'Pynford' Stool Method of Underpinning

This method can be used where the existing foundations are in a poor condition and it enables the wall to be underpinned in a continuous run without the need for needles or shoring. The reinforced concrete beam formed by this method may well be adequate to spread the load of the existing wall or it may be used in conjunction with other forms of underpinning such as traditional and jack pile.

Needle and Pile Underpinning

This method of underpinning can be used where the condition of the existing foundation is unsuitable
for traditional or jack pile underpinning techniques. The brickwork above the existing foundation must be in a sound condition since this method relies on the `arching effect' of the brick bonding to transmit the wall loads onto the needles and ultimately to the piles. The piles used with this method are usually small diameter bored piles.

Jack Pile Underpinning

This method can be used when the depth of a suitable bearing capacity subsoil is too deep to make traditional underpinning uneconomic. Jack pile underpinning is quiet, vibration free and flexible since the pile depth can be adjusted to suit subsoil conditions encountered. The existing foundations must be in a good condition since they will have to span over the heads of the pile caps which are cast onto the jack pile heads after the hydraulic jacks have been removed.

Underpinning to Walls

To prevent fracture, damage or settlement of the wall(s) being underpinned the work should always be carried out in short lengths called legs or bays. The length of these bays will depend upon the following factors:

1. Total length of wall to be underpinned.
2. Wall loading.
3. General state of repair and stability of wall and foundation to be underpinned.
4. Nature of subsoil beneath existing foundation.
5. Estimated spanning ability of existing foundation.

Generally suitable bay lengths are:

1000 to 1500 for mass concrete strip foundations supportingwalls of traditional construction.
1500 to 3000 for reinforced concrete strip foundations supporting walls of moderate loading.
In all the cases the total sum of the unsupported lengths of wall should not exceed 25% of the total wall length.
The sequence of bays should be arranged so that working in adjoining bays is avoided until one leg of underpinning has been completed, pinned and cured sufficiently to support the wall above.

Underpinning - General Precautions

Underpinning the main objective of most underpinning work is to transfer the load carried by a foundation from its existing bearing level to a new level at a lower depth. Underpinning techniques can also be used to replace an existing weak foundation. An underpinning operation may be necessary for one or more of the following reasons:

1. Uneven Settlement this could be caused by uneven loading of the building, unequal resistance of the soil action of tree roots or cohesive soil settlement.

2. Increase in Loading this could be due to the addition of an
extra storey or an increase in imposed loadings such as that
which may occur with a change of use.

3. Lowering of Adjacent Ground usually required when
constructing a basement adjacent to existing foundations.

General Precautions before any form of underpinning work iscommenced the following precautions should be taken:

1. Notify adjoining owners of proposed works giving full details and temporary shoring or tying.

2. Carry out a detailed survey of the site, the building to be underpinned and of any other adjoining or adjacent building or structures. A careful record of any defects found should be made and where possible agreed with the adjoining owner(s) before being lodged in a safe place.

3. Indicators or `tell tales' should be fixed over existing cracks so that any subsequent movements can be noted and monitored.

4. If settlement is the reason for the underpinning works a thorough investigation should be carried out to establish the cause and any necessary remedial work put in hand before any underpinning works are started.

5. Before any underpinning work is started the loads on the building to be underpinned should be reduced as much as possible by removing the imposed loads from the floors and installing any props and/or shoring which is required.

6. Any services which are in the vicinity of the proposed underpinning works should be identified, traced, carefully exposed, supported and protected as necessary.

Pneumatic Caissons

These are sometimes called compressed air caissons and are similar in concept to open caissons. They can be used in difficult subsoil conditions below water level and have a pressurised lower working chamber to provide a safe dry working area. Pneumatic caissons can be made of concrete whereby they sink under their own weight or they can be constructed from steel with hollow walls which can be filled with water to act as ballast. These caissons are usually designed to form part of the finished structure.

