Installation Conditions
16.1 Trench conditions
These installations are suitable for SINTAKOTE welded steel pipelines and SINTAJOINT rubber ring joint (RRJ) steel pipelines as described.
The following terms and their definitions are referred to in this section. See Figure 16.1.
Bedding - the zone between the foundation and the bottom of the pipe
Haunch support – the part of the side support below the spring line of the pipe
Side Support - the zone between the bottom and the top of the pipe
Overlay – the zone between the side support and either the trench fill or the embankment fill
Trench Fill – fill material placed over the overlay for the purpose of filling the trench
Trench width
The trench width should be as narrow as practicable consistent with the need to ensure:
• Proper laying and jointing of the pipe • Application of joint wrapping if relevant
• Where a change of direction is being made using the lateral deflection permissible at the joints, the trench should be sufficiently wide to allow the joint to be made in line and then the pipe laterally deflected
• Where the virgin soil does not provide the pipe with the required side support, the trench must be wide enough to allow the selected back-fill to be placed and compacted in such a manner which will adequately spread the load into the surrounding ground Common size backhoe/excavator bucket widths are 300, 450, 600, 750, 900, 1100 and 1200mm.
As a guide, the following trench minimum widths are reasonable: OD + 400mm for pipe diameters ≤ 450mm
OD + 600mm for pipe diameters > 450mm, ≤ 900mm OD + 700mm for pipe diameters > 900mm, ≤ 1500mm OD + 0.5 x OD mm for pipe diameters > 1500mm, ≤ 4000mm
Trench depth
The depth of the trench will depend on a number of factors apart from pipe diameter. Other considerations include:
• External loadings. Pipes usually have a greater depth of cover when subject to vehicle loading
• Location of other services, particularly in urban areas
• Future change in levels due to road re-grading or other civil works The minimum depth of cover recommended is 0.6m provided none of the other considerations require a greater depth. In rock, the trench should be excavated to ensure that at least 50mm of compacted bedding is achieved under the pipe after it is bedded. Where an unstable sub-grade condition unable to support the pipe is encountered, an additional depth should be excavated and backfilled as discussed below.
Bedding
Bedding provides support to the pipeline enabling it to withstand external loads. The higher the external loading (depth of trench plus any vehicle loading) the greater the degree of care necessary with the backfill in this zone. Any part of the trench excavated below grade unintentionally or because of rocky ground should be backfilled to grade with a thoroughly compacted approved material.
In the case of additional depth due to unstable sub-grade the extra depth should be backfilled with crushed stone or other suitable material to achieve a satisfactory trench bottom.
For open field loading where traffic and superimposed loading will be low, the bedding angle (total depth of bedding) can be limited to approximately 70°. For roadways or heavy traffic and
superimposed loads total depth of compacted bedding may need to be increase to the spring line (centre line) of the pipe to increase the bedding angle to 180°, maximise support and minimise deflection. See Spangler & Handy (ref 3).
In order to prevent damage to SINTAKOTE a compacted zone of 50 mm below the pipe should comprise non-cohesive native soil, imported fill or sand such that the maximum particle size does not exceed 13.2 mm.
SINTAJOINT ( RRJ ) pipelines
Bellholes should be excavated in the foundation to prevent the socket from bearing on the foundation.
SINTAKOTE welded joint pipelines
Construction holes should be excavated at the joint to facilitate welding and coating reinstatement. Bedding should then be restored. Haunch support, side support and overlay
It is essential that backfill for haunch support, side support and overlays be well compacted between the sides of the pipe and the trench. Particular care should be taken in compacting the material under the haunches of the pipe.
pipe. Whilst the depth of such layers should be established at the commencement of the laying for any particular material to be used, it should not normally exceed 150 mm. Backfilling in layers should proceed until 150 mm above the top of the pipe or as otherwise specified where vehicular traffic is encountered.
Backfill provides material to support the pipe and prevent sharp objects imparting high loads onto the pipeline coating. The material used should be non-cohesive native soil with no particles larger than 25mm, or imported sand or gravel of nominal size not larger than 20mm with the maximum size not to exceed 25mm.
When select backfill or bedding is used with pipes which are to be cathodically protected, the material should not be too high in electrical resistivity as this will reduce the effectiveness of the protection. Generally, sand or native soil is suitable. Stone and gravel can be too high in resistivity. Hence a graded mix of sand and gravel should be used on cathodically protected lines where imported backfill is required.
Compaction should achieve the effective combined soil modulus E’. Non-cohesive soils
Cohesionless soils are often specified for bedding and side support areas of buried conduits.
