206 - Excavation and Backfill for Structures
GENERAL DESCRIPTION
Foundation: A foundation is the lowest load-bearing part of a structure, typically below ground level, that transfers the weight of the structure to the underlying soil or rock. Foundations distribute the load evenly to prevent settlement and provide stability for the entire structure. There are different types of foundations, including shallow foundations (such as a spread) and deep foundations (such as piles or drilled shafts), which are selected based on factors like soil conditions, building loads, and structural requirements.
Pump Station: A pump station, also known as a pumping station or lift station, is a facility equipped with pumps and other equipment designed to move fluids (such as water, wastewater, or storm water) from one location to another. Pump stations are commonly used in water supply systems, wastewater treatment plants, storm water drainage systems, and irrigation networks. They play a crucial role in maintaining the flow within a system and are often equipped with control systems to regulate pump operations.
Retaining Wall: A retaining wall is a structure built to resist the lateral pressure of soil or other materials and to hold back or retain earth, rock, or other materials in a sloped or uneven area. Retaining walls are commonly used to create terraces on steep slopes, prevent erosion, or provide structural support for roads, railways, or other infrastructure. They can be constructed using various materials, including concrete, stone, brick, timber, or modular blocks, and may employ techniques such as gravity walls, cantilever walls, or reinforced soil walls, depending on the specific site conditions and design requirements.
Sound Earth: Sound earth refers to soil that is stable, compact, and free from contamination or other undesirable properties. In construction and engineering contexts, sound earth often denotes soil that provides a suitable foundation for infrastructure projects, with adequate bearing capacity, drainage properties, and stability to support the intended structures.
Sound Wall: A sound wall is structure built along a roadway to act as a noise barrier. Sound walls are used to reduce the noise of traffic from a roadway as it is dispersed to the surrounding area. Sound walls are often located in urban areas. They can be constructed using various materials, including concrete, stone, brick, timber, or modular blocks.
General Overview
Excavation and backfill operations are foundational processes in the construction of any structure or infrastructure project like a bridge or a highway. These operations involve a series of steps to prepare the ground for construction and ensure the stability and integrity of the final structure.
The process of removing earth, rock, or other materials from a site to create space for the construction of foundations or other structural elements begins with site preparation, including clearing vegetation and debris, marking out the area to be excavated, and conducting surveys to identify any underground utilities or hazards. Depending on the scale of the project and the nature of the soil, excavation may be carried out using heavy machinery such as excavators, bulldozers, or trenchers.
MATERIALS
All materials must meet acceptance requirements in the Materials Source Guide and be listed on the Contractor-provided Material Source Lists in the project files.
Description of Materials
Granular Material (Class II)
Granular backfill, often comprised of sand, is used to fill the space around the underdrain system components. This material provides support and stability to the drainage system while facilitating water movement. Generally, Class II sand is used.
Sound Earth
Sound earth refers to soil that is stable, compact, and free from contamination or other undesirable characteristics that could compromise stability.
Aggregate
Aggregate is a collective term for a variety of inert granular materials, such as sand, gravel, crushed stone, or recycled materials like crushed concrete or slag, that are used in construction applications. Aggregates are typically sourced from natural deposits, quarries, or recycled from demolition waste. They are commonly classified by size, shape, and gradation.
Geosynthetics
Geosynthetics are synthetic fabrics made from materials like polypropylene or polyester, primarily used for separating materials, such as a sand subbase and an aggregate base. Geosynthetics, such as biaxial geogrids, can provide stabilization to a road base by interlocking with aggregate materials to improve overall stability. In some cases, geosynthetic fabrics are used to facilitate the drainage of storm water that infiltrates the road base by directing the water to an underdrain system or to ditches. These materials are important because they enhance the structural integrity of infrastructure, reduce the need for additional material layers, improve drainage, and extend the lifespan of roadways by reducing soil movement and providing necessary reinforcement.2
Woven Geotextile Fabric
Woven geotextile fabric can be made of polyester or polypropylene slit films, fibrillated yarns, or monofilaments. Individual threads are woven to create strong geotextile fabric used for separation and reinforcement. The texture is an easily seen coarse weave. Woven geotextiles are semi-impermeable. Due to their poor permeability, woven geotextiles should not be used in areas where vertical drainage is critical.
Non-Woven
Non-woven geotextile fabric is made of synthetic materials such as polyester or polypropylene fibers. Short and long fibers are bound together through needle punching or other methods, and heat treatment is usually applied to improve strength. Non-woven fabric is permeable and is used for filtration and separation.
Geogrid
Geogrid is comprised of extruded polypropylene and can have varying strength axes depending on the application. It comes in uniaxial, biaxial, or multiaxial forms. The openings in the geogrid allow aggregates to interlock with the geogrid. This provides confinement and restrains aggregate from lateral movement. The geogrid is sized for the aggregate that will be placed over it. The main difference between each form is the strength axis.
- Uniaxial geogrid has strength in one direction and the placement orientation is dependent on this direction. Care must be taken to ensure the strength axis is placed appropriately in the field.
