Thursday 3 June 2010

PILING, DIAPHRAGMS AND RETAINING WALL SYSTEMS

PILING, DIAPHRAGMS AND RETAINING WALL SYSTEMS

VIBRO-REPLACEMENT AND VIBRO-COMPACTION

Introduction

Where structures cannot be safely founded on loose soils or fill material, piling may be considered as a means of transferring the loads to suitable levels. However, piled foundations are not the only means of achieving satisfactory foundations in such situations - the Engineer may wish to consider geotechnical processes or vibratory processes.

The most economical is likely to be cement injection. However, vibratory processes can also be used to consolidate and strengthen ground conditions at a comparatively low cost; this is achieved by stabilising the soil so that greater loads can be carried without risk of settlement. It also allows simple, shallow foundations to be used on otherwise poor sites. Vibratory processes can also be used on recently filled sites which contain brick rubble, soil, concrete or other miscellaneous material. This means that sites which were considered totally unsuitable for construction operations can now be considered.

Methods and materials

Two principal methods are employed for the compaction of the ground, namely vibro-­replacement and vibro-compaction.

Vibro-replacement

In the vibro-replacement method stone columns are constructed through weak soils to improve their load-bearing and settlement capacities. There are three processes, the dry process, the bottom feed process, and the wet process. In the dry process a heavy vibratory unit is allowed to penetrate the weak soil to the designed depth and the cavity is filled with stone, the stone is compacted in stages. The vibrator is up to 4 metres long and weighs about 2 tonnes. It is either suspended from a crane or held in crane leaders. In the bottom feed process the vibrator is mounted on a specially designed track ­mounted machine, which employs leaders to guide the vibrator. A heavy duty tube on the outside of the vibrator carries stone to the tip of the vibrator. Stone is fed into the tube via a stone reservoir, which in turn is fed by a skip travelling up and down the leaders. The vibrator is powered by a diesel generator mounted on the machine. The vibrator remains in the ground during the construction of the stone columns - the stone being supplied to the tip of the vibrator. Stone columns up to 15 metres deep can be formed in this way.

In the wet process the vibrator is suspended from a crane and the weak soils are removed by water jetting. Stone backfill then replaces the weak soil and is compacted into the surrounding ground. This process requires a water supply of 10,000 to 12,000 litres per rig hour. The stone columns formed by these processes can carry loads of 10 to 40 tonnes, but a safe bearing factor of 3.0 is applied to the calculated ultimate loading.

Vibro-compaction

This process is based on the fact that non-cohesive soils such as sand, can be compacted into a denser state by vibration. The action of the vibrator, which is often accompanied by water jetting, reduces the forces between the granular material allowing it to consolidate to the optimum density. This form of compaction is permanent and can be used in loose soils up to 29 metres in depth. The sequence of operations is shown in the sketch. The high relative density leads to high safe bearing capacities and permit the use of shallow foundations designed to bear pressures between 250 kN/m2 and 500 kN/m". The process is not suited to cohesive soils simply because such soils do not respond to vibration.

In addition to the techniques described above, vibrated concrete columns may be employed instead of stone and gravels.

Economic considerations

These processes may be an economic alternative to piling and grouting methods used to improve bearing capacity. However, the site must be large enough to justify the use of the special equipment involved in the process. Since the depth compaction using these methods is approximately 12 metres, it can be used satisfactorily only on sites which will provide suitable resistance at these depths. Vibro techniques are capable of achieving safe bearing pressures of up to 500 kN/m2.

While these pressures are suitable for most spread foundations, they may prove unsuitable for concentrated loads such as those found in framed buildings, and the formation of extensive capping beams may be uneconomic. In both cases large quantities of fill material have to be used. Where stone is used for forming columns in very soft soil, the quantity of material used may produce cost figures which are only marginally cheaper than conventional piles, and the latter will give much higher safe bearing capacities. The vibro-compaction process is particularly valuable for consolidating loose sands prior to the formation of raft foundations and may be used in conjunction with raft construction more economically than piling.

