retaining wall(الجدران الاستنادية).- scant piles

1

Dr Youssef Hammida

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Pilaster masonry wall

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Steel sheeting provides resistance during installation stresses. The sheets must be driven into the ground and they have high resistance to the force of being driven down.

It is extremely light weight and makes it easier to lift and handle. Steel sheeting is reusable and recyclable. There is a long life for it both above and under water. It only requires light protection

to keep it maintained. The pile length is easily adaptable and can be welded or bolted to make it work. They have stronger joints that can withstand the force of being driven into place.

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Steel Sheet Piling Construction Steps

First, lay out the sheets in sections to make sure that the piles will interlock

correctly.

Drive each sheet to the depth that has been mapped out.

Then drive the second sheet that has the interlocks between the first sheet and

the second locked sheet.

Repeat until the wall is completed.

If the wall requires complex shapes use connector elements to ensure that the

integrity of the wall is maintained.

Vibratory hammers are used for the installation of steel sheet piles. An impact

hammer is used if the soil is too dense for the vibratory hammer.

At sites where vibrations are not recommended the sheets are pushed into place

using hydraulics.

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Sheet Pile Walls...

Sheet pile walls are another method for construction of basements and temporary

excavations, however they are increasingly being used as permanent structures with

the correctly specified surface coating.

بيتم تنفيذها قبل حفر الموقع ألن وظيفتها سند جوانب الحفر الساندة واألوتاد, لخوازيقا يوم على تنفيذ آخر خازوق ساند 82واليتم الحفر قبل مرور

والخازوق الساند طوله مرتبط بقيمة الحفر واليصل لمنسوب تربة التأسيس

.

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For free earth support method

, the soils at the lower part of piling is incapable of inducing effective restraint so that it

would not result in negative bending moments. In essence, the passive pressures in front

of the sheet piles are insufficient to prevent lateral deflection and rotations at the lower

end of piling. No passive resistance is developed on the backside of the piling below the

line of excavation.

For fixed earth support method

, the piling is driven deep enough so that the soil under the line of excavation provides the

required restraint against deformations and rotations. In short, the lower end of piling is

essentially fixed.

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Anchored sheet pile wall

Anchored sheet pile wall in cohesionless soil Anchored sheet pile wall in cohesive soil

Design using free earth support method

1. Sheet pile is rigid, and lateral deflection is small. 2. The lateral pressure distributes according to Rankine’s or Coulomb’s theories

3. The tie back is strong, and sheet pile rotate about the tie rod anchor point at failure 4. Bottom of sheet pile is free to move.

http://www.ce-ref.com/Foundation/Sheet_pile/Anchor-Sheet_pile/Anchor_sheet_pile.html#Anchored%20sheet%20pile%20wall%20in%20cohesionless%20soil

http://www.ce-ref.com/Foundation/Sheet_pile/Anchor-Sheet_pile/Anchor_sheet_pile.html#Anchored%20sheet%20pile%20wall%20in%20cohesive%20soil.

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The embedded depth can be determined by summarizing horizontal earth pressures and moments about the anchor.

Fx = 0 [1]

Mo = 0 [2]

the lateral earth pressure is a function of embedded depth. Both equations are highly nonlinear. A trial and error method has to be used to determine the root.

For structural design, the sheet pile needs to be able to withstand maximum moment and shear from lateral pressure. A structural analysis needs to be done to determine maximum moment and

shear.

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Anchored sheet pile wall in cohesionless soil

Design length of sheet pile

Calculating active earth pressure

The method for calculating active earth pressure is the same as that in cantilever sheet pile wall. The lateral forces Ha1 is calculated as

Ha1= Ka h2/2+q Ka h

The depth a can be calculated as

a = pa / (Kp-Ka)

The lateral forces Ha2 can be calculated as

Ha2=pa*a/2

Calculating passive earth pressure

The slope from point C to E in the figure above is (Kp-Ka). The passive earth pressure at a depth Y below a is calculated as

Pp = (Kp-Ka) Y

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The passive lateral force

HCEF = (Kp-Ka) Y2/2

Derive equation for Y from Mo = 0

Mo = Ha1*y1 + Ha2* y2 – HCEF* y3 = 0

Where

y1 = (2h/3-b)

y2 = (h+a/3-b)

y3 = (h+a+2Y/3)

The equation needs to be determined by a trial and error process.

Determine anchor force T from Fx = 0

Fx = Ha1+ Ha2– HCEF-T = 0

Then,

T = Ha1+ Ha2– HCEF

Design size of sheet pile

The structural is the same as cantilever sheet piles in cohesionless soil.

Maximum moment locates at a distance y below T where shear stress equals to zero.