Typical Caissons Details

These are box-like structures which are similar in concept to cofferdams but they usually form an integral part of the finished structure. They can be economically constructed and installed in water or soil where the depth exceeds 18000 There are 4 basic types of caisson namely:

Installing Steel Sheet Piles

To ensure that the sheet piles are pitched and installed vertically a driving trestle or guide frame is used. These are usually purpose built to accommodate a panel of 10 to 12 pairs of piles. The piles are lifted into position by a crane and driven by means of percussion piling hammer or alternatively they can be pushed into the ground by hydraulic rams acting against the weight of the power pack which is positioned over the heads of the pitched piles.

Steel Sheet Piling

Apart from cofferdam work steel sheet can be used as a conventional timbering material in excavations and to form permanent retaining walls. Three common formats of steel sheet piles with interlocking joints are available with a range of section sizes and strengths up to a usual maximum length of 18000

Typical Cofferdam Details

These are temporary enclosures installed in soil or water to prevent the ingress of soil and/or water into the working area with the cofferdam. They are usually constructed from interlocking steel sheet piles which are suitably braced or tied back with ground anchors. Alternatively a cofferdam can be installed using any structural material which will fulfil the required function.

Concrete Production - Specification

A composite with many variables, represented by numerous gradings which indicate components, quality and manufacturing control.

Grade mixes: C7.5, C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, and C60; F3, F4 and F5; IT2, IT2.5, and IT3.

1. Standard Mix - BS guidelines provide this for minor works or in situations limited by available material and manufacturing data. Volume or weight batching is appropriate, but no grade over C30 is recognised.

2. Prescribed Mix - components are predetermined (to a recipe) to ensure strength requirements. Variations exist to allow the purchaser to specify particular aggregates, admixtures and colours. All grades permitted.

3. Designed Mix - concrete is specified to an expected performance. Criteria can include characteristic strength, durability and workability, to which a concrete manufacturer will design and supply an appropriate mix. All grades permitted.

4. Designated Mix - selected for specific applications. General (GEN) graded 0-4, 7.5-25 N/mm2 for foundations, floors and external works. Foundations (FND) graded 2, 3, 4A and 4B, 35 N/mm2 mainly for sulphate resisting foundations.

Paving (PAV) graded 1 or 2, 35 or 45 N/mm for roads and drives.

Reinforced (RC) graded 30, 35, 40, 45 and 50 N/mm2 mainly for prestressing.

See also BS EN 206-1: Concrete. Specification, performance, production and conformity, and BS's 8500-1 and -2: Concrete.

Concrete Production - Weight (Weigh) Batching

This is a more accurate method of measuring materials for concrete than volume batching since it reduces considerably the risk of variation between different batches. The weight of sand is affected very little by its dampness which in turn leads to greater accuracy in proportioning materials. When loading a weighing hopper the materials should be loaded in a specific order:

1. Coarse aggregates - tends to push other materials out and leaves the hopper clean.
2. Cement - this is sandwiched between the other materials ,since some of the fine cement particles could be blown away if cement is put in last.
3. Sand or fine Aggregates - put in last to stabilise the fine lightweight particles of cement powder.

Typical Densities: cement - 1440 kg/m3 sand - 1600 kg/m3 coarse aggregate - 1440 kg/m3

Water/Cement Ratio: water in concrete has two functions

1. Start the chemical reaction which causes the mixture to set into a solid mass.
2. Give the mix workability so that it can be placed, tamped or
vibrated into the required position.

Very little water is required to set concrete (approximately 0.2 w/c ratio) the surplus evaporates leaving minute voids therefore the more water added to the mix to increase its workability the weaker mis the resultant concrete. Generally w/c ratios of 0.4 to 0.5 are madequate for most purposes.

Concrete Production - Volume Batching

Concrete Batching: a batch is one mixing of concrete and can be carried out by measuring the quantities of materials required by volume or weight. The main aim of both methods is to ensure that all consecutive batches are of the same standard and quality.

Volume Batching: concrete mixes are often quoted by ratio such as 1 : 2 : 4 (cement : fine aggregate or sand : coarse aggregate). Cement weighing 50 kg has a volume of 0.033 m3 therefore for the above mix 2 x 0.033 (0.066 m3) of sand and 4 x 0.033 (0.132 m3) of coarse aggregate is required. To ensure accurate amounts of materials are used for each batch a gauge box should be employed its size being based on convenient handling. Ideally a batch of concrete should be equated to using 50 kg of cement per batch. Assuming a gauge box 300 mm deep and 300 mm wide with a volume of half the required sand the gauge box size would be - volume = length x width x depth = length x 300 x 300

For the above given mix fill gauge box once with cement, twice with sand and four times with coarse aggregate.

