Site Investigation and Pavement Design - Assignment Example

NAME: xxx INSTITUTION: xxx UNIT CODE: xxx UNIT NAME: xxx TITLE: SITE INVESTIGATION AND PAVEMENT DESIGN LECTURE: xxx ©2016 Table of Contents Objectives 1 Introduction 1 CBR Test Description 2 Results 3 Calculations of the amount of water to be added or removed from test portion 3 Readings and Calculations at Compaction before Soaking 3 Penetration Test Readings 4 Discussion 5 Calculated CBR Value 6 Design Traffic Loading 6 Pavement Design 6 Construction Methodology 8 Conclusion 8 References 10 Objectives 1. To determine the importance of field investigation in a road project 2. To determine a subgrade design CBR 3. To select a design traffic load 4. To propose a Thin Sealed pavement design 5. To propose a basic construction methodology Introduction Before an engineer can embark on any project, it is paramount that he understands the arrangement and properties (physical, mechanical and chemical) of the materials under the proposed construction site with reasonable accuracy and currency [Roy09]. This understanding is crucial since the durability and functionality of any engineering project, both structural and roads, relies heavily on the conditions of the underlying foundation and the ability of the structure to adapt to any changes that might occur in those conditions. Site investigation (otherwise referred to as field investigation) is conducted in a bid to allow for the assessment of the ground conditions (geo-environmental and geotechnical) and the environmental and engineering considerations that need to be analyzed with regard to the proposed development [LiX04]. In the case of roads projects, field investigation is very important given that the project spans across different kinds of underlying soil strata, unlike structural projects, which tend to occur on a single type of sol structure. Therefore, different road design is required for each section to accommodate these changes in soil strata. In road projects, this site investigation is done at periodic intervals so that the construction team is well aware of the conditions they are dealing with. The main information acquired from a site investigation is [Civ14]; The sequence and nature of the soil and rock strata (layers) The hydrological conditions at the site(ground and surface water conditions) The physical, chemical and mechanical properties of the soils and rocks found under the proposed construction The extent of the site investigation for a road project is often limited to the actual area where the road will be constructed. However, in some cases, the team will be required to carry out tests over the full road reserve width to determine if the adjacent materials could have any adverse effect in the foundation of the road, or the road structure itself. The depth of the field investigation is limited by the depth to which the effects of the construction of the road will be felt by the underlying strata, or until an incompressible stratum is reached, usually the base rock layer. This layer can support the loads subjected to the road structure by vehicle passing over it without any adverse effects. One of the tests used to carry out this field investigation is known as the California Bearing Ratio (CBR) test, which looks to determine the bearing pressure that the underlying soils strata can support. This report focusses on the CBR test. CBR Test Description The California Bearing Ratio (CBR) is a penetration test used to evaluate the mechanical strength (load bearing strength) of existing natural ground, pavement subgrades and base courses under constructions in new carriageways. The California Department of Transportation developed the test, thus the name. The pressure (load) required to penetrate the substrate under consideration is measured. This pressure is applied using a plunger of standard area. The measured pressure is then compared to the load required to penetrate standard crushed rock material to the same level (13.2 kN and 19.8 kN for 2.5mm and 5mm penetration respectively). The two load values are divided to give the CBR value. CBR was developed mainly to determine the load- bearing capacity for roads, but can also be used for unpaved airstrips and the soils under paved airstrips. The higher the CBR value for a substrate, the higher the load- bearing capacity. It is possible to achieve a CBR of more than 100 for some well-compacted materials The spacing for the sites where the CBR test sites are located for different pavement constructions are as follows;[Eng08] 100m for urban projects, minimum of 2 tests 250m for rural projects, minimum of 3 tests Results Calculations of the amount of water to be added or removed from test portion Record the Mass of Wet Material () 500 g Note: Clay sample minimum of 5000g required, Crushed Rock minimum of 8000g required Determine the Moisture Content of Test Portion () Container + Wet Sample () 230.87 g Container + Dry Sample () 189.7 g Container Weight () 21.48 g Moisture Content – 21.53 % Calculation of Mass of the Dry Material in the test portion 4098.032 g Target Compacted Dry Density (from compaction) 1.57 Maximum Dry Density MDD (from compaction) 1.57 Target Compaction