T- Ka (y+b)2/2=0

Solve for y, we have, y = -b+2*T/( Ka)

The maximum moment is

Mmax = T y - Ka (y+b)3/6

The required section modulus is S = Mmax / Fb

The sheet pile section is selected based on section modulus

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Design of tie rod and soldier beam

The sheet pile design above is based on a unit width, foot or meter. The tie back force T calculated from sheet pile design is force per linearly width of sheet pile. The top of sheet pile often supported with

soldier beams and tie rods at certain spacing.

Assume the spacing of tie rod is s, the tension in the rod is T times s. The required area of tie rod is

A = T s / Ft

Where Ft is allowable tensile stress of steel and is equal to 0.6Fy in AISC ASD design.

The soil beam is designed as a continuous beam that subjected to tie back force T. The maximum moment in the soldier beam is calculated from structural analysis. The required section modulus is equal to S

= Mmax / Fb.

Design procedure

1.Calculate lateral earth pressure at bottom of excavation, pa and Ha1.

pa = Ka H, Ha1=pa*h/2

2.Calculate the length a, and Ha2.

a = pa / (Kp-Ka), Ha2=pa*a/2

3.Assume a trial depth Y, calculate HCEF.

HCEF = (Kp-Ka) Y2/3

4.Let R = Ha1*y1 + Ha2* y2 – HCEF* y3

y1 = (2h/3-b)

y2 = (h+a/3-b)

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y3 = (h+a+2Y/3)

Substitute Y into R, if R = 0, the embedded depth, D = Y + a.

If not, assume a new Y, repeat step 3 to 4.

5.Calculate the length of sheet pile, L = h+F.S.*D, FS is from 1.2 to 1.4.

6.Calculate anchored force T = Ha1+ Ha2– HCEF

7.Calculate y = -b+2*T/( Ka)

8.Calculate Mmax = T y - Ka (y+b)3/6

9.Calculate required section modulus S= Mmax/Fb.

10. Select sheet pile section.

11. Design tie rod

12. Design soldier beam.

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Example 3. Design anchored sheet pile in cohesionless soil. Depth of excavation, h = 10 ft

Unit weight of soil, = 115 lb/ft3

Internal friction angle, = 30 degree

Allowable design stress of sheet pile = 32 ksi

Yield strength of soldier beam, Fy = 36 ksi

Location of tie rod at 2 ft below ground surface, spacing, s = 12 ft

Requirement: Design length of an anchored sheet pile, select sheet pile section, and design tie rod

Solution:

Design length of sheet pile:

Calculate lateral earth pressure coefficients:

Ka = tan (45-/2) = 0.333

Kp = tan (45-/2) = 3

The lateral earth pressure at bottom of excavation is

pa = Ka h = 0.333*115*10 = 383.33 psf

The active lateral force above excavation

Ha1 = pa*h/2 = 383.33*10/2 = 1917 lb/ft

The depth a = pa / (Kp-Ka) = 383.3 / [115*(3-0.333)] =1.25 ft

The corresponding lateral force

Ha2 = pa*a/2 = 383.33*1.25/2 = 238.6 lb/ft

Assume Y = 2.85 ft

HCEF = (Kp-Ka) Y2/3 = 115*(3-0.333)*2.852/3 = 830.3 lb/ft

y1 = (2h/3-b) = (2*10/3-2)=4.67 ft

y2 = (h+a/3-b) = (10+1.25/3-2)=8.42 ft

y3 = (h+a+2Y/3) = (10+1.25+2*2.85/3) = 13.15 ft

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R = Ha1*y1 + Ha2* y2 – HCEF* y3 = 1917*4.67+238.6*8.42-830.3*13.15 = 42.5 lb

R closes to zero, D = 2.85+1.25 = 4.1 ft

Length of sheet pile, L = 10 + 1.2* 4.1 = 14.9 ft Use 15 ft

Calculate anchor force,

T = Ha1+ Ha2– HCEF = 1917+238.6-830.3 = 1326 lb/ft

Calculate location of maximum moment,

y = -b+2*T/( Ka) = -2 ft + 2*1326/(115*0.333) = 6.32 ft

Mmax = T y - Ka (y+b)3/6 = 1326*6.32 – 115*0.333*(6.32+2)3/6 = 4.7 kip-ft/ft

The required section modulus S= Mmax/Fb = 4.7*12/32 = 1.8 in3/ft

Use PS28, S = 1.9 in3/ft

Design tie rod, the required cross section area,

A = T s / (0.6*Fy) = 1.326*12/(0.6*36) = 0.442 in3.

Use ¾” diameter tie rod, A = 0.442 in3.

