Everything a civil engineer designs and constructs rests on, or in, the earth. Soil mechanics play a pivotal role in laying pipes.By Mike Smart Engineering soils are produced by the erosion of the earth’s crust. They change in character by being moved from the source. Particle shapes change from angular to rounded, and are then sorted and graded. Water is the most productive of all soil-forming agencies (see Arthur Casagrande’s Soil Classification). Classification of engineering soils The classification of engineering soils is a complex and exacting science that requires extensive investigation, testing and analysis to precisely define a given soil (see COLTO classifications G1 to G9). There are four basic methods of designing a road structure. These methods, based on AASHTO Soil Classification, are not applicable to bedding, embedment and backfill of pipelines:
- Group Index Method: US Highway Engineers’ empirical quantity calculated from the soil’s grading and Atterberg limits.
- Shear Strength Method: links it to theoretical stress imposed on pavement and subgrade that ignores repeated loading effects.
- Californian Bearing Ratio (CBR) Method: subgrade empirical property measured by empirical penetration test.
- Modulus of Elasticity Method: considers this the critical soil property.
- coarse-grained or non-cohesive soils
- fine-grained or cohesive soils
- organic soils (peat); extremely treacherous and troublesome.
- Effective size – maximum particle-size of the smallest 10%.
- Uniformity coefficient – ratio of the maximum of the smallest 60% to the effective size.
- 90% Modified AASHTO for bedding
- 93% Modified AASHTO in traffic areas for cohesive soils
- 98% Modified AASHTO in traffic areas for non-cohesive soils.
It is imperative that the compaction of the embedment supporting the haunch of the pipe is conforming to enable the compaction of the embedment at the ‘half-pipe level’ to be conforming to enable it to transfer loads on the pipe to the trench walls, to limit the deflection to the predetermined design magnitude. The ETH’s (Swiss Federal Institute of Technology Zurich) IGT (Institute of Geotechnical Engineering) produced a technical report in December 2004, ‘Correlation between the values of Compaction AASHTO-Standard and AASHTO-Modified’. The results of which are reproduced in Graph 1. Graph 1 Correlation Standard AASHTO and Modified AASHTO η = 0.95 = SC Well graded sand-clay mixture; excellent binder η = 0.97 = GP Poorly graded gravel/gravel-sand; little or no fines η = 0.97 = GW Well-graded gravel/gravel-sand; little or no fines <highlighted copy to go with graph> The relationship between the compaction factors is dependent upon the classification of the soil being compacted. A Casagrande Classification SC material will achieve 95% Standard Proctor Compaction with 90% Mod. AASHTO Compaction. Different classification soils will have different ratios for the two compaction methods. A comparison of the compactive effort of the test methods is shown in Table 2. Table 2 Standard Proctor and Mod. AASHTO Tests Comparison
|No. of Lifts
|No. of Blows
|ASTM D698 AASHTO T99
|ASTM D1557 AASHTO T180
Note: Both tests use a cylindrical mould, 4 inches diameter of 1/30 ft³ volume. Both samples receive equal energy if 56 blows are applied in Standard Proctor. Well-graded soils compact better than poorly graded soils. Moisture content in excess of optimum moisture content reduces dry density because the pore water pressure pushes the soil particles apart increasing the volume between them. Thermoplastic SWP manufacturers must understand, when the pipe has been manufactured, delivered and joined in the trench approximately half the work to construct a pipeline has been done. The other half is the construction of the ‘soil-pipe’ structure on-site that must be conforming if the pipeline is to function as designed. The SANS 2001-DP1 must be revised to take cognisance of the requirements of thermoplastic pipelines by replacing Modified AASHTO with appropriate Standard Proctor values. The thermoplastic SWP requirement, that a ‘soil-pipe’ structure is constructed, must be explicitly expressed in contract documents to establish its importance to the conformance and performance of the pipeline. The bedding, embedment and compaction of the pipeline is an integral part of the pipeline’s construction to enable it to deliver its designed service life, of not less than 50 years, as a sustainable buried conduit.
|56 per layer
|25 per layer