Borrow Pits In Construction Of Road Networks Engineering Essay

Over the last two decade there has been an unprecedented boom in the Irish construction industry in road infrastructure. It has become a common sight on Irish roads to see countless Lorries carrying materials to these sites ranging from crushed rock, sand, gravel and tarmac. At present these materials are formed from primary aggregates. The use of these primary materials reduces the natural resources of existing quarries. An alternative to this, is the use of borrow pits which will help in the supply of materials requirements both in general fill and aggregates which may help to avoid the use of reserves from established quarries which will help increase the lifespan of these quarries. As with the supply of material, borrow pits could also be used for recycling purposes also because most jobs would have inert materials that would have to be discarded of site to landfill.

The term’ borrow pit’ is generally utilized by the Contractor when a project requires a larger amount of fill material versus amount of usable material obtained from cut

sections. Borrow pits are typically located next to the construction site, and in the ideal situation are soon backfilled with waste materials, such as soft clay, that often have to be removed from the construction area – hence the material excavated is ‘borrowed’. Normally, large quantities of material, mainly bulk fill, are required over a short time.

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The author first became involved in the construction of road infrastructure during the construction of the N18 N19 Ballycasey to Dromland Dual Carriageway in early 2000 where there was a lot of import material brought into to the job from nearby quarries and materials that had to go to landfill.

From the authors initial research and work experience on various road jobs it was apparent that the use of borrow pits to provide an alternative source of material would be a worthy dissertation topic and that the knowledge gained would be invaluable for future use.

Hypothesis

The key question that this dissertation will address is

“Could the use of Borrow pits in the construction of the road network be a benefit”

Aim

To determine if Borrow pits can provide the necessary quality material required for the construction of roads and also provide alternative options for the disposal of inert waste produced on sites other than to landfill.

Outline of chapters

This dissertation is structured into and comprises of six chapters, which are briefly described as follows. Chapter 1 introduces the reader to the dissertation. Its shows the particular reasons why this topic was chosen. This chapter also outlines the aim, objectives and hypothesis of the dissertation. In chapter 2, the methodologies used to research the topic are discussed. It provides a review of both the primary and the secondary sources utilized for gathering information and the limitations encountered in carrying out the research are also discussed. In chapter 3 outlines the work that is required to ascertain if borrow pits are up to providing the material necessary for the construction of roads. It also looks at site investigation and soil testing.

Chapter 2 Research Methodology

Introduction

This chapter outlines the methods of research which were adopted in relation to the fieldwork research for this dissertation. It explains in detail the various methods of data collection that were used, indicating the methods of primary and secondary data collection methods and any limitations encountered during the research are highlighted.

Research Process

The proposal for my dissertation began to materialise when looking back on my previous working experiences in the construction industry. One of the main areas of the authors work was involved in road construction. During the initial research the author investigated on the availability of information for his topic and when satisfied, proceeded to expand on this proposal. There was a medium volume of primary and secondary data available for this topic and also from the authors work experience there was a readily available source of primary data.

Sources of Information

Primary Sources

Naoum (2007, p20) states that “Primary literature is the most accurate source of information as it publishes original research”

The author sourced valuable primary information from Conferences, Reports, Government Publications, (for example NRA) and information from his work experiences.

Secondary

Naoum (2007, p20) states that “Secondary literature sources are those that cite from primary sources such as textbooks and newspaper articles”

The subject of recycling and waste management in the construction is very topical at the present and the author was able to gather a substantial amount of information through secondary sources. The internet played a vital part in the collection of information. Through this median the majority of information was researched.

As well as the internet other sources of secondary information was gathered from

Publications/Reports

Trade Journal and magazines

Textbooks

The text books on the subject where sourced through the LIT and GMIT libraries

.

Literature Review

Once the literature search was completed a literature review was carried out to choose which material was most relevant to the dissertation. All this literature was placed in a literature file. Much of the material which was web based was stored on a computer file and was not printed out.