Design soldier beam:

The maximum moment of a continuous beams with 3 or more span is

M = 0.1*T s2 = 0.1*1326*122 =19.1 kip-ft

Required section modulus, S = M / (0.6*Fy) = 19.1*12/(0.6*36) = 6.4 in3.

Use W6x15, S = 9.72 in3.

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Anchored sheet pile wall in cohesive soil.

Calculating active earth pressure

Calculation of active earth pressure above excavation is the same as that of cantilever sheet pile in

cohesive soil. The free-standing height of soil is d = 2C/

The lateral earth pressure at bottom of excavation, pa = h – 2C, where is unit weight of soil. The resultant force Ha=pa*h/2

Calculating passive earth pressure

For cohesive soil, friction angle, = 0, Ka = Kp = 1. The earth pressure below excavation,

p1= p-a= 2C-(h-2C) = 4C-h

Assume the embedded depth is D, the resultant force below bottom of excavation is

HBCDF = p1*D

Derive equation for D from Mo = 0

Mo = Ha1*y1 – HBCDF* y3 = 0

Where

y1 = 2(h-d)/3-(b-d)

y3 = h-b+D/2

The equation can be determined with a trial and error process.

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Determine anchor force T from Fx = 0

Fx = Ha1– HBCDF-T = 0

T = Ha1+ Ha2– HCEF

Design size of sheet pile

Maximum moment locates at a distance y below T where shear stress equals to zero.

T- Ka (y+b-d)2/2=0

Solve for y, we have, y = -b+d+2*T/( Ka)

The maximum moment is

Mmax = T y - Ka (y+b-d)3/6

The required section modulus is S = Mmax / Fb

The sheet pile section is selected based on section modulus

Design of tie rod and soldier beam

Design of tie rod and soldier beam is the same as that of anchored sheet pile in cohesionless soil.

1.Calculate free standing height, d = 2C/

2.Calculate pa=(h-d)

3.Calculate Ha=pa*h/2

4.Calculate p1=4C-h,

5.Assume a value of D, and calculate HBCDF = p1*D

6.Calculate R= Ha*y1 – HBCDF* y3.

Where

y1 = 2(h-d)/3-(b-d)

y3 = h-b+D/2

If R is not close to zero, assume a new D, repeat steps 5 and 6

7.The design length of sheet pile is L=h+D*FS, FS=1.2 to 1.4.

8.Calculate anchored force T = Ha – HBCDF

9.Calculate y = -b+d+2*T/

10. Calculate Mmax = T y - (y+b-d)3/6

11. Calculate required section modulus S= Mmax/Fb. Select sheet pile section.

12. Design tie rod

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13. Design soldier beam.

Example 4: Design anchored sheet pile in cohesive soil.

Depth of excavation, h = 15 ft

Unit weight of soil, = 115 lb/ft3

Cohesion of soil, C = 500 psf

Internal friction angle, = 0 degree

Allowable design strength of sheet pile = 32 ksi

Yield strength of soldier beam, Fy = 36 ksi

Location of tie rod at 2 ft below ground surface, spacing =12 ft.

Requirement: Design length of sheet pile and select sheet pile section

Solution:

Design length of sheet pile:

The free standing height, d = 2C/ = 2*500/115 = 8.7 ft

The lateral pressure at bottom of sheet pile, pa = (h-d)=115*(10-8.7)=150 psf

Total active force, Ha=pa*h/2 = 150*10/2 = 750 lb/ft

p1=4C-h = 4*550-115*15 = 275 psf

Assume D = 11.5 ft,

HBCDF = p1*D = 3163 lb/ft

y1 = 2(h-d)/3-(b-d) =2 (15-8.7)/3-(2-8.7) = 10.9 ft

y3 = h-b+D/2 = 15-2+11.5/2 = 18.75 ft

R= Ha*y1 – HBCDF* y3 = 5438*10.9-3163*18.75 = -36 lb Close to zero

The length of sheet pile, L = 15 + 1.2*11.5 = 28.8 ft Use 29 ft

Anchored force per foot of wall, T = Ha – HBCDF = 5438 – 3163 = 2275 lb/ft

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Calculate location of maximum moment,

y = -b+d+2*T/ = -2+8.7+2*2275/115 = 13 ft

Maximum moment,

Mmax = T y - (y+b-d)3/6 = 2275*13 – 115*(13+2-8.7)3/6 = 24770 lb-ft/ft

Required section modulus of sheet pile, S= Mmax/Fb = 22.47*12/32 = 8.4 in3/ft

Use PDA 27 section modulus 10.7 in3/ft

Design tie rod

Cross section of tie rod required, A = T*s/(0.6*Fy) = 2.275*12/(0.6*36) = 0.91 in2.

