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Geotechnical Design Project The Medway Tunnel

Christian Smallwood (0345236) Page 1 of 32

Geotechnical Design Project

The Medway Tunnel

Issue Date: 8 th January 2007

Submission Date: 19 th January 2007

Candidate : Christian Smallwood (0345236)


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Table of Contents

1. LETTER TO CLIENT ................................................................................................. 3

2. FOUR POSSIBLE OPTIONS FOR CASTING BASIN............................................ 5

2.1 Option 1 ................................................................................................................. 62.2 Option 2 ................................................................................................................. 72.3 Option 3 ................................................................................................................. 82.4 Option 4 ................................................................................................................. 9

3. OUTLINE FOR SUSTAINABILITY PLAN............................................................ 10

4. TUNNEL LOADING SCENARIOS ......................................................................... 14

4.1 Loading in the Dry .................................................................................................. 144.2 Loading when Immersed......................................................................................... 154.3 Proof of Floatability................................................................................................ 16

5. ALLOWABLE BEARING CAPACITY................................................................... 17

5.1 Bearing Pressure Using Meyerhof Method............................................................. 175.2 Bearing Pressure from Terzaghi & Peck ................................................................ 175.3 Comparison............................................................................................................. 17

6. TOTAL & EFFECTIVE STRESS ANALYSIS ....................................................... 18

6.1 Total Stress: Taylors Method ................................................................................ 186.2 Effective Stress: The Method of Slices................................................................... 206.3 Discussion on Soil Parameters................................................................................ 21

7. COST OF THE SCHEME ......................................................................................... 21

7.1 Excavation Costs..................................................................................................... 217.2 Combiwall Costs..................................................................................................... 227.3 Craneage Costs........................................................................................................ 227.4 Total Cost Summary ............................................................................................... 22

8. SOIL NAILING .......................................................................................................... 22

9. SETTLEMENT FOR STOCKPILED SPOIL ......................................................... 24

10. CRANE PILE DESIGN............................................................................................ 26

10.1 Driven Pile ......................................................................................................... 26

10.2 Under-reamed Pile ............................................................................................. 2710.3 Piling Summary ................................................................................................. 28

11. DEWATERING ........................................................................................................ 28

12. TEMPORARY HAUL ROAD................................................................................. 29

13. STABILITY OF COFFERDAMS........................................................................... 30

14. BIBLIOGRAPHY..................................................................................................... 32


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1. Letter to ClientSuperman S.I. Inc

WonderlandScotlandEH1 2AB

Messrs Q.J. Leiper, M.C. Forde & Y. YangAssessors of 4 th year Design ProjectUniversity of EdinburghEdinburghScotland

8 th January 2007

Dear Messrs Q.J. Leiper, M.C. Forde & Y. Yang

RE: Project Risks & Concerns

Thank you for your letter dated the 8 th of January. I appreciate your concern and haveincluded below a list of possible risks that may arise as a result of the differentconstruction options for the Medway Tunnel.

Firstly I would like to address the possible geotechnical & construction risks associatedwith the construction of the tunnel.

Were a shallow submerged tunnel to be considered, our primary concern would be thedesign. As this is a relatively new technology in Britain, there will be little previousexperience to rely upon. This can be dealt with by bringing in an external expert to

advise, however a language barrier may cause further problems. It is also unlikely that hewill have a local knowledge of the soil conditions. Needless to say consultants will alsoincur high costs.

Other concerns and solutions are listed below, however this is by no means anexhaustive study (some of the risks below also apply to deep tunnels):

The placing of the tunnel segments could cause problems, as an unprecedentedlevel of accuracy is required. Included in this are the ballast calculations. Thesemust obtain an operational range which ensures that the tunnel will not sink or float uncontrollably.

Weather conditions are also to be considered, as these are outside our controland must be prepared for. Adverse wave patterns and increased flow rates mayincrease the difficulty in maintaining a stable position whilst dropping the tunnelsegment.

All seals and end caps are susceptible to thermal expansion in extreme weather conditions, and cracking in extreme cold. This may cause unexpected movementand possible leaks.

An area will have to be excavated and dewatered for the casting basin. Slopestability will have to be sufficient to avoid land slides and consequent erosions tothe bunds. However pump failure is the main concern here, and standby pumpsand generators will have to be made available to allow a constant dewateringthroughout the project construction.


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The retaining wall around the excavated area could be subject to blow-ins. As thisis a brown field site, there is also the possibility that the excavation may unearthsome old piles or slabs remaining from previous structures.

A naval base has been identified upstream and munitions can be expected to haveleaked into the soil. If this is localised then can be treated appropriately, however it is a risk that contamination may enter the river in a larger burst than thesurrounding ecosystem has been accustomed to.

A nearby building is listed. There is a possibility of structural damage due to theground settlement as a result of the dewatering process

If a deep tunnel is to be considered, the following risks should also be taken into account:

Unexpected rock formations or metal ore may be met as a result of drilling under the river bed

Due to the deep nature of the tunnel, it will surface much further from the river side than its shallow counterpart, as a result land may need to be purchased andincreased planning permissions obtained.

The risk of tunnel collapse is very low but present. Depending on the permeability of the rock, pump failure during drilling may be a

reason for concern. As will be the circulation of air inside the tunnel if theelectricity supply fails.

Secondly, regarding the impact on the local community, there has been local support for the project since its inception, given it will serve as a means to regenerate the area,bringing business, whilst freeing space on the already congested bridges upstream.