Moisture Content (from compaction) 27.5 % Optimum Moisture Content (from compaction) 27.5 % Calculation for the Mass of Mixing water to be added or removed 226.1 g Mould Diameter Cm Spacer Diameter 15.7 cm Mould Height Cm Spacer Height 6.1 cm Mould Volume Cm Spacer Volume 107.8 Volume of Mould minus the spacer 2091.2 Number of Layers Required N (Standard 3 Layers, Modified 5 Layers) 3 no. Calculations of the Wet Material Required to be added to the mould for each layer 1395 g Readings and Calculations at Compaction before Soaking Mass of Clean Mould Minus Baseplate () – 4.35 g Determine the Moisture Content of Test Portion at Compaction () Container Number 34 Container + Wet Sample () 105.76 g Container + Dry Sample () Container Weight () 2.44 g Moisture Content – % Mass of Mould and Specimen minus Baseplate () 829.5 g Mass of Mould and Baseplate and Specimen before Soaking () 161.0 g Reading of Measuring Gauge before Soaking () N/A mm Readings after Soaking Reading of Measuring Gauge after Soaking () N/A mm Mass of Mould and Baseplate and Specimen after Soaking () N/A mm Penetration Test Readings Penetration (mm) Load (kN) 0.5 0.52 1.0 0.75 1.5 0.91 2.0 1.02 2.5 1.10 3.0 1.16 4.0 1.27 5.0 1.35 7.5 1.43 10.5 1.53 12.5 1.60 Discussion The graph below shows the relationship between the penetration and load applied to the test sample;- Figure 1: Graph of Load against Penetration Calculated CBR Value kN reading at Penetration 2.5mm 1.10 kN reading at Penetration 5mm 1.35 Calculations Bearing Ratio at Penetration 2.5mm 8.33 % Bearing Ratio at Penetration 5mm 6.82 % Calculated CBR of material 8.33 % The subgrade design subgrade for the material tested is Design Traffic Loading The proposed road is to act as an access road in an urban area. Since the road does not exist at the moment, data regarding the actual traffic using the road is non- existent, and as such, the value for actual traffic is assumed to be with a design life of 25 years. Table 1: Design ESA's for roads with no Actual Traffic Data[Eng08] Pavement Design The properties for the proposed road pavement are as follows; 1. Carriageway with two lanes 2. Initial Traffic – . This value is at the completion of construction (Note: ESA’s can be equated to number of commercial vehicles on the road) 3. Traffic growth rate (TGR) – 5% (assumed) 4. Design life (n) – 15 years 5. Vehicle damage factor (VDF) based on axle load survey – 1 per commercial vehicle (assumed) 6. Design CBR value of subgrade soil – 8.33% Calculation Distribution factor (DF) – 0.5 Design traffic Figure 2: Pavement Design Chart, 10-150 msa[Con12] Using the pavement design chart, with a CBR of 8.3% and design traffic of 137 msa, the total thickness of the pavement is determined to be The pavement composition is obtained by interpolation from Pavement Design Catalogue[Con12] a. Bituminous surfacing – 25mm SDBC + 70mm DBM b. Road base – 250mm WBM c. Sub- base – 315mm granular material of CBR not less than 7.5% Construction Methodology The construction methodology for a thin sealed pavement design is done in several steps, namely; 1. Collection of Material The aggregates and asphalt/ Portland cement concrete proposed for use in construction are collected from their various locations 2. Laboratory Testing The materials collected for use in the construction of the road are sampled and all relevant laboratory tests are carried out to determine their suitability at handling the loads the new pavement will exert upon them after construction and commissioning. If the selected materials are not sufficient, it is required to find other alternative materials that can be used in the construction. 3. Design of Pavement Materials Once suitable materials have been identified, the road pavement is designed to the specifications of the guidelines, considering all the relevant properties of the materials chosen and their interaction with each other 4. Construction of Pavement Structure Once the design is completed and approved, construction of the pavement structure can then commence. 5. Evaluation of Pavement Structure Periodic tests are carried out during and after the construction to ensure the specified design is adhered to. This is important for quality control to ensure the road can serve the design traffic for the design life specified. 6. Maintenance Proper periodic maintenance done in a timely manner will ensure the pavement structure constructed remains viable until the design period, or even longer Conclusion Site investigation is the process of determining the hazards and challenges that may be encountered with construction of an engineering structure. This is done through data collection, appraisal of information, analysis and reporting the properties associated with the construction site. The California Bearing Ratio (CBR) test has been determined to be a suitable test to determine the mechanical strength of subgrade soils in pavements. Using the CBR calculated for the sample tested, the pavement thickness was determined to be 660mm. References Roy09: , (Royse, et al., 2009), LiX04: , (Li, et al., 2004), Civ14: , (Civil Engineering, 2014), Eng08: , (Engineering Design Guidelines, 2008), Con12: , (Congress, I.R., 2012), Read More