Diameter of tie rod, d = 4*A/ = 1.08 in

Use 1-1/8” diameter tie rod.

Design soldier beam

Maximum moment in solider beam, Mmax = 0.1*T*s2 = 0.1*2275*122 = 32760 lb-ft

Required section modulus, S= Mmax/Fb= 32.76*12/(0.6*36) = 13.1 in3.

Use W 8x18, section modulus S = 15.2 in3.

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types of deep support systems

are commonly used in metropolitan cities.

(i) Diaphragm walls

(ii) Pile walls (Contiguous, Tangent or Secant)

(iii) Soldier pile w

ith wooden lagging walls

(iv) Sheet pile walls

(v) Composite supporting systems – that is, any of the retaining

systems

Retaining systems like

diaphragm wall, contiguous pile walls;

and soldier piles with wooden lagging

described in this article has been successfully used. Case studies of their use indicate that

adequate quality control measures and instrumentation monitoring of these systems go a

long way in ensuring their safeand economic deployment at sit

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Contiguous Pile Walls General – Piled Retaining Systems

Abstract

Providing space for parking, public amenities,etc in multi-storey buildings at

town centres has created a need to go deep excavationsinto ground. Deep excavations are

supported by systems like conventional retaining walls, sheet pile walls, braced walls,

diaphragm walls and pile walls. This article describes various excavation supporting

systems that are in vogue essentially contiguous pile wall and its advantages. A detailed

design methodology of an excavation supporting system is furnished in this study.

There are different types of pile walls

(Fig.4).Diameter and spacing of the piles is

decided based on soil type, ground water level and magnitude of design pressures.

Large spacing is avoided as it can result in caving of soil through gaps. In

Contiguous bored pile construction, center to center spacing of piles is kept slightly greater thanthe pile

diameter.

Secant bored pilesre formed by keeping this spacing of piles less

than the diameter.Tangen

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Fig. 4: Schematic Arrangement of Contiguous Piled Retaining System.

Contiguous piles serving as retaining walls

are popular since traditional piling equipments can be resorted for their construction. They are considered

more economical than diaphragm wall in small to medium scale excavations due to reduction in cost of site

operations. Common pile diameters adopted are 0.6, 0.8 and

1 .0m. These piles are connected with a Capping beams at the top, which assists equitable pressure

distributions in piles. These retaining piles are suitable in areas where water table is deep or where soil

permeability is low. However, some acceptable amount of water can be collected at the base and pumped

out.

ARRANGEMENT OF CONTIGUOUS PILe

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Secant Pile Walls are formedby constructing intersecting piles. Secant bored pile walls are formed by keeping spacing ofpiles lessthan diameter. Secant pile walls are used tobuild cut off walls for the control of groundwater inflow and to minimizemovement in weak and wet soils. Secant Wall constructed in the form of hard/soft or hard/firm and Secant Wall Hard/hard wall. Secant Wall-hard- softs or hard/firm is similar tothe contiguous bored pile wall

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Soldier Piles and Wooden Lagging supported system

The supporting system comprised soldier piles

spaced at 1.8m c/c and with a closer spacing of 1.6m c/c near the launching shaft (Fig.8). Wooden laggings of thickness 100mm to 120mm were supported between the soldier piles.Three levels of Struts were provided at depths 3.285, 7.285, and 10.831m below the established ground level (EGL-209.80m). Additional level of Waler beam with pre-stressed rock anchors were provided 2m above the excavation level. Rock anchors with capacity of 86.4T,spaced at 3.6m c/c, were embedded 6m into the quartzitic bedrock to meet the bond strength consideration

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Soldier Piles & Laggings Wooden Supporting System

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1. Excavate trench for guide wall

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2. Lean concrete trench base

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3. Place guide wall mould

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4. Place guide wall reinforcement

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5. Casting guide wall

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6. Completed cast guide wall

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7. Remove guide wall mould after concrete set

guide wall 8. Completed guide wall

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9. Open pilot hole for double wall casing installation

10. Install 1st section double wall casing

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11. Continue to install more sections of double wall casings

12. Boring & installing double wall casings

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13. Boring to pile toe level with different types of tools

14. Reinforcement cage ready for installation

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15. Install reinforcement cage into completed pile bore

16. Concreting secant pile

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17. Extract double wall casing by casing oscillator

18. Completed secant piles after pile heads trimming

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Exposed Secant Pile Wall for Capping Beam Construction

Trimmed Secant Pile Wall

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Ready for Capping Beam Casting

Secant pile cap beam

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Dr Youssef Hammida

retaining wall(الجدران الاستنادية).- scant piles

Engineering