Shell showed their confidence in the compound by incorporating it in the foundations of the new Shell Nederland Chemie Plant at Moerdijk in Holland where some 9000 precast concrete piles of 15-19m length were coated with the compound, now known in the U.K as BITUMEN COMPOUND SL- its first large-scale use. Since then, BITUMEN COMPOUNS SL has been used internationally for many projects- primarily as a slip layer on piling but also for other buried structures where loadings due to soil movement must be effectively eliminated. A list giving brief details of many of these projects is available on request.

How it functions

BITUNMEN COMPOUND SL is a hot-melt bituminous compound which does not contain solvents. It is applied hot to pile surfaces and piles can be driven as soon as the compound has cooled to ambient temperature.

The material has visco-elastic properties. As a result, it acts like a hard rubber under the short loading times of piling hammer blows. This property enables it to be driven through a variety of soils without significant damage to, or stripping of the coating so that the full thickness remains intact throughout installation of the pile.

Once in position and when subjected to the much slower loadings resulting from consolidating or moving soil, the product assumes fluid properties and shears within itself. It is this viscous shearing action which all but eliminates load transmission through the coating, caused by relative movements between its two surfaces.

It is pointed out however, that use of BITUMEN COMPOUND SL in now way reduces the friction occurring between soil and pile during driving. It functions purely by shearing within itself after the pile has been installed.

Although primarily intended for precast or other driven piling, BITUMEN COMPOUND SL is equally suitable for in-situ piling where application is feasible.

Two grades are available-standard and tropical-covering most conditions except extreme heat and extreme cold.

How effective is it?

A later section deals with calculation of loading transmitted through a layer of BITUMEN COMPOUND SL. As a guide, however, the reduction of downdrag load due to negative skin friction or other soil movement when using an appropriate thickness would exceed 90%.

Downdrag loadings on uncoated surfaces due to soil movement are difficult if not impossible to predict with accuracy and these can in practise considerably exceed the anticipated value. By using BITUMEN COMPOUND SL as a slip layer, the effect of any excess loadings is largely eliminated as the coating would transmit only a small percentage of any extra load. In this way, it constitutes a valuable “Insurance Policy” against the potentially serious effects of loadings beyond the design limits of the piling.

Practical post-installation evidence of the effectiveness of BITUMEN COMPOUND SL is contained in a report on the instrumented performance of coated piles supporting a road bridge abutment at Newhaven, Sussex. Details are given in CIRIA technical Note No 109 and a summary of the relevant part is available from LAYBOND PRODUCTS on request.

Uses

Typical conditions where the use of BITUMEN COMPOUND SL can prove highly effective include:

• Piling subject to negative skin friction and superimposed soil loadings.
• Prevention of structural load transfer from piling to adjacent sensitive underground structures (e.g rail tunnels).
• Prevention of frictional loading on buried surfaces e.g basements, tunnels, old foundations and sheet piling.
• “Clay-heave” situations in piled building foundations.

And many more where relative movement between soil and buried surfaces can cause unacceptable additional loading.

Load transmission through Bitumen Compound SL

Where a pile or other structure is to be coated with BITUMEN COMPOUND SL, the frictional drag caused by moving soil, transmitted through the coating is a function of:

• Temperature of coating (i.e soil temperature).
• Expected rate of settlement or movement of soil.
• Thickness of coating.
• Flow characteristics of the coating under load.

Items 1,2 and 3 are generally relatively easily obtained or assumed. Item 4 is more complex and involves detailed knowledge of the behavior of the Compound under different conditions.

However, in order to avoid complicated calculations for every individual situation, a range of constants (K) which take into account the flow characteristics of the coating, have been calculated. These, when incorporated into the appropriate formula, enable a close approximation of the down-drag or other type of loading transmitted through a coating of BITUMEN COMPOUND SL, to be obtained.

In devising the formula, it has been assumed that there is 100% bond between the soil and the coating and between the coating and the surface to which it is applied. This is the worst situation and anything less than 100% bond at both faces will result in a reduced load transmission from the moving soil through the coating to the structure.

Other structures

Many other situations occur where relative movement between soil and structure creates excessive loadings. In the majority of cases, these loadings can be virtually eliminated by the use of a slip layer of Bitumen COMPOUND SL. Stress transmission calculations are made according to the formula previously indicated.

Suitable piling systems

BITUMEN COMPOUND SL can be used with most piling systems either as they are pr with appropriate modifications.

As a hot-melt compound which solidifies by cooling, it has to be pre-applied before piling is installed. This can be carried out on such piles as:

• Precast concrete (including “shell” piles)
• H-section steel piles
• Circular steel casings
• Steel sheet piling

And is usually applied on site.

It cannot be used on continuous flight auger or similar in-situ piling unless some form of precoated liner can be installed in the shaft prior to concreting. Such a liner need only be substantial enough to support the coating until concreting has been completed.

In-situ vertical surfaces

BITUMEN COMPOUND SL has been successfully applied to in-situ vertical surfaces such as concrete walls and columns and on brick walls and piers (e.g to isolate them from externally superimposed loadings).

A practical technique has been established for this which consists of “casting” panels of BITUMEN COMPOUND SL onto kraft paper then applying them while still warm to the primed surface. Details are available on request.

Installation through hard ground

In most cases, BITUMEN COMPOUND SL is required to be installed through relatively soft, compressible soils which are the ones most likely to create negative skin friction. In some cases, however, the coated piling has to be installed through denser layers of sand/gravel due to the characteristics of the strata concerned.

Firmly adhered BITUMEN COMPOUND SL provides a tough abrasion- resistant coating under hammer-blow loading times and when driven through sand of medium density should suffer little damage except at the leading edges.

The denser the sand or the harder the ground, the greater is the possibility of damage to the coating and where doubt exists, it is safer to drive test piles and /or consider alternative methods of installing the piles. The latter might for example consist of preboring using C.F.A rig and injecting bentonite slurry instead of grout to maintain the bore. The coated piles are then installed through the slurry.

Whatever the conditions, however, where coated piles have to be driven, the most important requirement is that the BITUMEN COMPOUND SL is firmly bonded to the pile surface. It is always wise to test the adhesion before driving the pile.

Typical Specification for inclusion in a Bill of Quantities

The length of pile to be coated shall be ___ extending from ___point to point___

Steel surfaces should be cleaned free of any loose rust and scale. Concrete must have curved sufficiently to achieve its required strength and should be clean and free from laitance. Surfaces to be treated must be dry.

Where priming is required. A priming coat of bitumen primer is to be applied by brush or spray at about 0.2litres/m2. The priming coat must be dry before the BITUMEN COMPOUND SL is applied.

The slip layer shall consist of BITUMEN COMPOUND SL manufactured by Consolidated Coating Ltd, Kaymar House, Budby Road, Cuckney. It shall be heated gently at a temperature not exceeding 180°C until melted, without any localised overheating. It shall be applied by pouring or spreading by trowel or squeegee or by casting in, moulds or by a combination of these methods, whichever is most appropriate to the circumstances. Rotation of circular piles my assist in even distribution.

More than one coat may be required to achieve the necessary thickness. Checks will be made by the Engineer on the thickness of the completed coating which shall be a minimum of ___mm.

Piles should be driven as soon as possible after the coating has cooled to ambient temperature to avoid sagging during prolonged storage of coated piles.

If coating has to be carried out in wet or other weather conditions which may adversely affect adhesion of the slip layer or primer, a weather-proof cover shall be erected over the appropriate length of pile during coating. Certain precautions may need to be taken for hot or cold weather working or transporting coated piles and the recommendations of Consolidated Coatings Ltd should be followed where appropriate.