Caracterización general de CMMI

The typical drawing of the embedment method for flexible pipe as described in Australian Standard AS/NZS 2566.2:2002 – Installation for Buried Flexible Pipelines is shown in Figure 8.7.

The embedment material for the flexible pipe shall be in accordance with AS/NZS 2566.2:2002 which:

1. Comply with the maximum particles size of Table 8.4 and be of particle size and grading that will allow the specified relative compaction to be achieved with the intended compaction methods;

Table 8.4: Maximum Particle Size of Embedment Material for Flexible Pipeline

Nominal pipe diameter,

2. Contain no organic material that will affect embedment material performance;

3. Be free of materials that would be physically and chemically harmful to any pipeline component, including any protective coating; and

4. For unprotected metallic pipes, be a granular fill with a resistivity greater than 50 ohm.m.

The higher the granular content of the embedment material, particularly higher gravel content, the more supportive it becomes to the pipe, where an equivalent compactive method is used.

Sharp granular embedment material should not be used with some pressure pipe materials as it can either score the external surface of the pipe, or damage the protective coatings and sleeving.

The materials for flexible pipeline embedment shall be of the following:

1. Native soils, especially cohesionless soils containing of sand or coarse-grained soil with less than 12% fines and particles size less than 20mm. Sand prone to ‘bulk’ when moist shall be avoided. Whatever materials are used, ensure that the required density is to be met.

2. Imported cohesionless material containing of processed aggregates with nominal size of graded aggregate not more than 20mm. Graded aggregates are considered more susceptible to segregation in transport and handling. Particular care shall be taken to remix or wash this material to minimize the effect of segregation.

3. Other materials containing the gravel gradings of:

a. Well graded crushed rock with particles size not more than 20mm;

b. Crushed rock dust with particles size not more than 10mm; and c. Sand with particles size not more than 5 mm.

The application of this type of material embedment is to facilitate the achievement of the soil moduli.

Where there is a possibility of migration of fines between the native soil and the embedment zone, a geotextile filter fabric, such as non-woven fabric made from filaments of synthetic fibres shall be provided to ensure that the integrity of the side support to the pipe is not compromised.

The compacted bedding material surface shall be continuous, smooth and free of stones larger than those 20mm, so as to provide a uniform support to the pipe. Following grading, and where required compaction, pockets for sockets, couplings, flanges or other projections shall be excavated in the bedding material, so as to ensure the pipeline is fully supported along the pipe barrels. The bedding shall be provided with joint holes to ensure that the pipe rests on the barrel and not the joint.

The side support and overlay material shall be placed in layers of appropriate thickness for the method of compaction, to achieve the relative compaction or soil modulus specified. The side support material shall be brought up evenly on each side of the pipe.

Ordinary fill shall be a material obtained from the excavation or imported and containing not more than 20% by mass of rock with size between 75mm and 150mm and none larger than 150mm.

Control of the relative compaction of soils in the side support zone during pipe installation is the usual means for ensuring the soil moduli will be at least equal to those assumed at design stage.

Each layer of the embedment material shall be compacted as recommended in Table 8.5 below in accordance with AS/NZS 2566.2:2002.

Table 8.5: Minimum Relative Compaction of Embedment Material for Flexible Pipeline

Trafficable Areas Non-Trafficable Areas Soil Type Test Method Embedment

Material %

Embedment material is to be compacted in layers not exceeding 250 mm or 0.4 of the pipe external diameter, whichever is less. Bedding is to be compacted with a vibrating plate.

Replacement of embedment material by alternative structural support may be required in the following circumstances:

a. Pipes are laid on steep grades;

b. Forces, due to hydrostatic or hydrodynamic pressure, may not be contained by the embedment material;

c. The foundation for the pipeline is inadequate;

d. The embedment material or native soil support may be washed away.

The concrete encasement shall be considered as an alternative embedment material where:

a. Gradients are 30% or greater;

b. Additional embedment material stiffness is required;

c. The trench foundation is inadequate;

d. Buoyancy considerations could result in excessive uplift forces; and e. The risk of erosion is high (such as through water course)

The requirements of the pipeline embedment for flexible pipe shall be in accordance with the Australian Standard AS/NZS 2566.2:2002 - Installation for Buried Flexible Pipelines.

8.3.3 DI Pipe

8.3.3.1 Pipeline Structural Design

The structural design sewer pipeline used with ductile iron pipe is referred to as flexible pipe with high ring stiffness design. The design combines elements of rigid pipe design and flexible pipe design.

The basis of structural pipeline design for DI pipe is to provide a high degree of security for the pipeline during its operating life. It is dependent on the factors of:

1. Pipe ovalization – is proportionately to diameter of pipe. It is limited to 4% for DN ≥ 800 with safety factor of 1.5 at minimum elastic limit in bending of 500MPa and maximum stress in the pipe wall of 330 MPa.

2. Pressure from earth loading;

3. Pressure from traffic loading;

4. Bedding factor – depends upon the soil pressure distribution at the top of the pipe;

5. Factor of lateral pressure;

6. Modulus of soil reaction – depends upon the nature of soil used in the pipe zone and upon the laying condition; and

7. Heights of cover.

The failure mode of ductile iron pipe is from fracture through ring bending. The ductile iron cross section can deform considerably before fracture, but not as great a deformation as that with plastic pipe. Sufficient load provided over the pipe can help to provide some resistance to the pipe deformation. For the loading situations normally encountered for sewerage pipelines, the side support plays a negligible role in resisting the overburden loadings.

The cement lining of ductile iron pipe can suffer cracking with small deformations of the pipe cross section. Hence, the limit placed on the vertical deflection due to cement lining governs the design and overrides any stress limit in the ductile iron due to bending.

This deflection limit, however, although low compared to thermoplastic pipe, is not expected to be exceeded under the worst loading conditions for sewerage. It is therefore only necessary to undertake a structural design analysis for unusual loadings such as when a high load is superimposed over a pipeline with shallow cover.

Structural design of DI pipe is to be performed in accordance with the British Standards BS 8010-2.1:1987 – Code of Practice for Ductile Iron Pipelines, BS EN 598:1995 (Annex C) – Ductile Iron Pipes, Fittings, Accessories and their Joints for Sewerage Applications.

Other than what has been required here, the structural design of DI Pipe shall be in accordance with AS 2566.1-1992 as described in Clause 8.3.1.

8.3.3.2 Pipeline Embedment

The main functions of the embedment with ductile iron pipe are to ensure that the pipeline retains any specified gradient and that the external corrosion protection system is not exposed to abrasion with sufficient coating or tear by providing a sleeving.

The DI pipe is a flexible pipe that has a very high stiffness of most diameters, with maximum allowable vertical deflection of 1.5%. This high ring stiffness means that the pipe does not have a need for the embedment to function as side support unlike other more flexible pipe such as plastic pipe. However, the bedding is still required to give uniform continuous support to the pipeline.

Crushed rock, even though it fulfils the side support requirements, is unsuitable as sharp rock edges can damage the external coating. Cohesive materials (e.g. clays, sandy clays, and silty clays) will not damage the coating but are difficult to place and compact to achieve sufficient side support. The preferred embedment medium is sand.

The requirements of the pipeline embedment for DI Pipe other than what is required here shall be in accordance with the Australian Standard AS/NZS 2566.2:2002 as described in Section 8.3.2.