Please refer to ‘Introduction to Civil Engineering Construction’ by Roy Holmes for a complete explanation.

Tunnelling Techniques

Introduction

Need to consider:

Type of ground

Method of tunnelling

Removal of spoil

Ground water control

Types & Methods

Used for road, rail, water, gas, sewerage, storm water and services

Techniques are numerous depending on ground & permanent use:

Hand

Trenchless technology

Traditional

New Austrian (NATM)

Boring

Immersed tubes

Traditional

Usually rock, up to approx 200m2 csa

Drill & blast

Spoil loaded into trucks

Can use benching

Propping may be required

NATM

New Austrian Tunnelling Method

Designed by the Swiss used under licence in UK by Caledonian Mining Company Ltd

Uses lattice girders, metal mesh and shotcrete

Ease of use

Boring

Rock needs to be classified

Whole face boring

Roadheaders

Immersed Tubes

Lowering prefabricated tubes onto a pre-prepared sea or river bed.

Sub-base and Roadbase Materials

Unbound materials:

Clause 803 Granular sub-base, Type 1

This has to be crushed rock, slag or other hard materials graded as follows;

Clause 804 Granular sub-base, Type 2

This is a much smaller sized material than type I, therefore natural sand and gravels may be used as this material. The Table below shows the grading requirements and variations between type 1 and type 2 stone.

Sieve size

Proportion passing sieve

Type 1

Type 2

75mm

100%

100%

37.5mm

85 - 100

85 - 100

10mm

40 – 70

45 – 100

5mm

25 – 45

25 – 85

600 μm

8 – 22

8 – 45

75 μm

0 – 10

0 - 10

μ = micrometer = one millionth of a metre

Clause 805 Wet-mix macadam

This is a ‘plant manufactured’ material using crushed rock or slag accurately graded and batched, and mixed with 2-6% water according to the nature of the aggregate. The purpose of the water is not so much to make a ‘wet’ mix as a ‘damp’ mix, with the result that segregation during transport and laying is minimized, while at the same time the material is more easily compacted. The material is usually laid in compacted layers not exceeding 200 mm, the aggregate grading being within the stated limits. Care should be taken to keep the moisture content within the optimum limits; drying out, or excess moisture will have a serious detrimental effect.

Bituminous-bound materials:

Dense Roadbase Macadam to BS 4987 (DOT Clauses 902/903)

The need for strong but truly flexible bases that will not crack has led to the use of these dense bituminous materials.

The main requirements in the composition of the dense macadam for use in road bases are that the materials have a fines content (aggregate passing 3.35 mm sieve) of 38% and are made with high-viscosity binders, i.e. 50,54 or 58°C evt tar, or 50 pen, 100 pen or 200 pen bitumen.

‘Evt’ is the abbreviation for ‘equi-viscous temperature’ which is the temperature in degrees Celsius at which a tar a has a viscosity of 50 seconds as determined by the British Standard Test (see Section 10.21). ‘Pen’ is the abbreviation for ‘penetration grade’ and is a measure of the hardness of a bitumen binder. It is obtained from the standard penetration test which is described in Section 10.20.

Rolled Asphalt to BS 594 (DOT Clause 904)

Rolled asphalt is the oldest established bituminous material used for road base construction and has load-spreading properties superior to those of other flexible road bases.

The composition of rolled asphalt for road base construction is a mixture to BS 594, containing 65% of coarse aggregate and normally made with 50 or 70 pen bitumen.

Concrete and cement-bound materials:

Clause 1030 Wet-lean concrete

Clause 1036 Cement-bound granular material, category 1 (CBM 1)

Clause 1037 Cement-bound granular material, category 2 (CBM 2)

Clause 1038 Cement-bound granular material, category 3 (CBM 3)

Clause 1039 Cement-bound granular material, category 4 (CBM 4)

The grades of concrete covered in the DOT Specification (Clause 1001) range from those suitable for unreinforced and reinforced slabs to Cement Bound Granular Material Category 4.