However an inevitable amount of dust and dirt will be expelled into the air. Localtraffic patterns will not be significantly altered as construction will take place on

previously unused land. I cannot imagine noise being a concern as most constructionoccurs below ground in an excavated pit, which acts as an inherent noise barrier.As the site was formerly an industrial estate, the destruction of local wildlife is

not a main concern.

Finally, we will require the following information to efficiently plan out possible risks toenable the smooth procession of the Medway tunnel:

An extensive boring exercise needs to be undertaken to provide more boreholelogs, thus providing lab test data on soil properties and contamination levels.

Predictions for the weather conditions over the next two years need to be

provided. A study must be done to establish what silts and soils have been deposited on the

river bed A record of average river traffic is needed to avoid obstruction River flow rates through out the year A flow duration curve for the Medway river

I look forward to hearing from you soon,Kind Regards,

Mr J. C. SmallwoodEngineer in Charge


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2. Four possible Options for Casting Basin

In an immersed tube tunnel, the principle construction is the casting basin,

therefore a lot of attention has to be given to its design. The dimensions of the casting

basin are as a direct result of the chosen size for the immersed tunnel length, and the

number of segments the immersed section is to be split into. A total tunnel length of 375

metres was derived from Figure 2: Medway Tunnel Plan and Borehole Locations, as

supplied with the brief. Possible segment lengths were investigated and are summarised

in the table below:

Segments Segment Length (m) Total Tunnel Length (m)1 375 3752 187.5 375 3 125 375 4 93.75 375 5 75 375

Table 1 Tunnel Section Lengths

To establish four initial layouts, tunnel section lengths of 125m and 93.75m have

been chosen and two varieties of each worked on. These were considered optimum

lengths as too long would raise problems with too much resistance against the flow of the

river, and too short would be uneconomical.

In selecting the site location, various criteria had to be taken into consideration.

The sites former use as a dock yard would ensure considerable foundations wherever the

casting basin was placed. To minimise the cost of removing foundation, the basin was

carefully positioned to avoid where possible the site of a demolished crane, or an

identified high pile risk area. With permission to demolish building no. 2, the choice was

taken in two cases to make use of this land, as it was assumed this would have fewer piles

than the no. 8 slipway. A brief overview of the 4 options can be seen in the table below:

Option Area (m 2) Tunnel Sections Key Comments

1 32940 3 x 125mDemolition required

Avoids slipwayOver potentially 2 demolished crane site

2 39777.7 3 x 125mDemolition required

Avoids slipwayOver 1 demolished crane site

3 33176.25 4 x 93.75mNo Demolition

Requires removal of SlipwayOver 2 demolished crane site

4 35656.8 4 x 93.75m

No Demolition

Requires partial removal of SlipwayOver 1 demolished crane site Table 2 Four Possible Basin Layouts


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32940m3 x 125m

2

Cost implications of the total excavated volume have also been taken into

account. Consequently, the smallest area was selected as the final option, taking into

consideration the associated foundation removal. This option can be seen below in

section 2.1.

2.1 Option 1

In terms of cost of excavation this is the most

preferable option. Due to its location it does not

run into problems encountering the numerous piles

reported in the slip way. However the demolition

of building no. 2 will be required, along with the

removal of its foundations, which may cause

problems. Monitoring to the adjacent listed

building will have to be undertaken as a

precautionary measure.


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39777.7m3 x 125m

2

2.2 Option 2

This option offers the same benefits as option

1 however it requires a greater excavation area,

incurring a larger cost, the excavation & muck-away

operation being the biggest price component of the

casting basin.


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33176.25m4 x 93.75m

2

2.3 Option 3

The 2 nd smallest area of excavation,

depending on the flow of the river this is a

close contender for the final choice. If it is

demonstrated that the 125m segments will not

be controllable in the river, then this would be

the alternative. It is located over the slipway,

which will cause problems as far as the

removal of piles is concerned. However it does

not impose on building no. 2, and therefore will not require its demolition, but the two

former crane foundations will have to be removed.


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35656.8m4 x 93.75

2

2.4 Option 4

Due to the slightly larger area, this option comes in

at slightly more expensive than option 3. however that cost

may be covered by the lack of need to remove so many

piles from the slip way, as it does not impose on it as much

as option 3. It also only requires the removal of one dock-

crane foundation. Building no. 2 has not been touched.


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3. Outline for Sustainability Plan

Increasingly, sustainable development is being promoted asstandard engineering practice, with more focus being put on energy

efficient designs, and minimal waste management. This has been broughtabout partially through the construction industrys transition from acorner cutting sector to a client oriented clean & honest industry.Using an environment conscious business plan promotes this clean &honest image.

However, and possibly more importantly, it is becoming standardpractice as contractors & commissioners realise the true financial benefitsof going green. Moving to a more sustainable business plan is still metwith scepticism from much of the engineering workforce. It is however theelite contracting groups who are benefiting from the extra business fromenvironmentally conscious clients, and the financial savings it offers, as

well as a moral and ethical advantage.Consequently, sustainable action plans (SAPs) are now key

components to any major civil or structural engineering project, andshould be incorporated at an early stage to avoid any unnecessary risks.Below, the key issues concerning the Medway Tunnel crossing SAP arediscussed:

Reduce Wastage

Site wastage encompasses a largerange of product wastage, whether it isthe polystyrene protection for a delicatepiece of equipment, polyethylenewrapping, common litter or the excavatedsoil. All of it has to be removed from thesite location to maintain a clean andefficient operation. With increasedlandfill taxes, contractors have had totake a new approach to how to deal withthe problem of waste.