Research Limitations

The author found that he was unfortunately limited as there were not a lot of books published on certain past of the chosen topic. This meant a lot more time was spent searching the internet for information required for the various chapters.

Borrow Pit Selection

Introduction

There a number of factor in the selection of a potential borrow pits Pantouvakis et al said ‘the overall objective in selecting a borrow pit is to determine the most economical and technically efficient location with the minimal impact’.

the next activity is to establish which are the best sites to develop. The main influences that have to be evaluated include:

Technical Considerations

Economic Considerations

Social and Environmental Considerations

A summary of all the main factors to be taken into account is presented in Figure 4.1. This Figure illustrates the large number of potential influences and their interrelationship. The decision to develop one material source rather than an alternative will often require engineering judgement in order to adequately balance the technical, economic, social and environmental considerations.

This Section provides guidelines for carrying out an evaluation of technical considerations and presents an introduction to economic influences. Detailed consideration of economic influences associated with developing a material supply strategy for sections of road are reviewed in Section 9. The detailed review of potential social and environmental impacts associated with material resource development is also presented separately, in Section 11.

Borrow Pit Site Investigation

Introduction

Site investigations are primarily carried out in order to establish reliable estimates of the quantities, quality and processing needs of potential road building materials. Quigley ‘For road projects, a principal aim of the site investigation is to classify the suitability of the soils in accordance with Table 6 / 1 from Series 600 of the NRA Specification for Road Works (SRW), March 2000’ see appendix A

The design of the site investigation must consider the following factors:

How much material is required? The investigation should concentrate on identifying sufficient workable material to supply expected requirements.

The method of site investigation will probably be influenced by the availability of suitable resources (plant and test equipment).

The hardness of the deposits may dictate the method of investigation.

The depth and nature of overburden deposits may influence choice of investigation methods (this can be seen later on in the chapter).

Borrow pit ground investigations should be carried out at regular intervals across the site. It is recommended that the depth of overburden, and the characteristics and thickness of the workable deposit is investigated on the basis of a 30 m to 50 m square grid. This distance should only be widened if the site is very large and the materials very similar at each test location. Unexpected changes in the ground profile should be examined locally by making additional intermediate investigation sites in the grid.

A site plan of each borrow pit to be investigated should be prepared, showing:

The main features of the site.

The means of access and the pit location with respect to haul road placement for extraction.

The location of each site been investigated and investigation borehole should be logged using modern GPS or similar devices. This information will be important when you go to work out the quantity of the resources in each potential site been looked at. This process will be looked at further on in this chapter.

Desk top study

A desk top study is one of the first steps undertaken when you look at a potential site because it can provide you information already available about a site. (Simons et al, 2002). ‘Within the construction industry, desk studies are well know but are an under-used method of gathering and assessing existing information about a site’. Desk studies generally include a visual inspection of the site and its surrounding area (usually called a walkover survey).

The desk study examines and draws together existing information from a variety of sources to form an initial appraisal of possible ground conditions. A good starting point is to use OS maps. These allow the site to pin pointed and an accurate grid reference to be obtained. This permits much more accurate references on the geological maps used later, which can be got from the Geological Survey of Ireland, which show existing geological records for the area and also historical maps could be a help. The walk-over survey of a site can give valuable insight into potential ground condition problems (for example slope instability or shallow groundwater) and contamination issues (revealed for example by vegetation dieback). On site visit, local resident can often provide information on groundwater and sources of raw material in the area. The combination of the desk study and the walk-over survey is an extremely cost effective first stage in an investigation. It provides early warning of potential problems and a sound basis for the scope of subsequent, more detail investigation which may follow. The desk study and walk-over survey can also provide early recognition of site issues such as ecology and archaeology which may have implications on site selection.

Traditional Ground investigation methods

There are number of different types of ground investigation methods available on the market and we will look at some of the most common method used.

Trial Pits.

Boreholes investigation,

Light Percussion Boreholes.

Rotary Drill holes.

Geophysical

Seismic Refraction Method.

