The crucial role of geotechnical engineers in design of heavy duty concrete pavement
Ehsan Moradabadi
CEng MIEI | PhD in Civil Engineering (UCD) | Registered Geostructural Engineering Consultant | Technical Design (Geotechnical and Structural) | Feasibility Studies | Team Building | Project Management | Training
Concrete is one of the most used materials in construction of site paving for industrial and trucking facilities (e.g. storage facilities, warehouses, ports, airports, stores, metro stations, car parks, etc.). Considering the role of cement production in climate change as the world’s third largest CO2 emitter (7% of global CO2 emissions, more than double the emission produced by aviation or shipping), there is no doubt that optimising the usage of concrete in construction industry not only reduces CO2 emissions, but also helps on avoiding more depletion of natural resources.?
The issue would be more sensible if we realise that the CO2 produced for manufacturing of cement used for concrete paving of only one small size storage/warehouse facility (~2000 m2) is equal to CO2 emitted from about a full return flight with 350 passengers between Dublin and New York. This may be exacerbated as it is well known that both design engineers and contractors may increase the thickness of concrete to reduce the risk of failure or increase the longterm servicing performance of pavement, mainly due to low level of confidence in sub-grade assessment.
Worthwhile to mention that only 5 cm safe reducing the thickness of concrete in a small size warehouse can eliminate the carbon footprint being equal to CO2 emitted for return flight of 50 passengers of DUB-NYC, and replacing 30% of cement with fly ash can triple this reduction.?
To elaborate the important role of geotechnical engineers in sustainable design of heavy duty rigid concrete pavement (HDRCP), especially in sub-grade assessment and sub-base design, in this edition of ‘Geostructural Design Process’, GDP, design process of conventional HDRCP for industrial and trucking facilities is presented.??
Design Process
Similar to other geostructures, long term performance of HDRCP highly relies on harmonised collaboration of geotechnical engineer/s, pavement designer/s, and construction group/s involved in the project. Any deficit in geotechnical assessment of sub-grade, suggesting faulty solutions for stabilising sub-grade?or sub-base or having error in design of concrete thickness can have similar consequence similar to either faulty selection of materials or poor construction of the design. Figure 2 shows some examples of failures in HDRCP.
As it is illustrated in decision diagram of figure 1, among 20 critical decisions need to be made during the design process of the pavement, at least 10 decisions need direct approval of a geotechnical engineer, emphasising the crucial rule of geotechnical engineers in sustainable design of HDRCP.?Below is a summary of design steps being required for sustainable design of HDRCP, also suggested in Figure 1.?
Desk study
After reviewing the information and contractual documents received from the client, a desk study may be necessary to:
The sustainability assessment of the project may reveal whether the concrete pavement option is a reasonable choice or another alternative maybe more sustainable and also cost effective for the project. The criteria which affect in this decision may include a) the service life expectation; b) durability and maintenance requirement (e.g. resilience to solar degradation, resilience to salt seawater, poor sub-grade conditions, resilience to fuels, oils , chemicals, solvents and so on), c) access to material and their embodied emissions and energy, d) The carbon footprint and island heat effect comparing to other alternatives, e) The effect of colours of pavement in light and safety and d) the aspects related to reusing and recycling the pavement once the pavement reaches the end of its service life.
The appropriate standard and guideline should be used for sustainability study and the rest of the design process, such as EC 1; EC 2, EC 7, DMRB manuals, MCHW manuals, TII manuals,?ACI 330, 117, 224, 228, AASHTO specification manuals, BS 7533 series, to name a few.
Sub-grade assessment
The desk study should provide sufficient data to design a site investigation program and laboratory tests. Depending to the importance and magnitude of the project, the site investigation can be limited or very extensive. The key point in the investigation program is to provide enough information of soil characteristics of sub-grade to help increasing the confidence level of the decision makers of the project including moisture content of the sub-grade,?Atterberg limits, sieve analyses and moisture-density relationship, the modulus of sub-grade reaction, California bearing test (CBR), resistant value (R-value), soil support value, resilience modulus and so on.
For the pavement should be suitable for the safe operation and manoeuvring of the construction machineries, a general check at the beginning of the design process may be necessary to ensure that whether a thick granular stabilised sub-base layer is needed as a working platform or not (For CBR<6 generally a working platform is necessary. For more details about the design process of the working platform see previous published article:?https://www.dhirubhai.net/feed/update/urn:li:ugcPost:6962882168377253888?updateEntityUrn=urn%3Ali%3Afs_updateV2%3A%28urn%3Ali%3AugcPost%3A6962882168377253888%2CFEED_DETAIL%2CEMPTY%2CDEFAULT%2Cfalse%29?)
If the CBR of the sub-grade was greater than 6, generally 4 different conditions may cause that the sub-grade geotechnically specified as non-suitable for supporting the pavement:
1- The soil is weak and wet.? The solutions for stabilising a weak soil layer can be:
2- The soil is susceptible to pumping (low value for modulus of sub-grade (k-value). To strengthen the sub-grade against erosion, the geotechnical engineer may suggest one of the below solutions:
3- The soil is frost susceptible. A sub-grade can be?strengthen against freezing by one of the below measures:
4-The sub-grade was defined as expansive. To protect the pavement against the volume changes of the sub-grade, the engineer may suggest to treat the sub-grade by one of the below methods:
Subbase design?
