walls – Diaphragm walls
The use of slurry-supported trench filled with tremied concreteto provide a wall for both temporary and permanent soil
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to provide a wall for both temporary and permanent soilretention.
important improvements in excavation and slurry cleaning
equipment. In particular, the use of cutter-mill excavationequipment based on the reverse circulation of soil cuttings andslurry has allowed the construction of structural walls more than60m with exacting standards of vertical tolerance (between 1:200and 1:400).
Early developments in diaphragm wall design included the use of precast post-tensioned wall elements and post-tensioned in-situwalls. Neither of these innovations has found favor although the
improved surface finish of precast elements and the reduction of reinforcement quantities in post-tensioned walls may proveadvantageous.
99
INSTALLATION OF DIAPHRAGM WALL
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Roof Slab: Importance of sealing / grouting this particular zone (circled in
green) to block the water path.
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Treatment of Diaphragm Wall Joint at Slab Connection
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Base Slab: Importance of sealing / grouting this particular zone (circled in green) to
block the water path
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Treatment of Diaphragm Wall Joint at Slab Connection
102
Plan view
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Treatment of Diaphragm Wall Joint at Slab Connection
Elevation view
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Treatment of Diaphragm Wall Joint at Slab Connection
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COFFERDAMS
The function of a cofferdam is to exclude soil and water from an excavation
to facilitate construction. Total exclusion of water is rarely necessary, but theff f i h ld b i l d d i h d i l l i Wi h
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effect of water ingress should be included in the design calculations. With
good design and construction, single skin cofferdams can be used in marine
conditions, but for large excavations in marine works, double skin earth filled
cofferdams may be preferable. The following requirements must be fulfilled:
1. must withstand the loads upon it
2. water entering the cofferdam must be controllable with reasonable
pumping3. the formation level must be stable and not subject to excessive heave or
to boiling
4. deflection of the cofferdam walls and any internal framing must not
interfere with construction of the permanent works, and must not bedetrimental to existing adjacent structures or services
5. the cofferdam must have overall stability against unbalanced earth pressure or ground movements such as circular slip
105
COFFERDAMS
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Types of single skin sheet pile cofferdams
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COFFERDAMS
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Double wall earth-filled cofferdams
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Types of walls - Sheet piles - Tie Back System
1 Plain tie-rod
2 Upset end tie rod
3 N t
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3 Nut
4 Turnbuckle
5 Coupling sleeve
6 Bearing plate
7 Bearing plate on concrete
8 Waling
9 Spacer 10 Supporting console
11 Splice
12 Splicing bolt
13 Fixing bolt
14 Fixing plate
15 Fixing plate
Temporary cofferdams generally use walers &struts to cross-brace the inside excavation.
Permanent or large retaining walls are often
tied back to an anchor wall installed at a
certain distance behind the wall.
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COFFERDAMS
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Types of internal support for cofferdams with straight sides
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COFFERDAMS
Types of circular cofferdams
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COFFERDAMS - Circular walings
Circular walings are ring beam. In practice they will probably vary from a
true circle and therefore subject to some eccentric loading. The followingequation is for calculating the size of waling.
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WhereW = Safe radial waling load in kN/m run
E = Young’s Modulus of waling material in N/mm²
I = Moment of inertia about x-x axis in cm4
R = Radius on centre line of cofferdam piles in metres
3 5
1.5/
10
EI W kN m
R
where W u is the ultimate radial waling load and k is a factor, the value of which is
dependent on the stiffness of the retained medium. 3 is the value for water , e.g.marine cofferdam. Progressively higher values are, in theory, applicable for
weak/medium/strong soils. However, it is common practice to use the value of 3,
to which a factor of safety of 2 is applied. Hence the value of 1.5 in the basic
formula .
3 5/
10u
k EI W kN m
R
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COFFERDAMS - Circular walings
The ring beam can tolerate very little distortion from a true circle before the onset
of catastrophic instability. Hence the empirical rule: d D/35where d is the depth of the ring beam, i.e. the difference between the outer and
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inner radii of the beam, and D is the diameter of the cofferdam (i.e. the diameter
of the inner face of the piles).
When the sheet piles or wall deflect to any great extent then the load on the
walings will be concentrated at the top or bottom of the waling and will inducetorsion. This should be checked in the design.
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Reinforced concrete walings for circular cofferdams
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The tabulated safe loads are based on:
1. The pemissible compressive stress in the concrete not exceeding 5.2 N/mm².
2. W = 1.5 EI/105R³ Where W = waling load in kN/m, E = Young’s Modulus
for concrete = 13,800 N/mm², I = Moment of inertia about ‘xx’ axis in cm4
3. R = Radius of cofferdam in metres
4. Depth of beam ‘d’ to be not less than D/35.
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