EARTH RETAINING SYSTEM FOR 

DEEPEXCAVATIONS

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Types of Earth Retaining Systems for Deep Excavations

Braced Walls or (mostly) strut support

1. Soldier pile and lagging walls with anchor or strut support

2. Sheet piling or sheet pile walls with anchor or strut support

3. Pile walls (contiguous, secant) with anchor or strut support

4. Diaphragm walls or slurry trench walls with anchor or strut support

5. Prefabricated diaphragm walls with anchor or strut support

6. Reinforced Concrete (Cast-in-situ or prefabricated) retaining walls (Excavation in stages mostly anchor support

7. Drop shaft sinking and caissons

8. Jet-grout walls (or deep soil mixing) + Anchors or Struts

9. Soil nail walls

10. Top-down excavation and support method

Wall Selection Criteria:            

1. Size of excavation

2. Ground condition

3. Groundwater level

4. Settlement of adjacent ground

5. Displacement criteria

6. Others (availability, speed of Work etc.)

1. BRACED WALLS + ANCHOR/STRUT SUPPORT

a. Excavation step by step

b. Steel frames with timbering or shotcrete (sometimes timber frames)

c. Usually strut support

d. Soils with some cohesion and without water table are suitable

e. Often limited to small dimension shafts and trenches with and without penetration below base (mostly without)

2. SOLDIER PILE AND LAGGING WALLS

(Example: BERLIN WALLS)

a. With H shape soldier piles and timbering

b. For small heights (up to 5m), cantilever  wall

c. May be restricted to non-sensitive areas or stiff soils due to soil displacement risks behind the wall

d. Some alternative solutions to the classical method; concrete piles or high inertia sheet piles for the soldier piles and shotcrete with mesh or cast-in-situ concrete between the soldier piles

e. Projects: above water table and soils with some cohesion, otherwise soil treatment like; dewatering, grouting, soil mixing

f. Very popular in European cities and economic, but not yet practiced in this country

3. SHEET PILE WALLS (SHEETPILING)

a) Their use is often restricted in urbanized areas due to environmental problems: effect of vibrations, driving difficulties in case of hard layers and boulders, presence of buried pipes, public utilities etc.

b) Struts or anchors are used. They are usually constructed in water bearing soils. Sheet pile discontinuities constitute the risk for water tightness.

Steel sheet piles are the most common.

Reinforced concrete sheet piles (driving is more difficult in stiffer soils.)

Few types of sheet piles are shown below.

Light vs. heavy sections.  (Sheet pile and H sections are imported to Turkey, not manufactured.)

4. PILE WALLS

i. Intermittent (Contiguous) Bored Pile Walls

a Cohesive Soils or soils having some cohesion are suitable.

b No water table.

c Spacing depends on moments and type of soil

d Common diameters : 60 , 80 cm

e May be more economical compared to diaphragm walls in case of no water table exists.

Tangent (Contiguous) Bored Piles

Used when secant piling or diaphragm walling equipment is not available. (i.e. in cases where ground water exists.) Grouting between the piles is common in water bearing soils. Poor workmanship creates significant problems.

ii) Secant Bored Pile Walls (S<D)

Watertight wall may be more economic compared to diaphragm walls (mainly because of the cost of the site operations for bentonite plant)

May be constructed “hard-hard” as well as “soft-hard”, “soft” Low cement content or bentonitic concrete.

5.DIAPHRAGM WALLS (Slurry Trench Walls)

a. Structural Diaphragm Walls

b. Cut-off Barrier Walls

Material:

I.      Soil-Bentonite

II.      Cement-Bentonite

III.      Plastic Concrete

IV.      Concrete

§  Quide Wall

§       panel dimensionas and arrangements thickness : 50, 60, 75, 80, 90, 100, 120, 150 (cm) (60-80 cm most common)

§       Panel lengths: 2m  to 10m short lengths (2-2.5m) in unstable soils or very high surcharges due  to high loads. Longer panels in  stable soils Types of panel;

Location of internal framing and temporary bracing, interior column layout, length of contractors’ buckets are among the factors affecting panel dimensions and arrangements.

§       Panel Joints

Additional information on diaphragm walls:

§       It is a classical technique for many deep excavation projects, large civil engineering works, underground car parks, metro pits etc. Especially under water table.

§       They may be used to provide structural support and water tightness.

§       Nowadays depth of  panels exceeded 100m, excavation depths exceeded 50m.

§       Panel excavation is made by cable(d) buckets or kelly equipment.  Recently soil cutters (hydrofraise)

§       Hard layers are excavated by trepaning or by using the recent drilling equipment “hydrofraise” or “cutter”.