Electrical Resistivity Method

Trial Pits

Trial pits are predominantly carried out using mechanical excavators which will give information of the ground while the digging is taking place. Throughout the process photos are taken of the ground conditions and samples of material Quigley (typically 30kg for engineering performance tests) are also taken are specified ground depths. Depending on the excavators reach this will determine depth or if bedrock is reached, ‘Cat 320D digging depth is up to 6.690m’ (Caterpillar 320D brochures). All field work is carried out in accordance with BS5930.1999.

Figure Picture of Trial Pit operation.

(Source www.prioritygeotchnical.ie)

Figure Pictures of samples taken

(Sourced www.prioritygeotchnical.ie)

(Quigley) points out that ‘A suitably experienced geotechnical engineer or engineering geologist should supervise the trial pitting works and recovery of samples’. It is important that all trial pits are backfilled after testing is completed. See appendix 2 for example of Trial Pit log.

Borehole Investigation

Light Percussion Boreholes (Shell and Auger)

Light percussion boreholes or otherwise known as Shell and Auger boring can be used in all types of soils and is mainly used for depths exceeding that of an excavator.

The rig consists of an engine powered winch and tripod frame that is easily collapsed for towing behind a four wheel drive vehicle. The boreholes are usually 150 to 200 mm in diameter and steel casing is typically required throughout most of the hole. Boreholes of 20 to 30m depth are commonly bored in suitable soils, however in mixed soil, depths can vary samples should be taken at ( Quigley) ‘1.00m intervals or change of strata’. See figure 3 for example of rig in working position.

Figure Light percussion boreholes rig

(Sourced www.southerntesting.co.uk)

Boring is achieved by repeatedly dropping an auger, consisting of a steel tube, to the bottom of the borehole. In sands and gravels a valve is fitted to the lower end of the tube to trap the material entering it, this is a shell or sand auger. Any large stones or small boulders encountered can be broken up with a heavy chiselling tool. Large disturbed samples may be obtained, but below the water table the washing action of the shell may remove some of the fine material from sand and gravel deposits.

Attachments to the rig allow the use of light rotary drilling equipment that will enable limited investigation of the characteristics of any bedrock encountered. See figure 4.

Figure Attachments for light rotary drilling

(Sourced Clayton et al,, 1996)

Rotary Drill Boreholes

Rotary drill rigs come in a variety of sizes from small skid mounted machines to large truck mounted machines. See Figure 5. They use a rotary action combined with downward force to grind away the material in which the hole is made. The primary use of rotary drilling techniques is to investigate rock quality by taking core samples. However, rotary drilling rigs can be used for non core drilling. A destructive tricone (rock roller) drill bit is generally used to advance a non-coring hole. Only the rate of advance and characteristics of the cuttings flushed to the surface by the drilling water give any indication of the strata penetrated. This can be supplement by small disturbed samples recovered from a driven sampler (i.e. SPT sampler). Non core drilling is therefore of limited use, but might be employed through the overburden covering a rippable rock or quarry stone.

Figure Rotary drill rig in operation

(Sourced www.prioritygeotchnical.ie)

Rotary coring involves the use of a water lubricated diamond or tungsten tipped hollow core bit attached to a core sample recovery barrel and a series of hollow drill rods. See figure 6. In weak or fractured rocks better quality core are recovered from larger diameter bits. Some ground investigation drillers have developed a technique for recovering disturbed soil samples by “dry drilling” with a single tube core barrel, but

This practice is not widely used and is less successful in coarse deposits with little plastic clay binder. Example of rotary log see appendix 3.

Further information on site investigation drilling and the description of drill cores is contained in BS 5930 “Code of Practice for Site Investigations” (1999).

Figure Layout of a small Rotary core unit.

(Sourced Clayton et al,, 1996)

Geophysical Exploration

This method of investigation is not commonly used as it is not suitable for all ground conditions and there are limitations to the information obtained. The main use of this form of exploration is it can give bedrock and water table levels and it can also detect sub-surface cavities. The two main methods are.

