During my placement year I worked for North Midland Construction Plc. They are a medium sized company with their main office situated in Huthwaite, Nottinghamshire. North Midland Construction (herein NMC) is operated as 5 divisions.
Civil Engineering Division
Highways and Special Projects
Utilities
North Midland Building Ltd
Nomenca Ltd
During my placement I was working within the Civil Engineering Division. Throughout the 15 month placement I worked on various projects.
1.2-Site Structure
The site structure generally followed this model.
Contracts Manager
Agent
QS
General Foreman
Site Engineer
Labourers
1.3 Work Undertaken During Placment
Health and Safety Executive Laboratories - Harpur Hill, Buxton, Derbyshire
July 08- Aug 08
The primary work at this location was to construct 32 concrete sleepers as a base for a metal tunnel to be placed upon. The tunnel was then filled with ballast and a track placed inside on wooden sleepers. The purpose of the tunnel is to act as a test bed for train collisions. My initial duties on the site were to set-out the boundaries and dig level where the sleepers were to be placed. Once these were excavated to level a layer of Terram Geotextile was laid to separate the soft subsoil and new quarry waste granular material to increase bearing pressure. 450mm underneath the sleepers a further layer of Tensar Triax grid was placed to further increase bearing pressure locally and quarry waste stone compacted on top. Once the foundation for the sleepers was completed a 50mm wet mix of Gen 1, ST2 mix concrete was placed for blinding underneath the sleepers. Three 1.8m3 steel moulds were used to cast the sleepers. Each day I was responsible for setting out the sleeper dimensions on the blinding then finally checking the alignment using a total station set upon the know centre line. The next day the sleepers were stripped and the process repeated.
Barlaston 132Kv Substation, Stoke-on-trent, Staffordshire
Aug 08-Nov 08
NMC were subcontracted on this project to Siemens Transmission to construct the site prior to the cabling installation. The works comprised of building a new 132Kv substation alongside the existing substation. I was the sole engineer on this site and reported in the first instance to my foreman, a sole Agent for NMC was responsible for the management on site. The duties on site included drainage and haulage road construction and stanchion construction. The road construction on this site was to a high loading specification to allow mobile cranes to lift heavy duty transformer and steel structures to be placed. The drainage on this site was designed to be self contained including large oil separation and attenuation tanks to contain a spillage in the event of a oil filled transformer leaking. A large part of my time on-site was spent on the construction of concrete bases to support transformer equipment and lighting columns. At this site I became responsible for sending weekly returns and booking and hiring plant equipment. This was useful as I started to become aware of the cost of materials and develop a financial awareness onsite. I was also responsible for compiling the as built on survey on this site, and liaising with the designers HBPW to update the drawings to form part of the handover documents to the client. The work on this site sometimes involved working inside the 'live' 132Kv substation special H&S procedures needed to be followed rigorously since this was a dangerous environment. Some of the procedures were wearing particular PPE through to no-go zones around live equipment. We had to be constantly aware of overhead lines and reduce our levelling staff and plant heights accordingly.
Scotter Sewage Treatment Works, Gainsborough, Lincolnshire
Nov 08-Dec 08
This job was a temporary 3 week position to cover the absent site Engineer, I worked with the foreman and the Agent on this site. The works were a small sewage pumping station. My responsibilities on site were to set out a dig for a 5m height pumping vessel. This involved the use of sheet piles and shoring equipment. The dig was particularly difficult because of the running sand ground conditions. The excavation was constantly refilling with the saturated water sand mixture 'flowing'.
Various
Dec 08-Jan 09
During the slow workload before the Christmas vacation I did various types of work. I worked for the estimating and tendering departments doing initial walk over surveys on potential new tender sites. This involved going to site for new tender and taking photographs and assessing a new site for potential things such as;
Access/Egress
Weight limits on roads
Schools/special consideration areas
Nearest waste transfer places
Overhead lines
Site geography/topography
Potential site accommodation
Service supplies (water,power etc)
I also spent time travelling to various sewage treatment works to survey and photograph existing treatment equipment to aid the Severn Trent design team renew or refurbish different sewage treatment processes.
