BRACED EXCAVATION

PREPARED BY: SAPTARSHI KUNDU

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CONTENTS Need For Braced Excavation Advantages Of Braced Excavation Types Of Vertical Cuts Depth Of Unsupported Vertical Cut Different Types Of Sheeting And Bracing Systems Modes Of Failure Of Braced Cut Deflection Pattern Of Braced Cuts Apparent Earth Pressure Distribution In Sand Apparent Earth Pressure Distribution In Clay Ground Settlement Due To Sheet Pile Design Of Various Components Of The Braced Cut Design Of Various Components Of Bracing Uses Of Braced Excavation Case Study Reference

INTRODUCTION

BRACED CUTA deep vertical cut

No slope is provided

Useful in such situations where space is limited

Supported by suitable bracing systems including wales, struts, sheet piles and soldier beams to prevent collapse by estimating the lateral earth pressure to which the braced cuts will be subjected to.

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These excavation support systems are used to, Minimize the excavation area

Keep the sides of deep excavations stable

Ensure that movements of soil will not cause damage to neighboring structures or to utilities in the surrounding ground.

ADVANTAGES OF BRACED EXCAVATION

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DEPTH OF EXCAVATIONShallow cuts

Deep cuts

In shallow cuts, the depth of excavation is less than 6 m.

In deep cuts, the depth of excavation is more than 6m and design of suitable bracings (supporting system) is necessary to safe guard the cut or trench against collapse.

TYPES OF VERTICAL CUTS

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CONTINUED….

METHOD OF EXCAVATION

Open cuts

Closed cuts

Temporary cuts

Permanent cuts

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Vertical Timber Sheeting: Steel Sheet Pile: Soldier Beams: Tie Backs:

DIFFERENT TYPES OF SHEETING AND BRACING SYSTEMS :

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Vertical timber sheeting are economical up to a depth of 4 to 6 m.

Planks about 8 to 10 cm thick are driven around the boundary of the proposed excavation to some depth below the base of the excavation.

The soil between the sheeting is then excavated. The sheeting is held in place by a system of wales and struts.

The wales are supported by horizontal struts which extend from side to side of the excavation.

For relatively wide excavations, it becomes economical to support the wales by inclined struts, known as rakers.

If the soil can temporarily support itself in case of an excavation of limited depth without an external support, the timber sheeting can be installed in the open or in a partially completed excavation.

Vertical Timber Sheeting:

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MODES OF FAILURE OF BRACED CUT

• Failure of soil by bottom heave (applicable to clay)

• Failure of soil due to piping (applicable to sand)

• Failure by buckling of struts

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The surcharge pressure tends to heave out the soil at the bottom.

When H / cu is less than 6, movement of bracing system and heave of clay are small. When, H / cu approaches about 8, the movement of a well designed bracing system becomes very large. When, H / cu exceeding 8, the bracing is likely to collapse because of large inward movement of clay outside the embedded portion/s of the sheet piles and excessive upward heave of clay beneath the excavation.

To maintain stability , if a soft soil is encountered below the cut , the sheet pile is extended up to a depth equal to minimum B.

If however , a stiff soil is present at the bottom of the cut , the sheet pile is embedded inside it.

STABILITY AGAINST BOTTOM HEAVE

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F.O.S against heaving =( Bearing capacity of soil at the excavation level)/(Vertical pressure or surcharge at that level).

Vertical forces on bd:-1) Self weight of the wedge bdef = H* γ* B/√22) Load due to skin friction between the wall and soil= c*H*α3) Cohesive force along de= c*(H-2c/γ)

Total vertical stress =

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Bearing capacity of soil at the excavation level = c*Nc GRAPH OF Nc

F.O.S=(c*Nc) / Total vertical stress

Should not be less than 1.5.12

STABILITY AGAINST PIPING FAILURE:

The bottom of a cut in sand is generally stable.

If water table is encountered , the bottom of the cut is stable as long as the water level inside the excavation is higher than the GW level.

If dewatering is needed , F.O.S against piping is to be checked by drawing flow nets and determining the maximum exit gradient at the points A and B.

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After B.M.DAS

PIPING FAILURE CONTINUEDF.O.S against piping may be expressed as=

Imax(exit)=

a=length of the flow element at A or B

h=water head

Nd=number of drops14

After B.M.DAS

DEFLECTION PATTERN OF BRACED CUTS Beyond the critical height Hc , soil gets transformed

from elastic state to plastic state. Hence , soil movement occurs which is restricted by

the struts. The failure surface is like a logarithmic spiral. The deflected shape is shown below:-

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After P.C.Vergheese

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APPARENT EARTH PRESSURE DISTRIBUTION IN SAND

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APPARENT EARTH PRESSURE DISTRIBUTION IN CLAY

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GROUND SETTLEMENT DUE TO SHEET PILE:Extent of settlement is to be found out , so that nearby buildings are not affected.

Determined purely by measurements and subsoil conditions.

Terzaghi and Peck extensively studied this phenomena on numerous deep excavation .

GRAPH

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After B.M.DAS

DESIGN OF VARIOUS COMPONENTS OF THE BRACED CUT

Struts Sheet Pile Wales

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Design of Various components of the braced cut

Beam with load support

After B.M.DAS

Tributary area method

Equivalent beam method

DETERMINATION OF LOADS AND MOMENTS

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DETERMINATION OF LOADS AND MOMENTS

In the tributary method , the load for a particular strut is calculated over an area equal to half of the spacing of the adjacent struts above and below that strut.Using these equivalent areas , the load is determined.

Tributary area method

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After B.M.DAS

In the Equivalent Beam method , the first and last strut positions are considered as simply supported , and the intermediate points as hinged.

DETERMINATION OF LOADS AND MOMENTS

Equivalent beam method

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After B.M.DAS

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Design of Various Components of

Bracing

STRUTS

The first and the last struts are considered as simply supported and rest as hinged.

We find the support loads. We choose struts of the required section

modulus from code.

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SHEET PILE

We find the maximum bending moment of each section and take maximum of them.

We obtain the required section modulus of the sheet piles. (Mmax /σall)

We choose sheet pile of the required section modulus from code.

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WALES

The reactions at the supports are determined and then the moments are calculated by (Ps*s/8)

We obtain the required section modulus of the sheet piles. (Mmax /σall)

We choose wales of the required section modulus from code.

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USES OF BRACED EXCAVATION IN THE FIELD

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Advantages and of the New EarthRetention System with Prestressed Wales

The new earth retention system with prestressed wales allows for wider strut spacing and a larger construction workspace.

Construction costs can be reduced with the new system because a wider spacing of struts reduces the number of struts required to support earth pressure.

If an excessive deformation occurs, hydraulic jacks can be used to reload the prestress on the prestressed wale system.

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Limitations of the New EarthRetention System with Prestressed Wales

The new earth retention system with prestressed wales requires skilled workmanship since it differs from any conventional support system.

The failure of steel wires will cause the serious structural damage of the earth retention system.

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