An allowance must be made for the bulking of damp sand which can be as much as 331/3%. General rule of thumb unless using dry sand allow for 25% bulking.

Materials should be well mixed dry before adding water.

Deep Basement Construction

Basements can be constructed within a cofferdam or other temporary supported excavation up to the point when these methods become uneconomic, unacceptable or both due to the amount of necessary temporary support work. Deep basements can be constructed by installing diaphragm walls within a trench and providing permanent support with ground anchors or by using the permanent lateral support given by the internal floor during the excavation period (see next page). Temporary lateral support during the excavation period can be provided by lattice beams spanning between the diaphragm walls

NB. vertical ground anchors installed through the lowest floor can be used to overcome any tendency to flotation during the construction period

Basement Construction

In the general context of buildings a basement can be defined as a storey which is below the ground storey and is therefore constructed below ground level. Most basements can be classified into one of three groups.

Complete Excavation

This method can be used in firm subsoils where the centre of the proposed basement can be excavated first to enable the basement slab to be cast thus giving protection to the subsoil at formation level. The sides of excavation to the perimeter of the basement can be supported from the formation level using raking struts or by using raking struts pitched from the edge of the basement slab.

Basement Excavations - Perimeter Trench Excavations

In this method a trench wide enough for the basement walls to be constructed is excavated and supported with timbering as required. It may be necessary for runners or steel sheet piling to be driven ahead of the excavation work. This method can be used where weak subsoils are encountered so that the basement walls act as permanent timbering whilst the mound or dumpling is excavated and the base slab cast. Perimeter trench excavations can also be employed in firm subsoils when the mechanical plant required for excavating the dumpling is not available at the right time.

Basement Excavations Open

One of the main problems which can be encountered with basement excavations is the need to provide temporary support or timbering to the sides of the excavation. This can be intrusive when the actual construction of the basement floor and walls is being carried out. One method is to use battered excavation sides cut back to a safe angle of repose thus eliminating the need for temporary support works to the sides of the excavation.

In economic terms the costs of plant and manpower to cover the extra excavation, backfilling and consolidating must be offset by the savings made by omitting the temporary support works to the sides of the excavation. The main disadvantage of this method is the large amount of free site space required.

Gabions and Mattresses

Gabion: a type of retaining wall produced from individua rectangular boxes made from panels of wire mesh, divided internally and filled with stones. These units are stacked and overlapped (like stretcher bonded masonry) and applied in severa layers or courses to retained earth situations. Typical sizes, 1.0 m long x 0.5 m wide x 0.5 m high, up to 4.0 m long x 1.0 m wide x 1.0 m high.

Mattress: unit fabrication is similar to a gabion but of less thickness, smaller mesh and stone size to provide some flexibility and shaping potential. Application is at a much lower incline Generally used next to waterways for protection against land erosion where tidal movement and/or water level differentials could scour embankments. Typical sizes, 3.0 m long x 2.0 m wide x 0.15 m thick, up to 6.0 m long x 2.0 m wide x 0.3 m thick.

Types of Soil Nails or Tendons

A cost effective geotechnic process used for retaining large soil slopes, notably highway and railway
embankments.

Function: After excavating and removing the natural slope msupport, the remaining wedge of exposed unstable soil is pinned or mnailed back with tendons into stable soil behind the potential slip plane.

Types of Soil Nails or Tendons:

• Solid deformed steel rods up to 50 mm in diameter, located in bore holes up to 100 mm in diameter. Cement grout is pressurised into the void around the rods.
• Hollow steel, typically 100 mm diameter tubes with an expendable auger attached. Cement grout is injected into the tube during boring to be ejected through purpose-made holes in the auger.
• Solid glass reinforced plastic (GRP) with resin grouts. Embankment Treatment ~ the exposed surface is faced with a plastic coated wire mesh to fit over the ends of the tendons. A steel head plate is fitted over and centrally bolted to each projecting tendon, followed by spray concreting to the whole face.