Outu Kumpo Stainless Steel Works, J33 M1, Sheffield
Jan 09
At this site again I was the sole engineer onsite this site was unique in the fact it had no foreman and I worked closely with the single site agent. The works consisted of constructing a multi level concrete foundation for steel processing machinery inside an existing large warehouse. I was responsible for all the setting-out in this job. The drawings and all dimensions were given from the centre line of existing columns and my first job was to establish the centre line of the new foundation. I then taped all distances and marked out the shape of the new structure. The tolerances on the job were small as the large machinery approx 80m long needed to be bolted to cast-in bolts along the foundation.
Network Rail Walsall Maintenance and Safety Training Centre, Walsall, West Midlands
Jan 09-Sept 09
This job was the first job I was involved with from the start onsite. The works consisted of site clearance and demolition of existing structures and construction of two new portal frame buildings to become a new training centre for Network Rail. My first job onsite before the demolition was to check and confirm the existing Ordinance Survey. I did this by doing a loop traverse of the known stations onsite and adjusting the given co-ordinates accordingly to eliminate misclosure. I was then responsible for setting out new survey pegs and stations onsite where they would not become disturbed as most of the original survey stations would be lost during the demolition of the existing structure.
The contract type was design and build. NMC were the principal contractor onsite and organised all aspects of the contract from design through to project completion. The design phased overlapped with the construction and a tight deadline was inplace. The worked moved along at a fast pace and decisions often had to be made quickly meaning decisions were often made on behalf of the client.
The site was adjacent to live track and was treated as a track environment we were required to follow all network rail procedures and treated as network rail employees where drugs and alcohol policies, PPE requirements and near miss/ accident reporting were concerned . I was also expected as a NMC employee to enforce safety onsite and remind subcontractors and labourers about health and safety issues especially wearing the correct PPE for the task in hand.
During the demolition of the structure it was discovered that several types of asbestos were present. The removal was a lengthy process and required Work Package Plans (method statements) to be written for the subcontractor to follow. I wrote several of these and they were overseen by the site Agent this introduced me to some of the management duties on site.
One of my main daily responsibilities onsite was keeping a Quality Assurance (QA) file. This meant recording works such as concreting, taking samples or measurements to check the work was to British Standards and getting the client to review the works and sign a completion form for the task. This meant keeping good records and being able to perform tasks such as drainage air testing or concrete cube making.
I was also responsible for inducting new staff onsite and gathering and collating all the associated paperwork.
The first duties onsite after the demolition onsite were to set-out the dig for the larger of the two buildings on site, then after subsequent blinding the concrete foundation beam. I setup a local control grid to set out the foundations to keep the co-ordinates simple numbers to aid field calculation and to reduce transposition errors. The local grid also made it easier to set out the bolt sets at 6m centres for the steel frame. The second smaller building was also set-out using a local grid system.
The next large job onsite was to install the drainage. The large surface area of the roofs on the buildings meant large diameter pipes were installed underground. The drainage onsite was all new and eventually broke into a large existing man holes on site. The drainage was subcontracted and this meant working closely with the labourers and checking rigorously to see the tasks were completed to a good standard.
A feature of the drainage on this site was the installation of a 68.4mx4mx1.2m underground attenuation tank. This was made from small hollow crates wrapped and sealer watertight in 1mm plastic sheeting. The system was designed as a storm surge arrester as the large amount of storm water drainage on site was calculated to be too great in the event of heavy rain to discharge into the main sewer underground.
