Shore Pile and Bracing System Design in Deep Excavation
Why do we need?
• It is legal obligation to protect adjacent infrastructures when excavating to any depth
Deep Excavation, shore pile and bracing system
· Earth pressure calculation
· Shore pile or sheet pile design
· Bracing system design
o Strut, wale, tie
· Capping beam design
· King post
Other important issues of deep excavation
•Heaving at base
•Connections of bracing system
•Foundation information of surrounding structures
Methods of Analysis and Design of Shore Pile and Bracing System
•Limit Equilibrium Analysis: Manual Calculation
•Modeling Shore Pile / Sheet Pile as Elastic Beam in any FEM Software (ETABS, STAADProetc)
•Modeling Shore Pile / Sheet Pile as Elastic Beam in any Geotechnical FEM Software (Geo5,Geostudio, Plaxis2D etc)
•Modeling soil, shore pile / sheet pile and bracing system in 3D GeotechnicalFEM Software (Plaxis3D, MIDAS etc)
Analysis of Staged Construction
•Shore pile / sheet pile, bracing system and deep excavation need staged construction
•Each stage should be analysed
•Any stage might be critical based on soil and surrounding condition
Analysis of Shore Pile and Bracing System
Limit Equilibrium Analysis
•FoSis calculated from Ultimate Resistance/Applied Load
•Serviceability can not be checked
•Failure means FoS<1.0
•C, phi is required
Finite Element Analysis
•Stress –Strain –Deformation is calculated
•Serviceability can be checked easily
•Failure is identified from excessive deformation or nonconvergence
•C, phi, E and other parameters required
Failure modes of shore pile and bracing system in deep excavation
1. Lateral deflection of shore pile, wale ******
2. Vertical displacement of retained soil
3. Shear or bending failure of shore pile, wale
4. Buckling failure of strut
5. Anchorage failure (anchor rod yielding, passive failure of anchor block / friction failure anchorage length
6. Connection failure of bracing system
7. Water seepage***** or leakage
8. Sand boiling in sand
9. Heaving at base in soft clay
10. Toe failure (kick out failure)
11. Uplifting of mat during construction
12. Deep seated slope failure
Stages of planning, design and construction
•Inventory survey (foundation, structure and existing situation of surrounding infrastructures)
•Analysis and design (meeting criteria of code)
•Construction sequence, safety and instrumentation planning
•Piling, Excavation and Bracing etc.
Two Approaches of Basement Construction
•Top-down method -X
Most favorable situation during excavation
•Unsaturated soil within excavation depth
•Water table is below base of excavation
So, who is the #1 enemy?
Excavation in Clay Soil
•Earth pressure is high if soil is soft
•Very low soil resistance below dredge line
•No water seepage
Excavation in Sandy Soil
•Water seepage and loss of ground with seepage water
•Better resistance under dredge line
· Settlement gauge
· Deflection measuring devices
Components of bracing system in deep excavation
•Shore pile / sheet pile
why don’t shore pile and bracing system fail in many instances even it is not properly designed?
Answer is: unsaturated soil, tree roots
· Excavation : An excavation means a man made cavity or depression in the earth’s surface formed by earth removal.
· Open excavation means an excavation in which the width is greater than the depth, measured at the bottom.
Deep excavation Vs Open excavation
Deep excavation: An excavation in soil or rock more than 4.5mis called deep excavation
Open excavation: It means an excavation without any retaining system by providing suitable slope 1:2 or 1:1, when surrounding allows
Shoring: is an assembly of structural members designed to prevent earth or material from falling, sliding or rolling into an excavation.
Support: structure means a temporary or permanent structure or device designed to provide protection to workers in an excavation, tunnel or shaft from cave-ins, collapse, sliding or rolling materials and includes shoring, bracing, piles, planks and trench cages.
Trench means an excavation that is deeper than its width measured at the bottom. (long -deep –narrow)
Trench Cage means a steel support structure designed to resist the pressure from the walls of a trench and capable of being moved as a unit.
Trench Jack means a screw or hydraulic jack used as a brace for a temporary support structure.
Tunnel means a generally horizontal excavation that is more than a meter long and located underground.