They offer the advantages of • ease of placement and handling • minimum compactive effort • free draining behaviour • minimum settlement • non shear stress memory
• maximum density over a wide moisture content range • high shear strength
• anchorage friction
For situations where trench water flow is possible, cross trench dams keyed into trench walls should be constructed to prevent erosion of backfill and bedding.
Trench fill
The trench can then be topped up with convenient fill. Where necessary it should be compacted to achieve the appropriate relative density for pavement support. The extent of compaction depends on the allowable future surface settlement. Under roads, pavements and in certain other areas the load bearing capacity of the ground surface is important and fill must be compacted in layers all the way to the surface.
Where the trench is across open land the compaction requirements are not normally so important and the surface can usually be built up to a degree to allow for some future settlement.
The material used would normally be the excavated trench material but where a high degree of compaction is needed in poor natural ground, imported material may be required.
16.2 Compaction
Compaction increases the density of the soil resulting in greater bearing capacity, stability and reduced permeability and settlement. Void space is reduced and interparticle contact is increased resulting in higher internal friction.
Generally, non cohesive soils require less compactive effort to achieve a given density as the interparticle cohesive forces to be overcome in rearranging the soil are a minimum.
Vibratory compaction uses equipment which incorporates vibration, normally by means of a rotating eccentric weight. The vibration
Figure 16.1 - Definition of terms
Trench wall Embedment zone Springline of pipe Finished surface Trench Embankment Trench fill Overlay Side support Bedding Haunch
support Haunchsupport
Embedment zone Springline of pipe Top of embankment (finished surface) Embankment fill CL CL
jostles adjacent particles and allows their relative movement to settle together in a denser state.
The three main factors to be considered in compaction are: • soil type
• moisture content
• compaction method and energy input
Soil classification
A commonly referred system of soil classification is the United Soil Classification System (USCS). Soils are categorised by this system in 15 groups identified by name and letter symbols. (ref Table 13.2)
Gradation
The gradation of a soil is a measure of the size and distribution of the constituent particles. This is assessed by sieving the sample through a series of screens of increasing fineness. The retained material on each screen is expressed as a percentage of total sample weight. These figures, when plotted on a graph show the gradation of the material. Refer to Graph 16.1.
A well graded material covers a wide range of particles filled by smaller ones. Higher densities are more easily achieved with well graded materials than uniformly graded materials.
Density index - non cohesive soils
Density index IDis a measure of compaction used for non cohesive (low fines) soils, and is specified in AS 1289.5.6.1 as:
ID= γmax (γ – γmin) x 100%
γ (γmax – γmin)
where
γmax = maximum dry soil density kN/m3
γmin = minimum dry soil density kN/m3
γ = measured dry soil density kN/m3
= 100ϕ (100 + wt)
ϕ = measured wet soil density kN/m3
wt = measured soil moisture content %
γminis determined by drying and pulverising the soil to a single grain
size and pouring with minimum disturbance into a container of a known volume. The sample is then weighed and γmindetermined. γmaxis determined after compaction with a drop hammer, tamper
or vibrator.
Dry density ratio - cohesive soils
Dry density ratio (RD) is a measure of compaction used for cohesive soils and is specified in AS 1289.5.4.1 as:
RD= 100γ γr Where:
γ = measured dry soil density kN/m3
γr= maximum dry density (adjusted for oversize material,
where applicable) kN/m3
as assigned or determined in the compaction test. Compaction equipment
The most common forms of compaction equipment used in pipeline construction are vibratory plate compactors and vibratory tampers. Their use depends very much on the surface loads to be carried by the installation. This load carrying capacity depends on the structural stiffness of the pipe and the degree of soil bedding and side support compaction achieved.
Very dry sand and gravel can be vibrated into place at a density of over 90% providing it contains little or no silt.
Sluicing and dumping
Where the material is of a granular nature and drains quickly, an alternative to using compacting equipment is to flood the backfill with water. Using this method with coarse sand a 60% relative density can be achieved, whilst with fine sand a 50% relative density should be attained.
100 90 80 70 60 50 40 30 20 Per cent passing 0.0001 0.001 0.01 0.1 1 10 100 Particle size in mm
Clay Silt Sand Gravel Cobbles Silt Clay Uniformly graded sand Well graded sand Road base material U.S. standard sieves
40 10 1.1/2" 200 100 50 30 16 8 4 3/8"3/4" 3" 6" 12"
124 |S E C T I O N 1 6
Clean selected aggregate will usually achieve 60% relative density or better by simply dumping around the pipe.