- Biaxial geogrid has strength in both axes. Biaxial geogrid provides more stabilization than uniaxial geogrid. This is the most commonly used type of geogrid on roadway projects.
- Multiaxial geogrid has strength in multiple directions and provides the most stability. The grid pattern and amount of strength axes can vary depending on the application.
Pictures of Materials
EQUIPMENT
The following equipment can be used:
- Excavator
- Excavator with plate compactor attachment (Ho-Pac)
- Front end loader
- Backhoe
- Bulldozer
- Dump truck
- Cutoff saw
- Plate or jumping jack compactor
- Shoring equipment (sheet piles, trench boxes, hydraulic shores, etc.)
- Transit or laser level
- Leveling/grade pole
- GPS or total station
Description of Equipment
The equipment used to excavate and backfill will need to be able to dig out the earth to create space for structures, utilities, or foundations and then refill that space once the work has been completed. Following is an overview of equipment typically used in the excavation and backfilling operation. The choice of equipment depends on factors such as the scale of the project, soil conditions, access constraints, and project requirements. Additionally, proper training and adherence to safety protocols are crucial for the efficient and safe execution of excavation and backfill tasks.
Pictures of Equipment
PRECONSTRUCTION
Prior to the start of construction, the Inspector should perform the following:
- Review the plans to verify the location of the work.
- Verify items called out be removed, and whether they will be completely or partially removed.
- Often, only portions of bridges or larger culverts are removed.
- Verify items called out be removed, and whether they will be completely or partially removed.
- Verify MISS DIG has flagged existing private and MDOT utilities.
- Review all pertinent environmental restrictions. This will be included within the project plans, special provisions, and/or permits.
- These could pertain to endangered or migratory animal species, or to prevent spread of disease between plants and hazardous materials such as asbestos.
- Review the plans and the Special Provision for Maintenance of Traffic. There may be restrictions or staging requirements. Installation restrictions are typically associated with staging requirements on a project and access to side streets and driveways.
- Review the plans and proposal for the following special provisions:
- Vibratory exclusion areas.
- Vibration monitoring.
- Demolition plan requirements.
- Asbestos removal.
- Verify all materials are approved for use and meet specifications, and verify stockpiled materials are protected from weather and sunlight as required by the specifications and manufacturer recommendations.
- Obtain photographs of material tickets, storage method, and location.
- Hold an onsite meeting with the Contractor to discuss:
- The construction methods that will be used to complete the work.
- Methods or operations that will be used if existing facilities must be maintained during the work.
- Required traffic control measures, proximity of traffic to the work area, and maintaining driveways during construction.
- Identification of right-of-way and/or grading easements required to complete the work.
- Review of existing features, such as sprinklers, signing, or landscaping in the vicinity of the work, and what methods will be used to protect them.
- Coordination and permit requirements for work on or near a railroad.
- Communication required if impacting residents or businesses during installation.
- Residents and businesses should be contacted if there will be disruptions to services. Coordination by the Contractor with impacted users may be required if disruption is necessary.
- Review of local ordinances.
- Disposal of excavated material and material stockpile locations. The Contractor should provide authorization letters to the Engineer noting material stockpile and disposal locations.
- Soil erosion and sedimentation control measures that will be utilized to minimize soil erosion and subsequent sedimentation.
- Environmental restrictions associated with the project site.
- Methods for securing the site during work operations and at the end of each workday.
Submittals and Shop Drawings
The following submittals or shop drawings must be approved by the Engineer prior to the start of construction, if applicable: • Cofferdam shop drawings that detail the design and construction of the cofferdam if it has been determined that a dry work area is needed during excavation near water bodies. • Sheet piling shop drawings. • Shoring or support of excavation plan. • Earth change plan.
CONSTRUCTION
Excavation
The Contractor should be familiar with the excavation standards set by the Occupational Safety and Health Administration (OSHA). Many factors affect the determination of an acceptable sloped excavation. Refer to OSHA Regulation 1926, Subpart P, Excavations, for all applicable sloping, shoring, and soil classification safety standards for excavation. Existing soil material, groundwater elevation, utilities, trees (roots), and proximity of paved surfaces (roads, sidewalks, and curbs and gutters) may all be considered by the Contractor when deciding how the trench will be excavated. At a minimum, excavated material must be stored 2 feet away from the excavation edges. Following are various sloping requirements in accordance with OSHA regulations.
Sloping Requirements of Soil by Type
Maximum Allowable Sloping of Excavation by Soil Type
Maximum Allowable Slope with Support System by Soil Type
Maximum Allowable Slopes by Soil Type
The Contractor will remove existing material, including existing portions of a structure, to the location shown on the plans. If noted in the plans or approved by the Engineer, footings can be excavated to the exact size of the footing. Excavation of rock should be completed separately from other excavation activities. All surfaces of the rock should be exposed to allow the Engineer to take measurements before excavation begins. Rock is to be excavated in accordance with the requirements of Subsection 205.03.B of the Standard Specifications for Construction.