Necessity of deep excavation
1. Mega structures like dams, power stations
2. Building with multiple basements
3. Tunnel construction
4. Basement required for high rise building to increase overturning stability
5. Nuclear waste containment
6. Utility lines under road
Bottom-Up Excavation Methods
1. Sloped open cut method
2. Vertical wall / shore pile / sheet pile
· Cantilever wall / shore pile / sheet pile
· Anchored (tieback) wall / shore pile / sheet pile
· Incline Propped (Raker) wall / shore pile / sheet pile
· Braced wall / shore pile / sheet pile
Sloped open cut method
· Sloped open cut method: Does not use retaining walls or struts. The construction site is excavated with sloped sides.
· Digging with slope and backfilling are necessary
· Preferable when excavation is not too deep
Cantilever method: Require construction of retaining walls without bracing system
VARIOUS WALLS FOR BRACED EXCAVATIONS
Types of Vertical Wall
• Braced sheet piling
• Braced RCC precast piling
• Soldier beam and lagging
• Bored-pile walls
• Diaphragm-slurry walls
How to make water tight
· Sheet piling
· Jet grouting in contiguous bored pile
· Shotcretingwith wiremeshin bored pile
· Secant pile
· DIAPHRAM WALL / SLURRY WALL
Watertight by bored pile and shotcreting
• Shotcretingis done from the ditch after excavation
• Wire mesh may be used if needed
• Weep holes may be kept with proper dewatering management
Watertight by Secant pile
Secant wall construction sequence:
1. Drill and cast female piles (soft primary piles without rebar)
2. Drill and cast male (secant) piles
Secant piles are reinforced with either steel rebar or with steel beams. Primary (female) soft piles are installed first without rebar. Then secondary (male) piles constructed in between primary (female) piles once the concrete gains necessary strength. Pile overlap is typically in the order of 80 mm and can go in depths of up to 45 meters. (Baxter, 2012)
Advantages & Disadvantages of using secant piles
The main advantages of secant pile walls are:
· Increased wall stiffness compared to sheet piles.
· Can be installed in difficult ground(soft soil, loose sand etc)
· Used in high water table conditions.
The main disadvantages of secant pile walls are:
· Verticality tolerances may be hard to achieve for deep piles.
· Total waterproofing is very difficult to obtain in joints.
· Increased cost compared to sheet pile
DIAPHRAM WALL / SLURRY WALL
· The continuous diaphragm wall (also referred to as slurry wall) is a structure cast in a slurry trench by tremieconcrete.
· The trench is initially supported by bentoniteor polymer based slurries.
DIAPHRAM WALL / SLURRY WALL
A) Trenching under slurry,
B) End stop inserted (steel tube or other),
C) Reinforcement cage lowered into the slurry-filled trench,
D) Concreting by tremiepipes
Short Term Critical
· Shallow foundation
· It applies a total stress increase to the clay subsoil, and positive pore pressures are induced
Idealized wall movements
Active: Rigid structure rotates away from soil about its base. Eventual soil failure involves a small mass of soil, which is partly supported by the shear stresses on the failure plane. Pressures are low.
Passive: Rigid structure rotates toward the soil about its base. Eventual soil failure involves a large mass of soil, with shear strength acting against the wall. Pressures are high.
Earth pressure at rest: Structure is rigid, does not move, and can be placed in the soil without allowing any lateral soil movement. Lateral pressures existing in the soil before wall installation are applied to the wall
Ka or Ko
· It is dangerous to assume passive pressure as resistance, because, to develop passive pressure need sufficient wall movement. Rather apply support spring at passive side.
· Same applies to active pressure.
· In reality, it is in between Kaand Ko.
· So, K = (Ka+Ko)/2
LIMIT EQUILIBRIUM ANALYSIS
Cantilever Shore Pile or Sheet Pile
· Cantilever pile may not fail totally, But excessive lateral deformation may pose danger to adjacent structures
· Pile must be embedded into sandy soil and stiff / hard clay
Alternative Solution of Anchored or Braced Shore Pile or Sheet Pile
Pressure distribution below dredge level
• Generally, there is a point of contra-flexure in the wall some distance below dredge level
• For very dense sands, the point of contra-flexure may be at, or slightly above, dredge level
• For loose sands, it will be lower
Steps of analysis
1. Calculate EI of shore pile / sheet pile. Define the beam section in software so that model EmIm= EI/s. Beam finite elements length should be 0.5-1.0 m
2. Compute pressure diagram using Effective unit weight and K =(Ka+Ko)/2 uptopile tip and take minimum Pressure = 0.2(H+q)
3. Add water pressure to earth pressure at both sides
4. Run analysis to get shear force, BM diagram and R1, R2 reactions
5. Calculate resultant force (Pp) of passive earth pressure below dredge line
6. Check that R2 < 0.5Pp
7. Use R1 to design bracing system
8. Use SF and BM of shore pile to design shore pile
CONVENTIONAL DESIGN OF SHORE PILE AND BRACING SYSTEM
Steps for design of shore pile and bracing system
· Compute the lateral pressure using peck’s apparent pressure diagram
· Determine strut forces
· Design the struts, sheeting, wales
1. They apply to excavations having depths greater than about 6 m.
2. Embedment length is tends to zero.
3. They are based on the assumption that the water table is below the bottom of the cut.
4. Sand is assumed to be drained with zero pore water pressure.
5. Clay is assumed to be undrained and pore water pressure is not considered.
6. Not applicable for layered soil
Cut in sand
σa= 0.65 γH Ka
γ= unit weight
H = height of the cut
Ka= Rankine active pressure coefficient
= tan2(45 –ϕ’/2)
ϕ’= effective fiction angle of sand
Apparent pressure diagram (Peck, 1969)
PRESSURE ENVELOPE FOR CUT IN LAYERED SOIL
Sometimes, layers of both sand and clay are encountered when a braced cut is being constructed. In this case, Peck (1943) proposed that an equivalent value of cohesion (ϕ = 0) should be determined according to the following formula
Solution for Layered Soil by Bowels
1. Compute pressure diagram using Effective unit weight, Kaand Ko
2. Make negative pressures to zero
3. Compute resultant Ra and Ro. Calculate avgR
4. Using avgR, make an idealized rectangular or trapezoidal pressure diagram
5. Add water pressure to the idealized pressure diagram
DESIGN OF VARIOUS COMPONENTS OF A BRACED CUT
• In construction work, struts should have a minimum vertical spacing of about 2.75 m or more
• Struts are horizontal columns subject to axial force and bending. The load-carrying capacity of columns depends on their slenderness ratio, which can be reduced by providing vertical and horizontal supports at intermediate points
• For wide cuts, splicing the struts may be necessary
• For braced cuts in clayey soil, the depth of the first strut below the ground surface should be less than the depth of tensile crack, zc
• Depending on the soil, assume one of the three apparent-pressure envelopes.
• Draw the pressure envelope next to the wall of the excavation.
• The struts are labeled A, B, C, and D, carrying compressive forces PA , PB , PC, and PD. They are placed at horizontal spacing of s.
• The connection between the strut and the sheet pile (or soldier beam) is assumed to be a hinge (i.e., carrying no moment) at all strut levels, except for the top and the bottom ones.
• Only the ends of struts B and C act as hinges. Some designers assume hinges at all levels except for the top.
•Each beam has two unknown strut loads, which can be determined from equilibrium considerations.
•the strut loads per unit length of the excavation are A, B = B1+B2, C = C1 + C2, and D.
With center-to-center horizontal strut spacing of s, the strut loads can be summarized as follows
PA = As (A = reaction per m, s = spacing of strut horizontally)
PB = Bs= (B1+B2)s
PC = Cs = (C1+C2)s
PD = Ds
Design strut section using the strut forces
Design of Wale Section
To compute the maximum bending moment and shear of wales, the wales can be Viewed as simply supported beams with struts as supporting hinges, or Viewed as continuous beam
• Wales conservatively treated as pinned at the struts
• Maximum moment for wales, which occur at the mid span are
At level A, Mmax= A*s2/8
At level B, Mmax= (B1 + B2)*s2/8
At level C, Mmax= (C1 + C2)*s2/8
At level D, Mmax= D*s2/8
• Now determine the section modulus S = Mmax/ σall
Case Study of Braced Cut and Apparent Pressure Diagram
Lambe(1970) provided data on the performance of three excavations for the subway extension of the MBTA in Boston (test sections A, B, and D), all of which were well instrumented.
The construction of the south half of the National Plaza in Chicago required a braced
cut 21.43 m deep.
Swateket al. (1972) reported the case history for this construction.
The area of actual pressure diagram = 2933 kN/m.
Area of Peck’s pressure diagram = 5280 kN/m
Peck’s pressure envelope gives a lateral earth pressure of
about 1.8 times that actually observed in this case.
So, this method is sometimes unsafe and sometimes conservative
This method is dangerous in the subsoil condition of Bangladesh where piles need to be embedded below dredge line
MODIFIED CONVENTIONAL METHOD OF SHORE PILE AND BRACING DESIGN,
Proposed by Professor Jahangir Alam, Department of Civil Engineering, BUET, Dhaka
• Any depth of excavation
• Water table
• Layered soil and
Solution for Layered Soil in Modified Method
1. Compute pressure diagram using Effective unit weight and K =(Ka+Ko)/2 uptopile tip and Minimum Pressure = 0.2H+q
2. Compute resultant R from pressure diagram
3. Using R, make an idealized rectangular pressure diagram
4. Consider water pressure diagram at both side
5.Draw passive pressure diagram and calculate Pp using FS=2.0
6. Apply Pp at D/2 distance from pile tip
7. Determine P1, P2…. Using simple beam model in ETABS
Simple FEM Model of Braced Shore Pile using ETABS or Any Structural FEM Software
In reality soil behavior is nonlinear, here it is modeled as nonlinear. If the lateral deflection is kept with code defined limit, the soil behavior is approximately linear
Steps of Simple FEM modeling
1. Calculate EI of shore pile / sheet pile. Define the beam section in software so that model EmIm= EI/s. Beam finite elements should be 0.5-1.0 m
2. Compute pressure diagram using Effective unit weight and K =(Ka+Ko)/2 uptopile tip and take minimum Pressure = 0.2(H+q)
3. Add water pressure to earth pressure
4. Estimate k1, k2…. below dredge line.
5. Assume strut sections and compute s1, s2….. Per unit length of wall
6. Run analysis to get shear force, BM diagram and s1, s2… and k1, k2… reactions
7. Rs1, Rs2… shall be used to design strut and re-calculate s1, s2……
8. Rk1, Rk2….. Shall be summed up to get total passive resistance Rt. Rt< (Pp –Pa)/FS [FS = 1.5-2.0]. If this criteria does not satisfy, increase D.
9. Modify the model for s1, s2, …. And k1, k2…. And RUN again
10. Solve all the stages similarly
11. Check deflections with code limits
12. Take the SF and BM to design the shore pile section
Estimate soil spring as per Chinese Standard
• Ref: JGJ 120-2012 (Technical specification for retaining and protection of building foundation excavations).
• Chinese Standard
m = proportional coefficient of modulus of subsoil reaction [kN/m4]
z = depth of the calculation point from the original ground [m]
h = depth of excavation at current stage of construction [m]
•modulus kh is linear with depth
•Proportional coefficient m should be determined from pile test with horizontal load.
If there are no test data, Chinese standard JGJ 120-2012 suggest an empirical formula to estimate this coefficient.
Vb= horizontal displacement of sheeting structure at the ditch bottom [mm]; (min 10 mm)
According to Matlock and Reese
Correlation between Modulus of Elasticity (Es) and N60
Linear Distribution (Bowles):
r= reduced width of pile[m],which is given the 2nd equation mentioned above
d= pile diameter[m]
β= angle of dispersion–is input with respect to the angle of internal friction in the range of φ/4~φ
K=soil parameter (modulus) according to Bowles [MN/m3]
r = d+2d.tanβ
Calculating spring constant of struts
S1 = P/d = (EA/L)/s
L = half of the width of excavation
S = horizontal spacing of struts
EA/L is for the width s
Modeling section of shore pile
•Calculate EI of one pile
•Model EmIm= EI/s
•Calculate depth of a rectangular beam of width 1 m or 1 ftwhich must have EmIm
MODELING BRACING SYSTEM IN FEM SOFTWARE
Kingpost Removal and Instrumentation in Braced Excavation