Seismic Refraction

This method involves the generation of an artificial shock waves in the rock or soil and then records the finding in a series of detecting station which are place across the area been investigated. The detecting stations are known as geophones. This method is often used to detect buried channels and identify bedrock levels. See figure

Figure Seismic Refraction Method

(Source www.geophysics.co.uk)

Electrical Resistivity Method

This method involves the driving of four equally spaced electrodes into the ground in a straight line. When this process is done and a current is passed through the outer electrodes and the difference in volts with the inner electrodes is measured. See Figure 8

Figure Electrical Resistivity Method

(Source www.discoveryprogramme.ie)

Soils Testing

The purpose of the soil testing of material from the borrow pits is to

Determine the engineering properties of the material.

To establish if the material is the same quality throughout.

To provide document records of the pit for the clients.

In order to fully assess the engineering properties of a borrow pit material it is necessary to have samples tested in a Materials Laboratory. The purpose of testing material is to see if the comply with the specifications of there indented use. The laboratory will use standard testing procedures to classify the samples taken. In Ireland the parameters set out in Table 6 / 1 of the NRA SRW include a combination of the following.

Typical tests undertaken are for soils and aggregates

Moisture content

10% fines

Particle size grading

Plastic Limit and Liquid Limit

CBR

MCV

Triaxial Shear Strength Test

(Laois County Council 2007)

Moisture content

Moisture content is the amount of water to amount of dry soil. The method is based on removing soil moisture by oven-drying a soil sample until the weight remains constant. The moisture content (%) is calculated from the sample weight before and after drying tests carried out to B.S.1377: Part 2: 1990 for soils and BS 812: Part 109:1990 for aggregates

Figure Soils Oven

(Sourced Testing equipment Supply)

10% Fines

This test is not too dissimilar from the test to determine the aggregate crushing value which determines the ability of an aggregate to resist crushing. But instead of using a standard force of 400kn. the force at which 10% of fines are produced is noted as the Ten Percent Fines Value. This usually requires a number of tests and a graph to establish the exact figure. This test tends to be used for softer aggregate where a force of 400kn. would crush most or all of the aggregate. This test is carried out BS 812 Part 111: 1990 for aggregates

Partially Grading Tests

The particle size distribution grading of a road surfacing material is an essential guide to the suitability of the material since the engineering properties are very dependant on grading. See Figure 7. Testing is carried out to B.S.1377: Part 2: 1990 for soils and BS 812: Part 103.1:1985 for aggregates.

Figure Sieve Test

(Sourced Soil Compaction Handbook)

Plastic Limit Testing and Liquid Limit Testing

The Plastic Limit (PL) is an indication of the percentage moisture content at which the soil fines change from a semi solid to a plastic state. In this test a 20 gram sample of material passing the O.425 mm sieve is mixed with a little water until it becomes plastic enough to be formed into a ball. The soil is then moulded between the fingers until the surface begins to crack. It is then repeatedly rolled on a glass plate into 3mm diameter threads until longitudinal cracking causes the tread to start to break up. This moisture content is the plastic limit.

Test are carried out to BS 1377: Part 2:1990, CL5

Figure Plastic limit test and equipment

(Sourced http://ral.train.army.mil) and (Sourced www.humbolgtmfg.com)

The Liquid Limit (LL) is an indication of the percentage moisture content at which soil changes from a firm plastic state to a soft liquid state. Two methods are available for determining the liquid limit: the “traditional” method using the Casagrande apparatus and the more recently developed cone penetrometer method.

Test are carried out to BS 1377: Part 2:1990, CL4.3 & 4.4

Figure Casagrande equipment Figure Cone Penetrometer

(Sources www.humbolgtmfg.com) (Sources www.sbe.napier.ac.uk)

The difference between the Liquid Limit and the Plastic Limit is called the Plasticity Index (PI). This index provides a good guide to the cohesive properties of a road building aggregate. A high PI may indicate the presence of an undesirable amount or type of clay.