Using earth as an example of wastage, in this case there is significantpotential for on site soil recycling. Part of the excavated material can be usedinitially to make the bunds, whilst the restcan be used as fill material once thetunnel is immersed and in its finalposition.

This is very positive form asustainable point of view in that assuminga rigid HGV carrying volume of 12 m 3 of earth, you have saved roughly 23,000

lorry journeys (see section 12). If thenearest landfill site is 20km away, thisequates to approximately 71,000 kg of

carbon dioxide not emitted throughtransportation. 1

It is also beneficial from a financialpoint of view, the contractor has saved onthe cost of operating rigid HGVs. He hasalso saved on material costs for severalaspects of the project. The sand can beused in an onsite batching plant for theconcrete, the gravel as ballast fill asalready discussed.

The positive publicity resultingfrom the successful management of wastewill incur public appreciation, and givethe contractor a serious competitive edgein future bids, thereby stabilising itsfuture. This is an attractive characteristicfor investors and shareholders alike, andwill only add value to the fund holdersportfolio.

Energy Conscious Design

In any project, there are three keyaspects: Cost , Time and Quality .2 Whenadditional attention is given to one, it is

1 Assuming Urban driving: 0.154kg CO 2/km.2 Simon Smith: Engineering Project Management.


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automatically taken from the other.Energy design falls under Quality, and incountries where energy is cheap, it is veryeasy to sacrifice on it in favour of speedand price of construction.

In this project operational costs willbe fairly high, a large portion of whichcan be blamed on energy consumptiondue to constant ventilation andmonitoring systems. The client will havepaid close attention to an energy efficientdesign, as this will have given rise to longterm savings. Thus, whilst the capital costof the project may be higher due to theinclusion of an efficient design, thesavings over the tunnels lifespan will alsobe increased. It is a win-win situation for both the contractor and the clientsorganisation.

It is also a positive strategy in termsof sustainable development. Having torely less on fossil generated energy, theconsequent emissions are vastly reduced.For instance, the design may incorporatea wind capture channel, to make use of the prevailing winds, and use them toease the strain on the permanent

circulation fans.

Efficiency & Education of Labour

Promoting efficiency among theworkforce is an important route tominimising wastage. Well trained,organised and caring personnel willachieve faster results at a higher quality.But the leadership to do so must comefrom the senior management. It is

therefore essential that everyoneunderstands the need for sustainabledevelopment, from the business orientedCEO to the job oriented brick layer. Asimple method of doing so is to introducesustainability into the site inductionprocess.

An efficient workforce will berecognised by institutions and clientsalike, and will serve positively in terms of sustainable development and financial.

Sourcing local materials

As with the discussion aboveconcerning the removal of earth from thesite, the same can be said for bringingmaterial to the site.

Huge environmental savings can bemade from selecting a local supplier rather than a cheaper internationalsupplier. An example of this: TheBouygues Btiment Internationales hotelconstruction in Trinidad shipped inbamboo flooring from China, even whileit is an indigenous wood to Trinidad, dueto the fact that China could source andprocess the material for less than Trinidadcould 3. The carbon emissions thatresulted from the transport of the bamboohalfway across the world aremonumental.

Whilst it may cost more to sourcelocally, the contractor is investing in thenational economy by doing so. This is apositive tact, in that it promotes a civicduty by the contracting firm, and raisesthe firms image in a local context.

However from a fund raisers point of

view, the cost here of being sustainablemay be too high. This will cause negativepublicity among the investmentcommunity, and raising funds throughprivate equity will cease to be such anavailable opportunity.

Tidal Electricity generation

Venturing into the extremities of asustainable development, the Medway

River is tidal. This raises the possibilityof using tidal generation as a means of energy supply for all the tunnelsservices.

Whilst this is in itself a separateproject, it could also be one of lucrativepotential. Coming up to 2010, Britainsaim to comply with the Kyoto protocolguarantees a booming sector for renewable energy contractors. It wouldalso serve to promote the client/contractor as proactive in the fight against global

3 Data from my work placement in Trinidad.


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warming, something which could beconsidered a competitive asset.

Any excess energy could be sold back to the grid, generating additional revenuefor the operating company, and itsstakeholders.

Minimising risk

Risk management is an importantcomponent of any project, and should bebrought in at an early stage.

From a sustainable point of view, itdecreases the wastage of materials thatotherwise shouldnt have been used if therisk had initially been averted.

From a financial point of view, theinclusion of risk aversion is a logicalbusiness step, minimising unnecessarycosts through averting mistakes whichcould have serious financial or health &safety consequences. Good practiceincites confidence in shareholders,especially when money has been saved asa result.

Lead, dont follow

Whilst sustainable development isstill met with a degree of scepticism, it isthe bigger entitys responsibility to leadthe way. As industrialised nations set atrend for developing countries, so toomust the larger contracting firms set adefault model for smaller organisations tofollow.

Enterprises like Carillion plc aredoing just this. By incorporating their

sun diagram into their business model,they are putting sustainability at theforefront of every project. By assumingthis pro-active role, they have gained anedge against their reaction-basedcompetition.

A market leader is a favourableposition to hold, and a very attractiveprospect in any tender bid. A clientorganisation seen to be working with sucha contractor will gain sympathy for acknowledging the need for sustainability, and this will reflectpositively on its fund holders portfolio.

Carillion: New Sun Diagram

Automated systems

Basic systems such as motion sensorsor timers can be used to good effect,dramatically decreasing the period of time an item is left switched on for. Thiswill save on energy costs, and give animpression of a technologically advancedsite, hence improving the corporateimage.