Concrete for road pavements will be covered under the Rigid Road construction.

Cement-bound Materials

Soil cement, cement-bound granular material and lean concrete are all categories of the group known as cement bound material (CBM).

The DOT has renamed these as cement-bound material categories 1, 2 and 3 (CBM1, CBM2 and CBM3). A new and stronger category, CBM4, has been introduced for roads of higher traffic category.

Cement-bound materials are mixtures of raw material and cement which have a moisture content compatible with compaction by rolling, and capable of meeting the requirements for surface level, regularity and finish.

CBM 1 is the finest and weakest of these materials and should have a minimum average crushing strength of 4.5 N/mm2 after 7 days.

CBM 2 is both coarser and stronger, being of 40 mm nominal size, with a strength of 7 N/mm2

CBM 3 and 4 may be either 40mm or 20 mm nominal size and must be made of natural aggregate or slag with a strength of 10 N/mm2 for CBM 3 or 15 N/mm2 for CBM 4.

The DOT Specification covers requirements for batching, mixing, laying, compaction and curing. The mix is determined from site trials and can include PFA (pulverized fuel ash) or ggbs (ground granular blast furnace slag), used in combination with ordinary Portland cement (OPC).

Mixing and Batching

CBMI and 2 can be mixed in place or mixed in a plant and can be batched either by weight or by volume.

CBM3 and 4 must be mixed in a plant and batched by mass.

The mix-in-plant method of construction requires the material, cement and water to be mixed in a central plant and then transported to the point of laying and spreading. The mixer should be capable of mixing evenly and have an output sufficient to meet the demands of the spreading and compacting operations.

Transporting

Mixed material must be transported as quickly as possible to the point of laying and also be protected from the weather during this operation.

Laying

All cement-bound material must be placed and spread evenly in such a manner as to prevent segregation and drying. Roadbase cement-bound material must be spread by means of either a paving machine or a spreader box approved by the Engineer.

Cement-bound material must be spread in one layer so that after compaction the total thickness is as specified. Joints between each day’s work, or, where work has had to be interrupted, should be vertical.

Compacting

Compaction must be carried out immediately after spreading. All loose segregated or otherwise defective areas must be removed and replaced to the full thickness of the layer.

Curing

Immediately after compaction the material should be cured for a minimum of 7 days. Curing may be by:

(1) covering with impermeable sheeting,

(2) bituminous spraying, or

(3) spraying with a curing compound.

Traffic should not be allowed on the surface until the 7 day strength has been attained.

Surface tolerance

All road bases should be laid to a surface tolerance of ±

15 mm.

Sub-base and Roadbase Materials

Sub-base and Roadbase Materials

Bituminous-bound materials:

Clause 902 Dense tarmac ad am roadbase

Clause 903 Dense bitumen macadam roadbase

Clause 904 Rolled asphalt roadbase

Concrete and cement-bound materials:

Clause 1030 Wet-lean concrete

Clause 1036 Cement-bound granular material,

category 1 (CBM 1)

Clause 1037 Cement-bound granular material,

category 2 (CBM2)

Clause 1038 Cement-bound granular material,

category 3 (CBM3)

Clause 1039 Cement-bound granular material,

category 4 (CBM4)

Rigid Pavements

Road Construction

Construction can be either: Flexible Pavement or Rigid pavement

RIGID PAVEMENTS

Introduction

· Specialist operation

· Requires complex and expensive machinery

· The concrete slab is equivalent to wearing course, base course of a flexible pavement

· ‘Rigid’ because it does not deflect under traffic loads

· Designed to last 40 yrs

· Constructed of high strength and high quality concrete

· Great attention placed on surface finish

· Expansion and contraction of concrete must be accommodated

· Transverse joints used - contraction and expansion joints, every 3rd being expansion

· Expansion joints used - as much as 30m apart

· The concrete carriageway, for construction purposes, is divided into individual panels by joints in both transverse and longitudinal directions


