Excavation: Preliminary Investigations

• Open excavation is frequently seen as simply digging a hole, but it necessitates careful consideration of geological factors, material behavior, and potential environmental impacts.
• Experienced engineers understand the excavation's complexity and the need for thorough planning, material knowledge, and preliminary investigation, all which lead to increased efficiency.
• Neglecting the steps can lead to unforeseen problems, increased costs, and decreased efficiency

Importance of Preliminary Investigations in Excavation

• Geological maps display estimated rock contour lines for strata that differ significantly from the surface material, affecting excavation methods and progress.

• Geotechnical profiles show estimated rock contour lines for different strata to affect excavation and visualizing the mass of material to be excavated.

• As-constructed records of all geological features are critical for validating design-basis information and warning of future troubles

• The first rail subway in Canada was built by the Toronto Transit Commission

Factors Affecting Excavation: Dip of Strata

• Strata can be horizontal or approximately so which dictates the excavation works should be relatively straight and side slopes can be determined with certainty.
• When bedding is inclined, excavation methods are affected, hazards increase, the side slopes must agree with the natural slopes of the bedding
• Faults crossing the excavation area may cause issues by juxtaposing strata of varying hardness and strength.
• Soft ground makes fault detection before digging difficult, increasing hazards

Material Classification

• Soil (unconsolidated material)
• Weak rock
• Rock
• Contract specifications must properly define materials for excavation because claims often arise from open excavation disputes

Major Example of Excavation: St. Lawrence Seaway and Power Project

• This project serves as a prime example of excavation challenges because, despite site investigation, contractors underestimated the challenge of excavating compact glacial till, leading to significant claims for extra payments
• One key issue was the misclassification of till, and therefore contractors weren't prepared for its hardness
• Unexpected material behavior was a key issue since "Cemented" sand and gravel required ripping or blasting, while marine clay was difficult to handle

Economics of Open Excavation

• Volume of material removed open cuts require more material removal, but unit costs are lower due to more efficient machinery

• Side slope stability: If side slopes can be trimmed at an angle, no support is needed, increasing costs

• Geological sections are essential for determining underground conditions, drainage, and potential structural issues

• Drainage considerations apply to open cuts since drainage can impact slope stability and require additional measures so engineers can perform the most economical extraction

• Sir Adam Beck No. 2 Project of the Hydro Electric Power Commission of Ontario is an example

Open excavation in soil

• Open excavation in soil varies from sharp gravel or glacial till to digging in soft, cohesive clay
• Cohesion is a property that is of significance in all types of soil
• Open excavation in soil requires consideration of geological factors, soil mechanics, and the interrelation

Preliminary Investigation

• The bottom grade level of open excavations will be determined either by relative elevation or by general economic considerations
• Side slopes impact the cost

Geologic Factors

• Full consideration in the preliminary and final determinations of side-slope geometry is required with slope designs based on past experiences
• Increased capabilities of large-scale excavation equipment have been marked that average unit prices for excavation on large North American jobs have changed little since 1950, despite a threefold increase in the general price index
• Vertical excavations are require in confined areas may be challenging, especially in clay soils
• Retaining wall usage has grown, but requires careful consideration of soil conditions
• Pile driving can cause vibration, which can affect clay soils, leading to unexpected behavior
• Geologic interpretation of exploratory borings is necessary to anticipate potential obstacles like boulders and rock

Example: Steep Rock Lake Excavation

• Over 230 million m³ of soil was excavated to access iron ore beneath the lake bed.
• The soil consisted of varved clays with high sensitivity and liquidity index.
• Careful cutting with high-pressure water jets and benching were used to achieve stable slopes.

Los Angeles Parking Garage Excavation

• A three-level garage was constructed beneath a 2 ha garden.
• Preliminary investigations revealed the soil conditions, including clean sand, sand and gravel, and Puente Shale.
• Excavation was carried out in two lifts, with precast concrete columns used for foundation construction.

Massachusetts Building Basement Excavation

• A single boring was insufficient to reveal the underlying geology.
• Bedrock was encountered at a depth of 4.5 m, requiring costly removal with a tractor-hauled ripper.
• A more thorough site investigation could have avoided the trouble and extra costs.
• All these case studies demonstrate the importance of geological knowledge in open excavation, regardless of project size.

Support for Excavation

• Worker safety is paramount, and detailed knowledge of subsurface conditions is necessary to ensure safe excavations
• Accurate subsurface information is crucial for designing support systems, especially in complex urban geology
• Common support methods include sheet steel piling, king piles, lagging, horizontal bracing, and inclined struts
• Steel tied-back anchor systems have been used since the 1960s to provide additional support.
• Urban areas with complex geology, such as New York City, require careful planning and subsurface investigation to anticipate potential problems.
• Examples from Los Angeles, Johannesburg, and New York City illustrate the importance of lateral support and subsurface knowledge in excavation projects.

Two Innovative Methods for Supporting Excavation Walls

• Slurry-trench method developed in 1950, uses a bentonitic clay slurry to support exposed soil during excavation. Then slurry gets replaced with concrete, creating a reinforced concrete wall.
• Tieback method involves tying temporary walls into the ground behind the excavation area, anchoring into bedrock for support

Overall Benefits of methods

• Improved safety with Accurate subsurface knowledge and tieback system

• Increased efficiency through working site and effective stabilization, which enables faster excavation

• Cost savings can potentially reclaim land

• Example: Excavation for the World Tour Center in New York

Sinking of Shafts

• This specialty is a technology shared by civil and mining engineers.
• The first requirement is of open-cut planning with accurate knowledge of the strata to be penetrated and the location and level of groundwater
• Shallow shafts can implement well points, though depth limits the methods efficiency
• Pumping from great depth is usually impractical because of cone of depression, possible subsidence effects of the pumping operation upon surrounding property, or at least in lowering local groundwater supplies

Freezing Method

• Freezing has been used in civil engineering for many years

• The first patent for the application for artificial refrigeration in engineering work appears to have been taken out in 1883 in Germany with the first application of that process to shaft sinking in that same year for a mine in Schneidlingen

• Theories of freezing action in soils have been well researched with applications steadily growing

• The use of the freezing process costs is costly, its success is wholly dependent upon the certainty and uniformity of ground conditions all down the shaft.

• The freezing method has been used in the Saskatchewan potash mines and the Antwerp vehicular tunnel.