CBR test

It is in essence a simple penetration test developed to evaluate the strength of road subgrades. ”How strong is the ground upon which we are going to build the road’. (Summers 2010) Tests are carried out either unsoaked or soaked. Unpaved road materials would normally be tested according to the 4 day soaked test procedure. An estimated CBR value may be derived from the results of grading and plasticity tests. Tests are carried out to B.S.1377:4:1990 Section 7

Figure CBR test apparatus

(Sourced Testing equipment Supply)

MCV Test

This test involves compaction of soil or fine aggregates using a hand held device. The compactive effort can be compared to that needed at the optimum moisture content. This enables a comparison with the actual moisture content of the soil. Test carried out to BS 1377: Part 4:1990 see Figure 13 for test apparatus.

Figure MCV apparatus

(Sourced www.ele.com)

Triaxial Shear Strength Test

The triaxial shear strength test subjects a soil specimen to three compressive stresses at right angle to each other with stress being increased until the sample fails in shear. This test is carried out to predict how the material will behave in a larger-scale engineering application. An example would be to predict the stability of the soil on a slope, whether the slope will collapse or whether the soil will support the shear stresses of the slope and remain in place. Tests are carried out to BS 1377: Part 6:1990. See figure 14 for test apparatus.

Figure Test apparatus for Triaxial Shear Strength Test

(Sourced www.namicon.com)

New Changes to Site Investigation and Testing

At present all Geotechnical investigation and testing is carried out to B.S. but come 31st March 2010 all test will be carried out using the new Eurocode 7 this will be for all publicly funded project. This does not apply to the private sector but there are likely to follow suit. Example of changes.

TC 341: Geotechnical investigation and testing

– 14688 Identification and classification of soil 3 Parts

– 14689 Identification and classification of rock 2 Parts

– 17892 Laboratory testing of soil 12 Parts

– 22282 Geohydraulic testing 6 Parts

– 22475 Sampling methods and groundwater measurements 3 Parts

– 22476 Field testing 13 Parts

(Sourced New Irish Geotechnical Standards and Selection of Characteristic Parameter Values 2009)

Resources Estimation

Introduction

In this chapter the author will look at how quantities of materials that can be potentially in a borrow pit and how they may be calculated.

One of the main primary objective of a borrow pit investigation and testing is to establish the extent of the different classes of materials and there suitability to the given project. With all the information in the next objective is to calculate the quantity of material that is in the potential borrow pit to see if it’s a viable option to proceed with. The potential quantity of material that can be extracted from a borrow pit should always be calculated in cubic metres (m3). A cubic metre should be seen as a box of material with a length (l) of 1 metre a height (h) of 1 metre and a depth (d) of 1metre.

Before and estimates can be taken off a detail topography survey of the potential borrow pits will have to be taken see figure 21 for typical contour survey. However, with all information from the investigation of the site it can be hard to estimate an exact amount so it is better to estimate on the conservative than on the generous. This may come about if the information between the trial pits may vary from data collected as seen early a grids of 30 to 50 meters should be used and for larger areas if could be up to 100 meters grids

There are different method of calculation the quantity of material that in the given site depending on the borrow pit make up but the most simple way is to break the area up into simple shapes such as rectangular prisms or triangular prisms. The volume of material in these shapes is estimated by first calculating a representative area in square meters (m2), and then multiplying this area by its depth or a length (in metres). It may need to be an average depth or length if the pit is irregular in shape or the deposit has a variable thickness. Formula for calculating the areas for different type of pit can be work out using the standard mathematically formula to get the quantities for the site. See figures 17,18,19,20

Trapezoid

Figure Formula for a Trapezoid shape

Volume = Base X Height

Figure Formula for Prisms

Volume = Length X Width X Height

Figure Formula for a Rectangular

Volume = 1/3 pr2x height

Figure Formula for a cone shape

(Sourced Area and surface Formulas)

Another method of course if available to you is different types of computer packages that can do the work for you like AutoCAD 3D civil packages.