Crib Retaining Walls

A system of pre-cast concrete or treated timber components comprising headers and stretchers which interlock to form a three-dimensional framework. During assembly the framework is filled with graded stone to create sufficient mass to withstand ground pressures.

Retaining Walls - Construction

A reinforced concrete base is cast with projecting steel bars accurately located for vertical continuity. The wall may be built solid, e.g. Quetta bond, with voids left around the bars for subsequent grouting. Alternatively, the wall may be of wide cavity construction, where the exposed reinforcement is wrapped in 'denso' grease tape for protection against corrosion. Steel bars are threaded at the top to take a tensioning nut over a bearing plate.

Staged post-tensioning to high masonry retaining walls

Retaining Walls - Masonry Units

These are an option where it is impractical or cost-ineffective to use temporary formwork to in-situ concrete. Exposed brick or blockwork may also be a preferred finish. In addition to being a structural component, masonry units provide permanent formwork to reinforced concrete poured into the voids created by:

- Quetta bonded standard brick units, OR
- Stretcher bonded standard hollow dense concrete blocks.

Climbing Formwork or Lift Casting

This method can be employed on long walls, high walls or where the amount of concrete which can be placed in a shift is limited.

Cantilever Retaining Walls

These are constructed of reinforced concrete with an economic height range of 1200 to 6000. They work on the principles of leverage where the stem is designed as a cantilever fixed at the base and base is designed as a cantilever fixed at the stem. Several formats are possible and in most cases a beam is placed below the base to increase the total passive resistance to sliding.

Mass Retaining Walls

These walls rely mainly on their own mass to overcome the tendency to slide forwards. Mass retaining walls are not generally considered to be economic over a height of 1800 when constructed of brick or concrete and 1000 high in the case of natural stonework. Any mass retaining wall can be faced with another material but generally any applied facing will not increase the strength of the wall and is therefore only used for aesthetic reasons.

Earth Pressures - Retaining Walls

1. Active Earth Pressures - these are those pressures which tend to move the wall at all times and consist of the wedge of earth retained plus any hydrostatic pressure. The latter can be reduced by including a subsoil drainage system behind and/or through the wall.

2. Passive Earth Pressures - these are a reaction of an equal and opposite force to any imposed pressure thus giving stability by resisting movement.

Retaining Walls up to 6-000 High

These can be classified as medium height retaining walls and have the primary function of retaining soils at an angle in excess of the soil's natural angle of repose. Walls within this height range are designed to provide the necessary resistance by either their own mass or by the principles of leverage.

Design the actual design calculations are usually carried out by a structural engineer who endeavours to ensure that:

1. Overturning of the wall does not occur.
2. Forward sliding of the wall does not occur.
3. Materials used are suitable and not overstressed.
4. The subsoil is not overloaded.
5. In clay subsoils slip circle failure does not occur.

The factors which the designer will have to take into account:

1. Nature and characteristics of the subsoil(s).
2. Height of water table † the presence of water can create hydrostatic pressure on the rear face of the wall, it can also affect the bearing capacity of the subsoil together with its shear strength, reduce the frictional resistance between the underside of the foundation and the subsoil and reduce the passive pressure in front of the toe of the wall.
3. Type of wall.
4. Material(s) to be used in the construction of the wall.

Small Height Retaining Walls

Retaining Walls up to 1m High-2: Retaining walls must be stable and the usual rule of thumb for small height brick retaining walls is for the height to lie between 2 and 4 times the wall thickness. Stability can be checked by applying the middle third rule.

Retaining Walls up to 1m High

The major function of any retaining wall is to act as on earth retaining structure for the whole or part of its height on one face, the other being exposed to the elements. Most small height retaining walls are built entirely of brickwork or a combination of brick facing and blockwork or mass concrete backing. To reduce hydrostatic pressure on the wall from ground water an adequate drainage system in the form of weep holes should be used, alternatively subsoil drainage behind the wall could be employed.