Section Two
Technical Report
2.1 Introduction
The construction of highways in the UK and overseas form a major part of the construction industry. Engineers face many challenges in the construction of highways with a major contributor being poor ground. Any structure including a highway needs to transfer its loads effectively to the ground. Poor ground does not provide enough of a support reaction and deems the sub base unusable. Ground bearing capacities vary greatly in the UK and sub strata can vary greatly over a construction site. There are however various methods to improve the condition of the ground for construction. Lime stabilisation is a particularly cost effective solution
2.2 The Problem
When excavating onsite, engineers often encounter the problem of wet or saturated pockets of ground. The traditional approach would be to excavate through the saturated layers to good bearing strata. This however is not economical since a large amount of backfill is needed and the excavated ground needs to be disposed of which sometimes means complete removal from site. The removal of soils from sites is becoming increasingly expensive due to landfill tax and transport costs.
2.3 A Solution
One of the many solutions for bad ground is lime stabilisation. It can increase the stability, and load bearing capacity of a sub base whilst decreasing the permeability. Lime is mixed into the bad ground to change its properties. Lime is particularity suitable for clayey soil of medium to high plasticity although it has less dramatic results in fine grained soil.
Stabilisation is the process in which lime is added to a reactive soil to generate long term strength through what is called a pozzolanic reaction. The lime-clay reaction creates stable calcium silicate hydrates and calcium aluminate hydrates as the calcium heavy lime reacts with the aluminates and silicates solubilised from the clay. The full pozzolanic reaction can continues for years or even decades as long as there is sufficient lime present for the pH to remain above 10. Key factors for good results in stabilisation are reactive soils, good mix design and good construction practice.
Benefits of stabilisation include:
Increases in resilient modulus of soils
Increase in shear strength
Continued gain of strength as pozzolanic reaction continues
Good durability through design life of roadway
Generally the addition of 1-3% lime is needed to modify and clay soil, whilst between 2-8% is needed for cementation. When the lime is added calcium ions are combined initially or absorbed by the clay minerals which lead to an improvement in workability. The ideal amount of lime added to the ground to increase the plastic limit is referred to as the lime fixation point; any further lime added causes cementation and increased soil strength.
Lime stabilisation chemically improves the soil in the following ways:
Reduces shrinkage and swell characteristics of clay soils.
Increases unconfined compressive strength by as much as 40 times.
Substantially increases load-bearing values as measured by such tests As CBR, R-value, Resilient Modulus, and Tri-axial tests
Creates a water-resistant barrier. Impedes migration of surface water from above and capillary moisture from below; thus helping to maintain foundation strength.
In addition to lowering the plasticity in most cases and initially strengthening the improved soil, the strengthening effect increases over time.
Figure 11 Figure 2 1 Figure 31
Figure 41
Figures 1 to 3 show different variables against lime stabilised ground. A common factor in figures 1 to 3 is the fact that with lime addition the sample material becomes stronger over time. Figure 4 shows the affect on unconfined compressive strength of different clays by percentage of lime added. It is of note that over a certain percentage usually 6-10% by volume the strengthening effect of the lime is reduced and decreases the more lime is added.
2.4 Lime Stabilisation in the UK
As afore mentioned the use of lime stabilisation is only well suited to particular soil type and a site investigation should be carried out before use. If a clayey soil is to be improved with this method the level of sulphates in the clay must first be determined if the levels are too high the lime will react with the sulphate and create calcium sulphate or gypsum. The gypsum then hydrates and heave occurs.
This was a major problem with the road pavement on one section of the M4 shortly after construction, the section failed badly due to this problem, and had to be replaced.
More recently large sections of the A10 Wadesmill Bypass failed and had to be reconstructed because of significant amounts of heave in the lime stabilised layer.
Other successful projects include the Eaves Green Link Road in Chorley. The construction of the road took place in 2007 by Birse. They used lime stabilisation along the majority of the 1.1KM road as the clay that was in the area was very wet and had low load bearing capacities.
Figure 6.Lime stabilisation is not difficult to achieve. Once proper mix design and testing is performed. In place mixing is usually used to add the appropriate amount of lime to soil, mixed to an appropriate depth. Pulverisation and mixing is used to thoroughly combine the lime and soil. For heavy clays, preliminary mixing may be followed by 24 to 48 hours (or more) of moist curing, followed by final mixing. For maximum development of strength and durability, proper compaction is necessary. Proper curing is also important. If sulphates are present at levels greater than 0.3 percent, special procedures are required.