As far as permanent works areconcerned, automatic stations linked intoa central monitoring network will removethe need for a permanent on sitetechnician. This can serve to not onlyreduce carbon emissions throughreducing travel to work, but also byalternating ventilation to meet minimumcarbon monoxide levels, instead of having a constant speed which covers alltraffic conditions.

Whilst again increasing the capital

cost of the project, the long term benefitis clearly identifiable.

Visual & Environmental Impact of Temporary Works

Whilst the human culture may protestlarge scale temporary works as disruptiveand unpleasant, the human is adept tochange. In the case where the flora andfaunas local habitat has been disrupted,

the respective species is not as adaptableas humans are.


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It is to this end that carefulconsideration must be paid to minimisingthe impact to local wildlife.

Negative attention from animalwelfare groups can minimise the chanceof winning a contract, thereby damagingthe contractors future financial potential.If the contractor is still selected, thenegative attention may be shifted to theclient, damaging in turn the clientspublic image and investment capacity.

Community consultation

A positive relationship with the localcommunity is a valuable edge to have infuture competition for any other councilfunded projects.

If a concern is raised by thecommunity, it should be heeded, andevery effort made to solve the problem.

This will remove possible complexitiesfrom future works, as well as give a cleancorporate record for local affairs.

ConclusionThus as can be seen through a

variety of means, sustainability can easilybe incorporated into the business modelof any enterprise, adding to the financialgrowth of the business. However moreimportantly, it acts to secure the future of our increasingly limited resources.

However it is not only large stepsthat can be taken to achieving asustainable goal, day to day activities canvastly reduce on wastage, be it visible or invisible. As in cricket, it is not the 4sand 6s that win the game, but the singleruns.


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4. Tunnel Loading Scenarios

4.1 Loading in the Dry

The dry load of the tunnel is found from calculating the area of the cross

section, and multiplying it by the materials density. In this case the density of the

concrete is 24kN/m 3, for the purpose of this calculation it is assumed that this density

incorporates the steel reinforcement, cooling ducts and sandflow pipes within the

concrete. The calculation also assumes a standard cross section throughout the whole

length of the immersed tunnel, as displayed below:

1200

1500

1200

25900

10450 10450

9500 6800

600

1000

Tunnel Cross-Section, to scale

Area Dimensions (mm) Output (m 2)Base Slab (1500 x 25900) 38.85Outside Walls (6800 x 1200) x 2 16.32Centre Wall (6800 x 600) 4.08Roof Slab (1200 x 23900) 28.68Total 87.93

Table 3 Dimensions of Tunnel Cross Section

Density of Concrete: 24kN/m 3 Area of Section: 87.93 m 2

Loading: 24 x 87.932110.32kN per metre run of tunnel

Pressure: 81.48 kN/m 2


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4.2 Loading when Immersed

The loading of the tunnel segments in their final immersed position is similar

to the dry load, with the addition of the ballast concrete within the tunnel and ballastgravel surrounding the tunnel. As can be seen from the diagram, toes have been used

to make use of the gravel fill on the sides. For the purpose of this calculation, 2 metres

of gravel fill and 1 metre of concrete fill have been assumed. Previous assumptions

still apply.

1200

1500

1200

25900

10450 10450

9500 6800

600

1000

2000

1000

Tunnel Cross-Section with Ballast Material, to scale

Area Dimensions (mm) Output (m 2)Base Slab (1500 x 25900) 38.85Outside Walls (6800 x 1200) x 2 16.32Centre Wall (6800 x 600) 4.08Roof Slab (1200 x 23900) 28.68Concrete Ballast (1000 x 10450) x 2 20.9Total 108.83Soil on Top (2000 x 25900) 51.8

Soil on Sides (8000 x 1000) 8Total 59.8

Table 4 Dimensions of Tunnel & Earth Cross Sections

Density of Concrete: 24kN/m 3 Area of Section: 108.83 m 2

Density of Fill: 16kN/m 3 Area of Fill: 59.8 m 2

Loading: (24 x 87.93) + (16 x 59.8)3568.72 kN per metre run of tunnel

Pressure: 137.79 kN/m 2


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4.3 Proof of Floatability

To ensure the tunnel sections can be sunk into their final positions, they must

first be able to float. This is a simple check of making sure the volume of water displaced by the sealed tunnel section is heavier than the sealed section itself.

1500

25900

95008000

Area of displaced water, to scale

Area Dimensions (mm) Output (m 2)

Base Slab (1500 x 25900) 38.85Tunnel Segment (8000 x 23900) 191.2Total 230.05

Table 5 Area of Displaced Water

Density of Water: 9.81kN/m 3 Area of Section: 230.05 m 2

Loading of water: 9.81 x 230.052256.8kN per metre run of tunnel

If Dry Load 1Wet Load

< , then the tunnel section will float when submerged.

2110.320.935

2256.8=

When it is sunk into place, enough ballast has to be supplied to ensure it stays

immersed. The factor of safety against it floating can be investigated through a similar

calculation:

Immersed Load 3568.72Factor of Safety 1.58Wet Load 2256.8= = =


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5. Allowable Bearing Capacity

5.1 Bearing Pressure Using Meyerhof Method

( )

* * *

*

*

* *

Bearing pressure, ' 0.536.7 38

36.7 5

Bearing pressure,

Bearing pressure,

Bearing p

0

36.7 45 (assuming Hansen's )

0 0 0.5 15.8 25

ressure,

Bearing pressure, .9

Bea

45

p c q

q

c

p

q cN q N BN N

N

N N

q

= + += =

= =

= =

= + +

2ring pressure 920 /, 7.45pq kN m=

However this value of the bearing pressure incorporates a factor of safety of between

2.5 to 4 4:

2

2

9207.452301.9 /

49207.45

3683.0 /2.5

p

p

q kN m

q kN m

= =

= =

5.2 Bearing Pressure from Terzaghi & Peck

Using K. Terzaghi and R. Pecks curve5, a value of the allowable bearing

capacity can be established using just the N-Value from the SPT test.