Pile Testing

It is advisable to test load at least one pile per scheme. The test pile should be overloaded by at least 50% of its working load and this load should be held for 24 hours. The test pile should not form part of the actual foundations. Suitable testing methods are:

1 . Jacking against kentledge placed over test pile.
2. Jacking against a beam fixed to anchor piles driven in on two sides of the test pile.

Pile Caps

Piles can be used singly to support the load but often it is more economical to use piles in groups or clusters linked together with a reinforced concrete cap. The pile caps can also be linked together with reinforced concrete ground beams. The usual minimum spacing for piles is:

1 . Friction Piles † 1„100 or not less than 3 x pile diameter, whichever is the greater.

2. Bearing Piles † 750 mm or not less than 2 x pile diameter, whichever is the greater.

Cast In-situ Piles

An alternative to the driven in-situ piles.

Driven In-situ Piles

Used on medium to large contracts as an alternative to preformed piles particularly where final length of pile is a variable to be determined on site.

Steel Tube Piles

Used on small to medium size contracts for marine structures and foundations in soft subsoils over a suitable bearing strata. Tube piles are usually bottom driven with an internal drop hammer. The loading can be carried by the tube alone but it is usual to fill the tube with mass concrete to form a composite pile. Reinforcement, except for pile cap bonding bars, is not normally required.

Steel Screw Piles

Rotary driven and used for dock and jetty works where support at shallow depths in soft silts and sands is required.

Steel Box and `H' Sections

Standard steel sheet pile sections can be used to form box section piles whereas the 'H' section piles are cut from standard rolled sections. These piles are percussion driven and are used mainly in connection with marine structures.

Preformed Concrete Piles

Jointing with a peripheral steel splicing collar as shown on the preceding page is adequate for most concentrically or directly loaded situations. Where very long piles are to be used and/or high stresses due to compression, tension and bending from the superstructure or the ground conditions are anticipated, the 4 or 8 lock pile joint [AARSLEFF PILING] may be considered.

Preformed Concrete Piles

Variety of types available which are generally used on medium to large contracts of not less than one hundred piles where soft soil deposits overlie a firmer strata. These piles are percussion driven using a drop or single acting hammer.

Timber Piles

These are usually square sawn and can be used for small contracts on sites with shallow alluvial deposits overlying a suitable bearing strata (e.g. river banks and estuaries.) Timber piles are percussion driven.

Displacement Piles

These are often called driven piles since they are usually driven into the ground displacing the earth around the pile shaft. These piles can be either preformed or partially preformed if they are not cast in-situ and are available in a wide variety of types and materials. The pile or forming tube is driven into the required position to a predetermined depth or to the required `set' which is a measure of the subsoils resistance to the penetration of the pile and hence its bearing capacity by noting the amount of penetration obtained by a fixed number of hammer blows.

Grout Injection Piling

A variation of continuous flight auger bored piling that uses an open ended hollow core to the flight. After boring to the required depth, high slump concrete is pumped through the hollow stem as the auger is retracted. Spoil is displaced at the surface and removed manually. In most applications there is no need to line the boreholes, as the subsoil has little time to be disturbed. A preformed reinforcement cage is pushed into the wet concrete.

Replacement Piles

These are often called bored piles since the removal of the spoil to form the hole for the pile is always carried out by a boring technique. They are used primarily in cohesive subsoils for the formation of friction piles and when forming pile foundations close to existing buildings where the allowable amount of noise and/or vibration is limited.

Piled Foundations - Classification

These can be defined as a series of columns constructed or inserted into the ground to transmit the load(s) of a structure to a lower level of subsoil. Piled foundations can be used when suitable foundation conditions are not present at or near ground level making the use of deep traditional foundations uneconomic. The lack of suitable foundation conditions may be caused by:

1. Natural low bearing capacity of subsoil.
2. High water table - giving rise to high permanent dewatering costs.
3. Presence of layers of highly compressible subsoils such as peat and recently placed filling materials which have not sufficiently consolidated.
4. Subsoils which may be subject to moisture movement or plastic failure.

Classification of Piles, piles may be classified by their basic design function or by their method of construction: