Deep basements & cut & cover - 2

8/19/2019 Deep basements & cut & cover - 2

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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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Forms of gabion retaining walls

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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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Forms of reinforced concrete cribwork

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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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Green Reinforced Earth Walls13


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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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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


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Forms of mass or

RC concrete wallsMass concrete walls are suitable

for retained heights up to 3 m.

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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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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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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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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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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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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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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


2. Bearing capacity

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Design Criteria

External Stability


2. Bearing capacity

3. Overturning

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Design Criteria

External Stability


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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Typical temporary anchorage in soil during stressing

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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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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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Typical rock bolt fully bonded over free tendon length

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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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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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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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Externally supported Retaining System

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Short-term design stress-strain curve for normal andlow relaxation products – Prestressing strand & bar

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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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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

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Empirical method for approximate location of fixedanchor zone in soils

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Empirical method for approximate location of fixedanchor zone in soils

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Typical anchorage geometry using wedge method ofanalysis (BS8081)

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Stability analysis for determining the free and totalanchorage length.

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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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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



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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



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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



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Overall stability, circular slip instability caused by Surcharge Overloading of Embankment

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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

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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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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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Sheet piles - Nippon SteelComposite & Combined Properties


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Sheet piles - Nippon Steel


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92

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

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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.

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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.

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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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101

Base Slab: Importance of sealing / grouting this particular zone (circled in green) to

block the water path


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102

Plan view


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Elevation view

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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

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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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Deep basements & cut & cover - 2

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