The engineer/s may execute a cost-benefit analysis for selecting appropriate method to stabilise the sub-grade, in case of need. When the engineer became confident that the sub-grade can sufficiently support the sub-base and the final concrete pavement,?by considering sustainability and economy factors in addition of availability of materials, the engineer can choose whether to suggest a stabilised sub-base layer or appropriate natural granular layer to support the pavement.?for selecting appropriate sub-base materials?the engineer should:
In case that the available natural soil is not satisfied the above criteria, the engineer may suggest cementitious or other stabilised sub-base layer. However, in case of using an stabilised layer the engineer should ensure that:
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A thickness between 100 to 300 mm or more may be chosen by the designer depending to the drainage design and earthwork needs for grading the paving area. The thickness of the sub-base in addition of the material characteristics of the sub-base and sub-grade can provide enough information to calculate the resilience modulus and k-values from tables or empirical formulas that are necessary for the rest of design processes.
In case of major revisions in grading and drainage design, the engineer/s may revise the desk study report, renew the sustainably report, and ask for a new drainage and earthwork design, or even, in rare situations, suggest another pavement alternative to the project’s owners.
Joint design
To design concrete panels, joint’s type, and distance should be specified in advance. Worthwhile to mention that if considering?the effect of specific ambient conditions above normal environment, may be necessary for the design or limited joint or joint-free pavement is required for the project, the designer/s may be dictated to use appropriate commercial software package or to be consulting with a pavement specialist for being advised about the possible measures for mitigating the specific condition in design or using continuous reinforcement.?
For separating the pavement from structure, the designer should specify the isolation joints. These joints are normally filled with?premolded?compressible materials.?
The construction joints are also specified for preplanned or emergency stopping of construction. These joints may be similar to isolation joints or contraction joints depending to the planing of the construction phases.
The contraction joints? are used to minimise random cracking and to control the location of cracks of the concrete and the effects of wrapping and curling.?
Due to significant interaction between joints spacing, thickness of panels, and joint stability, the designer?should optimise the design based on performance requirements of the pavement. This may cause repeating the previous steps until the optimum design is reached, also satisfying the performance criteria. The sealant’s type and width and depth of saw-cut should be specified by designer/s. In certain circumstances that saw-cut is not tolerated or aspect ratio of panels exceeds 1.5, the designer may suggest either using minimum reinforcement or adding macro synthetics or steel fibres to minimise random cracks.
Concrete Panel Design
The previous steps provide minimum information needs to define the design criteria for concrete panels including:
Regarding he above criteria, the designer/s can estimate the appropriate thickness using, tables, figures, empirical formulas, or a commercial FEM software package, and check if the?calculated shear and bearing stress in panels are satisfied against the pre-specified performance criteria.
If edge load is anticipated, the designer may need to increase the thickness of the panels’ edge to satisfy the structural performance of the pavement. Subsequently, the design should be proved for stability against concentrated load, showing satisfactory results for punching shear and bearing stress.
The designer/s should satisfy that the result is optimised when performance criteria are satisfied. This may need sub-processes of optimisation either to increase or decrease the thickness of sub-base or panels or altering the method of stabilising of the sub-grade.
The designer/s may also need to specify other features, such as curbs and island, surface drainage, etc. This may also cause to completely revise the drainage design to fulfil the panel specifications.
In certain conditions that panels are susceptible to sliding, the designer may suggest one of the below solutions:
When thickness of panels is greater than 150mm and aggregate interlock is found insufficient, the designer/s should specify the appropriate dowel type or load transfer devise. This may dictate a new vendor assessment to choose a cost effective product for the project. It is also probable that the designer defined different thicknesses for different area of the pavement. The gradual thickness should be specified for transition between panels with different thicknesses.
Executing construction and post-validation
Subsequent to finalising the drawing, a design risk assessment should prove that the design is safe for construction stage. A conference between stakeholders of the project may be necessary before commencing the construction to have agreement about:
An arrangement between PSDP (project supervisor for the design process) and PSCS (project supervisor for construction stage) may be needed. A typical arrangement were suggested before that can be found at https://www.dhirubhai.net/feed/update/urn:li:ugcPost:6961008687209443328?updateEntityUrn=urn%3Ali%3Afs_updateV2%3A%28urn%3Ali%3AugcPost%3A6961008687209443328%2CFEED_DETAIL%2CEMPTY%2CDEFAULT%2Cfalse%29
The knowledge of above procedure was partially or completely implemented in below projects by the author:
1- Heavy duty concrete pavement for Kurdistan Barez tire fabrication plant (Andishevarzan Santa Jam consultant company)
2- Detailed Design of heavy-duty pavements for a warehouse in Logstrup,Tuam, for Coolsivna (Lally Chartered Engineers)
On the whole, the above process shows that how geotechnical engineers can significantly affect the process of design of?HDRCP.?As a rule of thumb,?carbon footprint of increasing 5cm of thickness of concrete pavement for a small warehouse?is equal to the CO2 emission of return flights of 50 passengers between Dublin to NewYork. Thus, for any new design, a wise suggestion when sustainability matters, would be optimising the project as much as possible, and this is not possible without increasing the confidence level about the ground, highlighting the crucial role of geotechnical engineers in pavement design.?
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