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.