§       Excavation very close to existing buildings is possible.

§       Integration to permenant  structures is possible

§       Watertightness is generally sufficient for provisional structures, and may be improved for permenant structures by special joints.

§       If deviating panels occurs, flux of water into the pit may be experienced in water bearing permeable soils.

§       Slurry technique is a specialized technique and specialized contractors do the job.

§       Equipment:  Crawler cranes, excavation equip. (buckets etc), extractor (stop and pipes), tanks, pumps, desanding equipment, air lifts, screens, cyclones, silos, mixers

Concrete mix: – Ultiimate strength 200-350 kg/cm²

- 20cm slump (field value)

- natural gravel preferred, 20mm max. Size, well-graded mixture, a more sandier mix is preferred.

- plasticizers, water reducing agents, air  entrainment agents & fly ash recommended

• Bentonite slurry:

-  Fresh bentonite upon mixing  Gs = 1.03

PH    : 7 to 11

Viscosity: 32 sec. (Marsh Cone)

-         Should be stored at least one day for hydration in tanks, pits etc.

-         Max. Bentonit content 29 – 34 kg/m3

• Before tremie

Concreting  :  Gs  £ 1.10   .

Sand content:  < %5

Viscosity < 50  (1.5m above the bottom)  (40 recommended)

One unit (panel volume) spare bentonite mix should be ready

• Verticality should be checked: %1 (0.5-1.25); in loose soils (fills) the tolerance may  be increased 50-100% (i.e. 2%)
• Depth should be checked at the start of the day to determine if cave-ins occured overnight.
• If slurry is O.K. concreting starts and it is a continious operation until fresh concreteoverflows at the top (concrete servicing organization must be safe!)
• Check quality and quantity of concrete.  Single tremie pipe 20-25cm dia. at the center of the panel recommended  10cmx10cm mesh screen at the hopper inlet is used.
• Tremie pipe should always be in concrete 2m (min) 5m max.
• Truck deliveries are calculated and a plot is prepared

6. Prefabricated Diaphragm Walls

a. Same principles apply as the diaphragm   walls, with bentonite-cement suspension in the trenches.

b. Then prefabricated panels are placed inside the trench and the slurry mixture is set.

c. The panels are excavated to the depth required for tightness, while the prefabricated elements are placed only to the depth required for ground retaining.

d. Concrete prefabricated panels may be designed quite thin by the use of good quality concreting and reinforcementt. A better property for permanent structures. These may allow cheaper alternative projects than conventional diaphragm walls.

e. Alternative to concrete panels: Sheetpiles or prefabricated steel slabs.

f. For small heights with no or small water pressures bentonite-cement grout (c/w ratio 0.1-0.4) reinforced by steel mesh and vertical steel sections proved to be practical.

Joints again need to be cared specially.

7. Reinforced Concrete (Cast in-situ or prefabricated) Retaining Walls:

Excavation in stages

a. Soil with some cohesion

b. No water table or appreciable amount of water

c. Sometimes minipile support (if required)

d. Drainage

Cofferdams are used in the following areas;

i. Bridge piers and abutments in rivers, lakes etc.

ii. Wharves, quay walls, docks

iii. Breakwaters and other structures for shore protection

iv. Large waterfront structures such as pump houses, subjected to heavy vertical and horizontal loads.

Caisson construction is restricted to major foundation works because of large construction cost.

Cases where caisson may be advantageous to other systems;

1. The soil contains large boulders which obstruct penetration of piles or bored piles.

2. A massive substructure is required to extend to or below riverbed to provide resistance.

3. The foundation is subjected to large lateral forces.

Concrete plug (seal) when the level is attained. Deep shafts up to 10 m.

Diameter safe, quick and cheap method especially in poor soils (no water lowering and soil improvement, grouting).

8. Jet Grout walls or other soil methods like deep soil mixing

a. Experiences with one line of vertical columns were not always satisfactory due to their small bending stiffness, when no steel reinforcing element is placed inside the column.

Problems;

Deep columns in water-bearing sands: If 1-2 % deviation of the borings occurs ( may be unavoidable) or discontinuities (mainly at the interfaces between hard and soft layers) occur sand-bearing flows into the excavation are potential. (Highly specialized contractors needed.)

9. So?l Na?l Walls

Similar to the method (7).

a Excavate step by step (1.5 to 2m high)

b Shotcrete is common for facing + wiremesh

c Ordinary steel bars (20 mm – 36 mm)

d Drilling and placing grout (no pressure) or pushing, driven nails

e 1 nail/few m2 may be critical in sensitive areas.

Requirements;

i. No water table.

ii. Soil should be somewhat cohesive.

10. Top Down Construct?on