Figure Typical Contour Survey

(Sourced Volume Calculations)

Material Wastage, Shrinkage and Bulking

Material Wastage

Wastage can occur if site investigation reports of material don’t add up to what in the ground and material been use for work it was not envisaged to do, like having to create bund not accounted for some.

Material Bulking

Material bulking will also to be taken into account when hauling materials of the bulking-up factor. When materials are excavated they are loosened and sometimes broken down, this result in an increase in volume per unit weight known as “bulking”. The bulking factor of a material is the bulk density in the pit face (bank) divided by the loose bulk density. A bulking factor of 1.25 indicates that 100 m3 of bank material will become 125 m3 of loose material in the stockpile.

Material Shrinking

Compacted material, material that has been excavated from it natural state and placed some place else for compaction. With compaction the volume reduces. The same can apply to shrinkage when material is been compacted what was originally 1m3 in a compacted state with shrinking factor is 0.85m3.

Table 1 Typical Material Density, Bulking and Shrinkage Factors.

(Source Earthworks)

The Resources Estimation has a dual purpose with given you the quantities of resources out but also on the other hand the amount of material which if can be disposal back into to fill the pits from surplus or unsuitable material that cannot be use on site.

Legalisation

Introduction

This chapter discusses of both Planning legislation and Waste legislation in the terms of Construction and Demolishing Waste in the construction sector. The planning legislation we will be looking in relation to opening up of a borrow pit for bulk material (soil) and the production of aggregates. In C&D waste legislation for the disposal of inert material. In the area of waste management there has been a number of direct and indirect (both national and EU) legislative influences on how operations should be carried out with the regards to C&D waste in road construction

The Planning and Development Act 2000 and associated Regulations 2001

The Planning and Development Act 2000 is the main act for planning permission in Ireland and this process is overseen by the different Local Authorities in the country. Regulations 2001 is the statutory legislation that applies to new and existing extractive developments and ancillary facilities in Ireland.

Quarries (including sand and gravel pits) operating before the existence of planning legislation in Ireland that was implemented in 1964 did not have to obtain planning permission and were essentially unregulated. Despite this, much self-regulation took place such as imposition of emissions (noise, blasting, and dust) limits and the implementation of Environmental Management systems accredited to IS0 14001.

However, under Section 261 of the Planning and Development Act 2000, a new system of once-off registration for all quarries was introduced. Only those quarries for which planning permission was granted in the five years before section 261 (i.e. after April 1999) became operative are excluded. Section 261 has also resulted in some extractive sites being required to submit an Environmental impact statement and to go through the formal planning process. This review process has resulted in up-to-date environmental and biodiversity management conditions being enforceable on all authorised sites; this process is largely complete at this time.

C&D Waste legislation

As spoken earlier on C&D waste legislation in Ireland is influenced by European waste policy and legislation. The EU waste management is based on the waste hierarchy which gives preference to waste prevention, then to waste recovery (including reuse, recycle and energy recovery). And finally to waste disposal (to landfill) See figure 22 for hierarchy

Figure Waste hierarchy

(Source Fas CIF Handbook on Construction Demolishing and Waste)

European Legislation

The majority of European waste management directives are based on the ‘Symond’s report 1999’ which evaluated the recycling of C&D Waste in each of the member state and prepared 55 recommendation.

The following is a list of the most important recommendations of the report

Fly tipping subject to sanctions

Land filling of C&D waste should be discouraged by member states.

C&D Waste derived aggregates should not be discrimination in the market place, primary aggregates and C&D Waste derived aggregates should be both be considered as raw materials

The client should demand that Environmental Impact Analysis be undertaken for each project which could form part of Environmental Management Plan.

Governments to draw up National Waste Management plans for C&C waste.

The main based EU based legislation that influences C&D waste management on a national level are

Framework Directive on waste 75/442/EEC plus amendments

Framework Directive on Hazardous Waste,1991 91/689/EEC