Bearing Pressure Values & ResultS.P.T. at 10.35 OD 4, 8, 6, 9, 9, 10Width of footing >>6mN value 34Allowable Bearing Capacity 340 kN/m 2

Table 6 Values for Bearing Capacity

5.3 Comparison

Terzhaghi & Becks method is generally considered very conservative, and

with Meyerhofs method producing significantly greater results, it is understandable

why. However even the conservative result of 340 kN/m 2 is far greater than the 82

kN/m 2 required by the tunnel section, thereby ensuring the soil is safe to work on.

4 Dr Yummin Yang: Foundation engineering pile design5 Copy can be found in attached spreasheet


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6. Total & Effective Stress Analysis

To ensure the slopes in the excavation of the casting basin will not fail, a stressanalysis must be performed. The method of Slices, Taylors method and the u=0

method have been employed to determine the associated factors of safety relevant to

each slope 6, as per the diagram below:

Taylor's MethodTotal Stress

Method of SlicesEffective Stress

Fill

Sandy Silt

Silty Clay

Gravel

Chalk

0m OD

Excavation Level

Cross-Section of slope down to excavation, to scale.

6.1 Total Stress: Taylors Method

10672,85

354200

8171,02

3131,38

Section to be analysed by Taylors Method

6 Craigs R.F. Soil Mechanics , 6 th Edition, pp 377 - 381


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Using the u=0 Method

Values as taken from above Diagramcu 21 kN/m 3*L

a

10.67285 mr 8.17102 m 20 kN/m 3 W 352.65 kN/md 3.13138 mArea 17.6325 m 2 * c u from scatter diagram in brief

Table 7 Values for Factor of Safety Calculation

where

21 10.67 8.171.66

352.65 3.13

u ac L r F W areaWd

= =

= =

Estimated Minimum (Taylor Method):

Values as Derived from Graphs 35Ns 0.14cu 21

20H 4.2Table 8 Values for Stress Analysis Calculation

211.79

0.14 20 4.2

u

s

cF

N H =

= =

It is at this stage that an anomaly occurs. Taylors method should return a

value smaller than the u=0 Method, however this is not the case, this points to two

possibilities. There is an irregularity in the above calculations, this is quite possible

due to the inaccuracy of the scatter diagram in supplying a c u value; or it simply

demonstrates how outdated and inaccurate the Taylor method is in acquiring a

suitable factor of safety.


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6.2 Effective Stress: The Method of Slices

The Method of slices analyses the slope in two dimensions. An assumed

failure surface is drawn, and divided into an arbitrary number of slices. The forcesacting on each slice are then individually analysed. Here the slope has been divided

into ten slices, each of 1.2m width. A much more in depth table of the results can be

found in the Slope Stability spreadsheet on the cd-rom.

Individual ArcLengths Individual ArcAngles

Slope to be analysed by Method of Slices

Slice h cos h sin u l ulNo. (m) (m) (kN/m 2) (m) (kN/m)

W W cos W sin W cos - ul

1 0.409 0.331 0.000 1.557 0.000 9.967 7.746 6.273 7.7462 1.175 0.763 1.970 1.429 2.814 26.559 22.274 14.465 19.4603 1.854 0.944 8.365 1.343 11.232 39.444 35.145 17.907 23.9134 2.062 0.792 12.507 1.283 16.052 41.885 39.103 15.010 23.0515 2.121 0.568 14.230 1.243 17.689 41.629 40.210 10.774 22.5216 2.025 0.357 13.772 1.217 16.764 38.989 38.397 6.770 21.6337 1.795 0.126 11.337 1.204 13.645 34.117 34.034 2.380 20.3898 1.428 0.025 7.030 1.201 8.441 27.082 27.078 0.473 18.6379 0.937 -0.115 0.768 1.218 0.936 17.904 17.770 -2.182 16.83510 0.337 -0.072 0.000 1.218 0.000 6.536 6.393 -1.359 6.393Sum 14.14 3.72 12.91 87.57 339.43 70.51 251.86

Table 9 Table of Values for Stress Analysis Calculation

Using the values from the table above, the following equation can be used to

establish a factor of safety as above:


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( )

( )

' tan ' cos

sin

' 0

' 37

0 0.7536 251.86

70.51189.790

2.6970.51

ac L W ul F W

c

+ =

=

=

+ =

= =

6.3 Discussion on Soil Parameters

Total Effective

cu (kN/m2

) u () c (kN/m2

) ' ()21 0 0 37

Table 10 Effective & Total Stress Parameters

These values are much derived from engineering judgement. The value of c u is

taken from the scatter diagram executed in alluvial clay. The value of 21 can be

considered a conservative estimate. As it is an alluvial clay, the undrained angle of

friction is taken as 0. The effective cohesion of clay is also taken as 0, however the

effective angle of friction can be estimated using the N-value from the standard

penetration test. In this case, an N value of 34 returned a ' value of 37 .