Backfilling
Before backfilling can begin, the Contractor must ensure the required curing, surface finishing, and structure waterproofing is complete, and all weep holes are covered by a geotextile blanket. Structures must be backfilled uniformly. Uneven backfilling can cause asymmetric loading on the structure, which can cause the structure to fail.
Backfill is considered structure embankment if it is under a foundation, footing, or structural component directly bearing on the backfill material and within a 1-on-1 slope from the edge of the foundation, footing, or structural component. Structure embankment must be placed and compacted to the proposed cross-section in lifts not exceeding 6 inches after compaction. All other backfill must be placed and compacted in lifts not greater than 12 inches in accordance with the proposed cross-sections. The Contractor must ensure the outer limits of the load bearing backfill are met before the remaining backfill is placed.
INSPECTION & TESTING
Inspection Procedures
The Inspector should utilize, at a minimum, the following tools to perform the required inspection: • Measuring wheel • 100-foot tape measure or longer • 25-foot tape measure • Chaining pin • 6-foot stick ruler • 4-foot level • Working plans • Survey equipment • Camera The Inspector should take the following steps during construction: 1. Monitor excavation to ensure it meets all requirements noted in the project documents. 2. Check for any signs of groundwater seepage or instability that may require additional support measures. 3. Observe delivery, placement, and compaction of the material for conformance to the project documents. 4. Verify that backfill layers are placed in a uniform thickness and compacted to the specified density. 5. Inspect the backfill for any voids, pockets, unsuitable material, or areas of inadequate compaction that may compromise the stability of the structure. 6. Perform a moisture test on the aggregate material if payment is based on weight. 7. Complete the required daily verification of compaction efforts using standard density testing procedures. Structure embankment should meet 100% compaction and all density testing should meet 95% of the maximum unit weight with a moisture content not greater than optimum is required as established with: a. The Michigan Cone if natural or slag aggregates are used; or b. The AASHTO T-180 if crushed concrete is used as detailed in the Density Testing and Inspection Manual. c. All excavated material should be either removed from the site or stockpiled for testing to be approved for re-use onsite. 8. Complete quality assurance verification of the trimmed grade and document results of these checks. Possible checks include: a. Cross-section checks. b. Top surface elevation checks. • If AMG is used by the Contractor, a surveyor with a Total Station should be utilized to check surface elevations of the subbase. c. Depth checks. 9. Verify all the excavation is secured per the project documents if backfilling is not completed the same day as excavation. 10. Document any deviations from the original plans or specifications and ensure that they are properly addressed before proceeding with further construction activities.
Testing
For evaluation of aggregate materials, use only certified aggregate sampling and testing technicians. Refer to the Procedures for Aggregate Inspection manual and project specifications for testing requirements and frequencies. The Engineer should track the approved testing of installed aggregate and coordinate with the respective testing lab to ensure the required number of aggregate tests for the project are being completed. For every new source, new day, or suspected change in material, a moisture check needs to be completed. Obtaining moisture readings from a nuclear density gauge is an acceptable method to obtain moisture results of material. The Inspector should ensure the minimum number of required density tests are being performed per the Density Testing and Inspection Manual. Density for backfill needs to be achieved for each lift before subsequent layers are built up: • Structure Embankment: The maximum lift thickness permitted is 6 inches after compaction. Density needs to achieve a minimum of 100% compaction. • Non-Load Bearing Backfill: The maximum lift thickness permitted is 12 inches after compaction. Density needs to achieve a minimum of 95% compaction. Regardless of the intended load bearing capacity, density checks on backfill should occur on every lift. All salvaged granular material should be tested for adherence to Class II requirements before placement.
MEASUREMENT, DOCUMENTATION & PAYMENT
Measurement and Payment
In situ material from the job site may be used for backfilling after the Contractor completes gradation testing of samples taken from an onsite stockpile. The respective MDOT material lab will confirm the gradation of the onsite material meets the requirements for the desired backfill material. In general, all items are paid for by the cubic yard regardless of whether excavation or backfill of a structure is occurring. Backfill that is measured compacted-in-place will be documented and paid for as described in Section 206.04 of the Standard Specifications for Construction. When the pay unit is loose measure, the volume of the hauling units will need to be determined. The Contractor must provide load tickets for each load.
Documentation
The Daily Work Report (DWR) should show all computations, measurements, depth checks, and supporting documentation needed to verify the quantity submitted for payment. Primary supporting documentation includes moisture and grade checks. Additional documentation once the work is completed should include: 1. Length, width, and height being paid on each date. 2. Survey or other approved methods for verification the foundation was constructed in conformance with the contract documents. 3. Volume and tonnage of bedding and backfill material placed. Form 1900, Aggregate Inspection Daily Report, will need to be completed for every sublot of aggregate to track gradation acceptance. Form 0582B, Moisture & Density Determination - Nuclear Method, is used to record density testing results. Delivery tickets must be verified for the source of material against the approved Material Source List. Each ticket must include the required information. Refer to Subsection 109.01.B.6 of the Standard Specifications for Construction for the information required on each ticket.
LOCAL AGENCY PROJECTS
Reserved
RAIL PROJECTS
Reserved