The Rigid Road

· Unreinforced

· Jointed reinforced

· Continuously reinforced

· A separation membrane must be provided between the road slab and the sub-base

· For continuously reinforced slabs, a bituminous spray is used as a waterproof membrane


Foundation Requirements

· Based on the number of commercial vehicles usage per day

· Design based on DoT HD 14/87

· Foundation requirements mean that the sub-grade must be tested to determine its CBR value

· If CBR below 15% then capping layer required

Sub-base materials

· Sub-base material: hard, durable, chemically inert, compacted to high density and not be susceptible to frost heave

· If capping layers needed, then cement-bound or wet-lean concrete must be used


Road Slab thickness

· Determined by quantity and type of traffic likely to use it

· 150 mm for residential estates

· 300 mm for heavily trafficked trunk roads and motorways


Concrete Quality

· Increased traffic meant that designers had to increase the strength of concrete

· OPC used, Grade 40 concrete which requires a minimum 320 Kg cement per cubic metre of concrete.

· Coarse and fine aggregates need to be chosen carefully: natural (to BS 882), crushed air-cooled blast furnace slag (to BS 1047)

· Workability must be sufficient to allow concrete to be fully compacted and finished without undue flow

· Concrete for the top 50 mm must be air-entrained.

Strength of Concrete

· C40 concrete at 7 days should give 31 N/ mm2 (with OPC cement)


Reinforcement

· Hot rolled steel bars or mesh (Grade 250 or Grade 460)

Surface finish

· Must be brush-textured in a direction at right angles to the longitudinal axis of the road

· Applied evenly by wire brush 450 mm wide

· Minimum texture depth should be 0.75 mm, measured between 24 hrs and 7 days after construction


Curing

· Curing essential to prevent protection from evaporation and against heat loss by radiation

· Cured by keeping concrete damp by: covering with plastic sheeting, spray on plastic, spray resin based aluminised curing compound

Joints in Pavements

· Rigid pavement divided into panels by joints in both transverse and longitudinal directions


Transverse Joints

· Formed at right angles to longitudinal axis: see sketches

· Spacing (distance between them) depends on weight of longitudinal reinforcement

· If concreting done in summer 21 April to 21 October then not necessary to provide expansion joints, at other times every 3rd joint must be expansion.

Longitudinal Joints

· Where carriageway is more than 4.2 m wide it is necessary to provide longitudinal joints: see sketches

· The tie bars prevent the joint from opening by more than a fraction of the mm, so maintaining the interlocking of aggregate particles on the two sides of the crack

· Arranged to coincide with lane divisions

· Manholes and gullies need to be treated as localised slabs


Machine Laid Concrete

· Slip-form pavers: conforming plate or oscillating beam models

· Concrete trains


Conforming Plate

· Built around a pair of parallel side forms which are linked together by a horizontal top plate

· Ready mix concrete is fed into the hopper at the front of the machine

· The waterproof underlay is laid from rolls immediately in front of the machine

· As machine moves forward the weight of the concrete in the hopper forces the material under the conforming plate.

· Vibrated by a row of vibrating pokers

· Adjustable to lay pavement widths maximum 13m

· Automatic dowel bar insertion mechanism


Oscillating Beam Slip-form paver

· In this machine side forms give shape and the oscillating beams compact and shape the top of the road slab

· Initial compaction by vibrators in the hopper

· Longitudinal joints cut mechanically as machine proceeds


Control System

· By two tensioned guide lines or wires on each side of slab

· At constant height and parallel to one edge

· Paver has electronic sensor which picks up signals from the wires

· These signals initiate the alterations to line and level


Concrete Trains

· Consist of a series of machines travelling in sequence along prepared side forms

· Jobs must be extensive for economic use

· Large scale batching plants required

· Rails to support the train have to be fixed - over several kilometres ?

· e.g. M6 Preston northwards: 20K, 11m wide c/way, 0.8 km per day, 8 No 10m3 mixer trucks and a batcher capable of producing 8000 tonnes per day.