7. Cost of the Scheme

7.1 Excavation Costs

Volume of Whole Cuboid:164.7m x 200m x 14.95m492,453 m 3

Volume under Slope:(376.1749 x 2(164.7)) + (376.1749 x 2(128))220,213 m 3

Total Excavated Material:492,453 220,213272,240 m 3

Rate: 19.50/ m 3

Total Cost of Excavation: 5,308,684.16


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7.2 Combiwall Costs

Length of combiwall: 441mHeight of combiwall: 21m

Rate: 275/m 2

Total Cost of combiwall: 2,646,775.00

7.3 Craneage Costs

Quantity: 1 Crane

Rate: 175,000/year

Total Craneage Costs: 402,500.00

7.4 Total Cost Summary

Item Rate Unit Quantity TotalExcavation 19.50 m 3 272,240 5,408,684.16Combiwall 275.00 m 2 9261 2,646,775.00Craneage 175,000.00 Year 2.3 402,500.00

Total 8,457,969.16Table 11 Associated Costs of Casting Basin

This cost however is purely indicative, and does not include for a huge variety

of necessary operations, for example the dewatering pumps. An extensive study into

the cost of the casting basin would return a larger value

8. Soil Nailing

The process of soil nailing, originally a French technique, is extremely simple

in concept. By driving metal rods into a slope, the apparent cohesion will increase due

to their ability to handle tensile loads, hence increasing the stability of the slope. It is

in essence an in-situ soil reinforcement.

The drive behind soil nailing is the same drive behind the development of

multi story buildings, the constantly increasing sparsity of useable land. It is highly

popular on road and highway lane expansions, where instead of purchasing new land

upon which to construct, they simply increase the incline of the cut, as below:


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Land Reclaimed through soil nailing

Potential reclamation of land through slope increase

However there are limitations to the application of soil nails, for instance the slopeconcerned must be able to stand along before the nails are applied. The type of soil isalso a factor in its viability, as summarised in the table 12:

Practical ImpracticalClays Soft, plastic claysSandy Soils Organics/PeatWeathered rock, Tallus slope deposits Loose, low density and/or saturated soilsHeterogeneous and stratified soils Fills (rubble, cinder, ash, etc.) 7

Table 12 Suitable & Unsuitable conditions for soil nailing

Thus whilst soil nailing would be an obvious solution in most scenarios, from

the bore hole log the soil is defined as Very soft becoming soft [] silty clay,

making it impractical for the casting basin excavation. Were it to be an option, it

would potentially enable the placing of the casting basin to avoid the extensive pile

foundation in the slipway and the demolition of building no. 2.

Estimated costs of soil-nailing are about 100/m 2, with approximately 3460 m 2

of soil to nail, the total cost of soil nailing would be 346,000.00 8. This is a meagre

cost in the total casting basin tally, and would probably be offset by the cost of the

removal of the piles in the slipway.

7 Table information sourced from Hayward Baker Services.8 Assumes 3 nailed slopes of 3:1 and 364.7m of longitudinal slope


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9. Settlement for Stockpiled Spoil

As appropriate data is not supplied for the entire clay stratum, assumptions

have been made concerning the coefficient of volume compressibility, asdemonstrated below:

Mv=1.2

Mv=0.69Mv=0.6

Assumed Mv=0.69

Assumed Mv=0.6

Assumed Mv=1.2

0m OD

The total settlement, if left for an infinite amount of time, is defined by the equation:

vdh h m d = , where ( )6 9.81 1.77d =

h m v d dh6.7 1.2 104.1822 837.62491.8 0.69 104.1822 129.39432.3 0.6 104.1822 143.7714

Total dh 1110.791Table 13 Settlement Values

With a total settlement of 1.11m, the coefficient of consolidation can be used

to determine a settlement after a set amount: Using the equations below, table 14 was

formed to give an idea of the rate of settlement :

2v

v

c t T

d = &

4 vt

T U

=


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T(years) Tv U t

Settlement(mm)

0.2 0.00 0.06 65.650.4 0.01 0.08 92.84

0.6 0.01 0.10 113.710.8 0.01 0.12 131.30

1 0.01 0.13 146.801.2 0.02 0.14 160.811.4 0.02 0.16 173.701.6 0.02 0.17 185.691.8 0.02 0.18 196.95

2 0.03 0.19 207.612.2 0.03 0.20 217.742.4 0.03 0.20 227.422.6 0.04 0.21 236.712.8 0.04 0.22 245.64

3 0.04 0.23 254.262.3 0.03 0.20 222.63

Table 14 Settlement over a period of time

Using this data, the following graph was produced to impose a visual

impression of the settlement over 3 years. Assuming the spoil will be stored on site

for 2.3 years, a maximum settlement of 223 mm is expected.

Settlement vs Time

-300.00

-250.00

-200.00

-150.00

-100.00

-50.00

0.00

0 0.5 1 1.5 2 2.5 3 3.5

Time (yrs)

Settlement (mm)


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10. Crane Pile Design

For the pile design, the following soil properties have been used:

Soil Characteristics1 1.62 varies1 0.45*2 0.9cu1 30cu2 80

*Nominal value for London clayTable 15 Soil Characteristics at -10.5m OD and below

For the following Driven and Under-reamed Calculations,

sQ was derived using the method:

s

u

Q f p L

f c

=

=

pQ was derived using Meyerhofs method

( ) ( )( )

* * *

*

*

'

0

0 1

0 9

80 9 0 1 720

p c q

q

c

p p

q cN q N BN

B

N

N

Q Aq A A

= + +

= =

= =

= = + =

10.1 Driven Pile

To calculate the basic properties of the pile, the skin friction and point bearingpressure of the pile must first be established. Once they have been established, a pile

length that delivers a loading capacity of 1012kN, which incorporates a factor of

safety of 1.5 for skin friction and 3 for point bearing capacity, can be established. An

assumption has been made that the price per metre of the pile is constant to an infinite

depth, and is proportional to the cross-sectional area of the pile. The assumption has

also been made that there is no critical depth for skin friction. Below is a summarised

table of the results. For the extensive calculation, please consult the spreadsheet on the

attached cd-rom.