2.5 How can heave be decreased?
Heave often occurs when there are high levels of sulphates and sulphides in the soils. Ground stabilisation can still take place by using GGBS (Ground Granulated Blast furnace Slag) and Lime together. Extensive research has been done in the UK for using GGBS. Lime + GGBS stabilisation is now an established technique in the UK and is becoming a preferred option where there are sulphates or sulphides present in the soil.
During the research period the use of lime and GGBS was tested in the construction of a distribution warehouse and the Twingewick Bypass. During the construction of the bypass traffic from the A421 had to be diverted this is where the process was to be tested. The soil on site was boulder clay containing high levels of sulphates and sulphides. The sub base for the diversion was made up of lime + ggbs stabilised clay and a 130mm bitumen cover. The contractor tested the ground bearing capacities and found it to be stable and continued to use the road as a diversion. There were delays on the project meaning the diversion had to be used for longer than anticipated and still the lime + ggbs mixture proved to be sufficient over time.
Figure 52.
Showing how different combinations of Lime and GGBS can affect the swelling of soils
2.6 Global Uses of Lime Stabilisation
Lime has been used over the past 30 years for the construction and expansions of the Dallas-Fort Worth International Airport. Nine inches of lime-treated sub-grade underlies runways and taxiways and 18 inches of lime-treated sub-grade provides support under terminal aprons. For the recent Houston International Airport expansion, the pavement system included a 24-inch lime stabilised sub-base and a lime/cement/fly ash base.
2.7 Cost Saving
Lime-stabilised layers can create cost-effective design alternatives. A recent highway project in Pennsylvania, for example, began with a $29.3 million traditional design. The engineers chose an alternate design incorporating lime stabilisation, consistent with mechanistic-empirical designs that cost only $21.6 million- more than a 25% saving.3
The construction of a 14 acre General Motors assembly plant in Arlington, Texas substituted lime stabilisation for 39 inches of aggregate fill, saving hundreds of thousands of dollars and enabling the contractor to achieve aggressive milestones on the fast track project.3
Section 3
Conclusion
3.1 Conclusion
From gaining 15 months in industry I feel that I have become more confident and can now make a significant contribution onsite. Initially I sometimes found the responsibility of being given new types of work daunting and would not often know the sources of help available. At the start of the placement I would often unwillingly waste time setting out when it had to be moved or destroyed during the construction. I soon learnt the best way to work with subcontractors or labourers is to ask them what they actually need to do the job and how they usually carry out their tasks. I learnt a lot of time saving methods and was sometimes reminded how construction sites were set-out before the advent of total stations. I was often left to use my own initiative and organise the work in a logical sequence
The time in industry also helped me understand site structure better and the various roles both on and off site. I was fortunate to be treated as an equal to the other North Midlands engineers and I was often the sole engineer onsite. The 'in at the deep end' approach often meant that I was directing works for people twice my age, and the experience of working with sometimes strangers helped me develop my communication skills and develop in part a management style. I found that a quick sketch was often the easiest way to communicate an idea whoever I was trying to express my idea to.
Working on many different projects enabled me to see various types of construction from waste water management through to commercial buildings. I now have a good understanding of the construction details I encountered and would be able to specify a solution to similar problems.
The placement was also beneficial in reinforcing my idea that I wanted to be onsite working for a contractor as opposed to working in a design office, I was also invited to ask staff about their career progression within the company. The work hours although longer onsite seemed to go quickly during fast paced periods of work. I did however sometimes find slow periods onsite an excuse for socialising rather than reading the drawings or completing my university commitments.
I understand that taking a placement year as part of my sandwich degree has increased my employability and career prospects; it has also helped to secure a bursary for my final year and given me many contacts within the construction industry.
