Construction Tech: Site Evaluation & Soil Investigation

• SEHS2330 is a construction technology and materials course.

Site Evaluation

• Before construction or design, it is essential to obtain information about the soil.
• Site Evaluation has two stages: Site Investigation and Soil Investigation.

Objectives of Site Evaluation

• Suitability: Assess if the site is appropriate.
• Design: Facilitate an economic and suitable design.
• Construction: Plan the best building method and foresee potential issues.
• Effects of changes: Determine changes in the ground and environment and their impact.
• Choice of site: Advise on site suitability when alternatives exist.

Site / Soil Investigation

• Site Investigation: Per BS5930, it's the investigation of the site on the surface and its surrounding environment.
• Soil Investigation: Per BS5930, it’s the investigation of soil and rock below the surface to detect water.

Stage of Site Investigation (BS5930)

• Desk study: Initial research about the site.
• Site reconnaissance
• Detailed examination for design.
• Ground (soil) investigation.
• Topographic and hydrographic survey.
• Follow-up investigations during construction.
• The cost of site investigation is typically 0.5% to 5% of the project cost and depends on size of project.

Desk Study

• Involves preliminary study about the site
• It's mainly an information collection process from sources like Lease conditions, Buildings Department, and Town Planning Department.
• Relevant information is listed in BS5930.

Site Reconnaissance

• Includes a site visit and visual inspection of the site/soil.
• Tasks may include:
• Inspecting existing slopes for gradient and tension cracks.
• Inspecting existing structures, if any.
• Inspecting the surface condition (top soil).
• Observing and recording adjacent structures.
• Local enquiries.
• Checking traffic, obstructions, and access.
• Interviewing local inhabitants.

Soil Investigation

• The soil investigation's extent depends on
  • Ground character and variability, including soil and ground water
  • Project type.
  • Amount of existing information.

Approach to Soil Investigation

• Includes tests on-site or in a laboratory.
  • Shallow trial pit.
  • Deep trial.
  • Hand auger boring.
  • Mechanical auger.
  • Wash boring.
  • Permeability test.

Trial Pits

• Trial pits are machine-dug and used for low-rise building ground investigation and soil inspection.
• They should be located close to the proposed foundation.
• Pits of 4-5 meters deep are normally sufficient.
• The number of pits depends on development scale, soil variability, and developer risk tolerance.

Information from Soil Sample

• Provides data on several factors:
  • Thickness of the top soil layer.
  • Strata pattern, thickness, and depth.
  • Soil types of corresponding strata.
  • General properties of the sub-soil type.
  • In-situ strength and other soil tests feasible when exposed.
  • Laboratory tests for soil and ground water samples.
  • Water table level and fluctuation.
  • Quantity of water in the hole, with description.
  • Depth of any filling in the ground.
  • Density of the soil layer.
  • Moisture content.

Excavation

• It's involved in foundation and basement construction, and in slope stabilization works.
• Excavation involves some uncertainty
• It's unpredictable in terms of time delays, cost overruns, and site safety.
• Excavation's five categories:
• Stripping top soil (organic matters).
• Reduced-level excavation.
• Bulk excavation.
• Trench excavation (laying utilities pipes).
• Hole, pit, or tunnel excavation.
• Excavation is significant but vulnerable in construction.

Angle of Repose

• Angle of Repose represents the maximum angle at which soil remains stable without extra support.
• Drained clay: 45°.
• Wet clay: 16°.
• Gravel & dry sand: 40°.
• Wet sand: 20°.
• The angle of repose is not constant and depends on soil properties, moisture, water table, and soil loading in adjacent areas.

Stripping of Top Soil

• Removes the topmost soil layer with plant/animal matter that could contaminate or compress the structure.
• Top soil stripping depth ranges from 150mm to 300mm.
• Carried out by manual excavation or machines like scrapper or bulldozer.
• The excavated soil must be disposed of in approved landfill and not reused for backfilling.

Reduced Level Excavation

• It creates level ground as reference for subsequent construction.
• The level after excavation is the formation level.
• Site leveling may involve cut and fill.
• Excavation depth is prescribed by the engineer, based on soil and site investigation results.

Pit Excavation

• Used for site investigation (trial pit), manhole, and caisson construction.
• Can be manual or mechanical.
• Deep pit excavation demands a higher degree of safety, especially in manual operations like hand-dug caissons.

Trench Excavation

• Used for pipe laying and cable installation.
• Trench excavation ways:
  • Full depth, full-length excavation.
  • Full depth, successive stages.
  • Stage depth, successive stages.

Soil Support for Trench Excavation

• Shoring is needed for trench depths of 1.5m or more to maintain the cut below its angle of repose.
• Shoring materials include timber (timbering or planking) or steel (sheet pile).
• In Hong Kong, sheet pile shoring is common.

Support to Shorings

• Shorings are supported by:
  • Walings (horizontally along excavated surfaces).
  • Struts (lateral support to shoring).
  • Raking shoring (at an angle).
  • Flying shoring.

Bulk Excavation

• Bulk excavation: large-quantity excavation with substantial vertical/horizontal span.
• This excavation usually uses mechanical plants and requires staged excavation for productivity.

Backfilling of Soil after Excavation

• Ideal backfilling materials are the originally excavated materials if uncontaminated.
• Imported soil is used for back filling in Hong Kong, because construction sites congested and lack storage for proper protection against contamination.
• Compaction should be carried out in layers of 150mm to 200mm in thickness.
• Typically done with a mechanical (pneumatic) compactor or roller.

Mechanical Plant for Excavation

• Two ways of classifying include fixed position vs moving, and digging below vs digging above
• Mechanical plants used in excavation:
• Fixed Position: Face Shovel, Backactor (Backhoe), Grabbing crane, and Dragline.
• Moving Excavation: Bulldozer, Loader, Scraper, Grader, and Trencher.

Major Risks

• Excavation can disturb soil stability, especially when dewatering is needed.
• Risks include:
  • Worker safety (soil collapse, falling).
  • Machines/vehicles falling into pits.
  • Soil movement damaging nearby structures and facilities (MTR, utilities).
  • Landslide potential near slopes.