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Diameter Length L/D Q s Q s Mod Qp Qp Mod Q Tot /m 0.40 17.58 43.94 1472.76 981.84 90.48 30.16 1012.00 80.00 1408.000.45 15.65 34.78 1460.74 973.83 114.51 38.17 1012.00 101.25 1589.630.50 14.10 28.19 1447.31 964.88 141.37 47.12 1012.00 125.00 1762.50

0.55 12.81 23.30 1432.47 954.98 171.06 57.02 1012.00 151.25 1936.000.60 11.74 19.56 1416.21 944.14 203.58 67.86 1012.00 180.00 2106.000.65 10.81 16.63 1398.54 932.36 238.92 79.64 1012.00 211.25 2281.500.70 10.01 14.30 1379.46 919.64 277.09 92.36 1012.00 245.00 2450.000.75 9.31 12.41 1358.96 905.97 318.09 106.03 1012.00 281.25 2615.630.80 8.69 10.86 1337.04 891.36 361.91 120.64 1012.00 320.00 2784.000.85 8.13 9.57 1313.72 875.81 408.56 136.19 1012.00 361.25 2926.130.90 7.63 8.48 1288.98 859.32 458.04 152.68 1012.00 405.00 3078.00

Table 16 Extract from spreadsheet for Driven Piles

As can be seen, the more slender the pile, the cheaper it is. However a

maximum length to depth ratio of 20 has been assumed. With this in mind, the

cheapest pile design is a 0.6 m diameter pile, of 11.74m depth. The total piling for this

crane foundation will cost 2106/pile, or 8424 in total.

10.2 Under-reamed Pile

A similar method is used to find a suitable pile with an under-reamed base.

The method used here assumes that the base diameter should be twice the pile

diameter, and that it projects at 45 from the vertical. The skin friction takes into

account the under-reamed base. Below is an abridged table of the results of the

calculations using the equations and values above. A complete table can be seen on

the attached cd-rom.

Diameter* Length L/D Q s Q s Mod Qp Qp Mod Q Tot /m **0.40 9.09 22.72 1337.04 891.36 361.91 120.64 1012.00 80.00 1228.000.45 8.08 17.96 1288.98 859.32 458.04 152.68 1012.00 101.25 1320.130.50 7.76 15.52 1235.26 823.50 565.49 188.50 1012.00 125.00 1475.000.55 7.03 12.77 1175.88 783.92 684.24 228.08 1012.00 151.25 1558.750.60 6.39 10.65 1110.85 740.57 814.30 271.43 1012.00 180.00 1652.000.65 5.84 8.98 1040.16 693.44 955.67 318.56 1012.00 211.25 1725.250.70 5.34 7.63 963.82 642.55 1108.35 369.45 1012.00 245.00 1798.500.75 4.90 6.53 881.83 587.88 1272.35 424.12 1012.00 281.25 1878.130.80 4.49 5.62 794.18 529.45 1447.65 482.55 1012.00 320.00 1940.000.85 4.12 4.85 700.87 467.25 1634.26 544.75 1012.00 361.25 1981.130.90 3.78 4.20 601.91 401.27 1832.18 610.73 1012.00 405.00 2039.00

*Base Diameter = 2 x Diameter **Cost is inclusive of generic 500 cost for under-reaming.

Table 17 Extract from spreadsheet for Under-Reamed Piles


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Again assuming a maximum value of L/D of 20, here the appropriate choice

would be a pile between 0.4 0.45m in diameter, and 8 9m in length.

10.3 Piling Summary

Pile Type Diameter Length L/D CostDriven 0.6 11.74 20 2106Under-Reamed 0.43* 8.5* 20 1274**Values interpolated from Table 17

Table 18 Summary of Selected Pile Properties

From this table, it can be seen that the cost of a driven pile is considerably

more than an under-reamed pile. However the installation of a driven pile is much

faster, and less prone to accidents.

11. Dewatering

Dewatering is often needed in any large excavation, the reason is obvious: to

prevent the construction area from flooding. A variety of methods exist to free the

site from water, and they are all specific to the nature of the geology of the soil. For

example, sheet piling physically blocks the flow of water, but may be impeded by the

presence of boulders, or whilst a pump drains the water, the pressure may be

insufficient in a granular soil of high permeability. A presence of both may also be

used. Essentially, they work by artificially lowering the level of the phreatic surface

and disturbing the normal flow into the area concerned.

A physical cut off, such as the combi wall used in the Medway tunnel, can be

used in soils of any permeability as an effective long term flooding prevention system.

If the toe penetrates an impermeable layer, then a pump will only be needed to

initially drain the excavated area, and can then be left. However if the strata at the

base of the pile is permeable, then pumps will still be needed during the operation

period. However once the water has initially been cleared, fewer pumps will be

needed to maintain the drawdown.

From the borehole log of the casting basin site, it can be seen that the base of

the combi wall penetrates a chalk layer. This shallow chalk will have a high porosity,

so pumping will be required in addition to the physical cut-off. Due to the significant

depth of the excavation base, at -10.5m OD, wellpoints are not a suitable choice. As


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an alternative, deepwells or ejector systems, which have a sufficient depth range, are

the best option 9 for the Medway borehole log.

Installation cost () Running Cost (/week) Cost over 2.3 years.b Pump Feasible?

Min Max Min Max Min MaxCombi-wall Sump No -- -- 120 240 14352 28704Combi-wall Wellpoint No 2000 5000 250 400 31900 52840Combi-wall Deep Well Yes 1500 2000 60 105 8676 14558Combi-wall Ejector System Yes 250 850 500 750 60050 90550

Table 19 Possible Pumping Systems & costs per pump

The above table gives a summary of the key pumping systems, as well as their

approximate costs per pump. The deep well is by far the cheapest for the period of

time its being employed. However contingency systems have to be put in place for

both the deep well and ejector system, as both require electricity. Contingency pumps

will also be required for maintenance on other pumps, which can quickly become

expensive.

12. Temporary Haul Road

uc 21CBR = = = 0.91323% 23%

9 Table 1.3 summary of principal pumped well groundwater control methods, Ciria report C515, p34.


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Assuming that a standard HGV has a carrying capacity of 12m 3, the number of truck

journeys is:3

3

Excavation 272, 42022,686

Truck Capacity 12m

m= =

Assuming that a standard HGV has 4 axles, the total number of axles to pass over the

road is:

22,686 x 4= 90,746.04= 0.091x10 6

Therefore, from the above graph, sub-base thickness:375mm @ 2% CBR

525mm @ 0.913% CBR

13. Stability of Cofferdams

Cofferdams are very high risk structures, and when theyre built into a river, as

is the case with the Medway tunnel, a failure can have catastrophic consequences

leading to potential loss of life. A failure in the cofferdam in certain cases may also

make the land unrecoverable. Therefore, a lot of research has to be done into thegeology of the ground it is being installed in. Some of data collected might be:

Tidal data & rate of flow Scour behaviour River bed profiles Soil parameters Water quality Previous site use Collision risk from river traffic

River regulations.

The most likely threat to materialise is the risk of piping, whereby the water

passes beneath the base of the sheet pile. This can occur when the cofferdam is

founded on relatively permeable material, and flood the area of excavation.

Depending on the circumstances of the soil, there are two ways to avert this. If

the toe of the sheet piles penetrate a stratum of low permeability, such as clay, then no

additional major works need to be done. Once the excavation site has been dewatered

to 1m below the excavation level, then the lowered phreatic surface need only bemonitored. However significant investigation needs to be done to establish whether


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there are any sand pockets in the clay stratum, as these will allow the clay pore

pressures to return to hydrostatic pressures more rapidly.

If however the sheets are being installed into a permeable stratum, such as

sand or gravel, then dewatering will not be enough to keep the water table below

excavation level. Additional measures such as anchoring a concrete slab to the

excavation base can be used to seal off the excavation area. Alternatively a water tight

geotextile can be installed below the phreatic surface, and enough ballast sand fill put

on top to counteract the pore water pressure uplift.

In larger sites, such as the one needed for the east and west portal of the

Medway tunnel, more significant measures need to be used to oppose the overturning

force from the hydrostatic pressure of the adjacent river water. The two principle

options are to construct a cellular cofferdam, or to use a double wall cofferdam. Both

act in a similar way: they are effectively a retaining wall against the water.

The base stability is also affected by the lateral hydrostatic pressure. Therefore

the profile of the sheet piling can also play a significant part. For instance, a straight

sheet will be prone to significant bending stresses, where as a corrugated sheet will be

largely more effective. The layout of the excavated area can also have a significant

impact on the effectiveness of countering overturning pressure. For example, an arc

sheet wall will be less likely to collapse in the same way that a cathode ray tube

avoids implosion through having a curved screen.

The cofferdam structure used in the portals of the east and west end of the

tunnel will have had extensive investigation and analysis done prior to the

construction. It will have been designed to a high factor of safety, as the excavation

area is considerably large, and would have been in operation for a significant period

of time. Failure in this case would have been catastrophic!


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14. Bibliography

Craig, R.F. (2004), Soil Mechanics , 6 th Edition, E & FN Spon

Craig, R.F. (1997), Soil Mechanics , 7 th Edition, E & FN Spon

Engineering Standards, Designs, Practices & Procedures, 2nd Edition , McGraw-HillProfessional

Forde, M (2006), Foundation Engineering: Cofferdams , Unpublished: CourseMaterial

Hayward Baker (2003), Soil Nailing , [Online], Available:http://www.haywardbaker.com/services/soil_nail.htm (Last accessed: 18/01/2007)

Phear, A., Dew, C., Ozsoy, B., Wharmby, N.J., Judge, J. and Barley, A.D. (2005) Soil nailing - best practice guidance (C637) , CIRIA

Preene, M., Roberts, T.O.L., Powrie, W. and Dyer, M.R. (2000) Groundwater control - design and practice (C515) , CIRIA

Puller, Malcolm (1996) Deep excavations: a practical manual , Thomas Telford

The Phi Group (2006) Soil Nailing and Facing Systems , [Online], Available:http://www.phigroup.co.uk/downloads/soilnailing.pdf (Last accessed: 18/01/2007)

Tomlinson, M.J. (2001) Foundation Design & Construction , 7 th Edition, Prentice Hall

Yang, Y (2006), Foundation Engineering: Pile Design, Unpublished: Course Material

geotechnical design project

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