Site Investigations - Lec 12

Site Investigation

• Soil exploration is a part of site investigation.
• Deals with determining in general, the suitability of the site for the proposed construction.

Site Investigation (Objective)

• The nature and sequence of strata
• The ground water conditions at the site
• The physical properties of soil and rock underlying the site
• The mechanical properties such as strength of different soil or rock strata

Importance of Site Investigation

1. Minimize Damages

2. Saves on Cost

3. Determine Construction Costs

4. Increases Project Safety

Initial Information

1. Type of Structure

2. Structure Characteristics

3. Starting Date

4. Construction Method

5. Estimated Construction Period

6. Soil condition (Geological, Geotechnical or Aerial Analysis)

7. Behavior of Existing Structures

Site Investigation (Failures)

• Retaining Walls Failure
• Settlement Failure
• Sinkholes

Site Reconnaissance

• First stage of site investigation
• Researching the site to gain as much geological and historical information as possible

General Site Exploration

• To obtain an approximate picture of sub-soil conditions at low cost.
• Soil sample (experimental borings and shallow test pits)
• Simple laboratory tests (moisture content test, density, unconfined compressive strength test, etc.)

Preliminary Site Exploration

• Approximates values of soil's compressive strength.
• Position of the groundwater table.
• Depth and extent of soil strata.
• Soil composition.
• Depth of hard stratum from ground level.
• Engineering properties of soil (disturbed sample)

Detailed Site Exploration

• Complex projects, major engineering works, heavy structures
• Field tests (in-situ vane shear test, plate load test, etc.)
• Laboratory tests (permeability tests, compressive strength test on undistracted soil samples)

Open Excavation (Test Pits)

• Permits visual inspection of subsurface conditions in natural state.
• Max. depth limited to 18 -20 feet.
• Especially useful for gravelly soil where boreholes may be difficult.
• Sampling/testing done on exposed surfaces.

Auger Borings

• Simplest method of exploration and sampling.
• Power driven or hand operated.
• Max. depth 10 m
• Suitable in all soils above GWT but only in cohesive soil below GWT
• Hollow stem augers used for sampling or conducting Standard Penetration Tests

Power Auger

• Used for boring to a depth of about 10 to 30 m
• Set with a drill rig can be used to obtain samples from deeper strata.
• Some rigs can be used to drill a hole to 100 m depth.

Wash Boring

• Casing is driven with a drop hammer
• Soil is loosened and removed from the borehole using water or a drilling mud jetted under pressure
• The water reaches the ground level where the soil in suspension is allowed to settle and mud is re-circulated.

Rotary Drilling

• A heavy string of the drill rod is used for choking action
• Suitable for boring holes of diameter 10cm, or more preferably 15 to20cm in most of the rocks
• Depth of various strata can be detected by inspection of cuttings.

Percussion Drilling

• Done by alternatively lifting and dropping a heavy drilling bit that is attached to the lower end of the drilling which is attached to the cable
• The stroke of bit varies according to the ground condition
• 45-100 cm in depth with rate of 35-60 drops/min.
• It cannot detect thin strata

Dams

• Geological character and thickness of the strata
• Channels
• Materials
• Induced seismicity, destruction of flora and fauna, resettlement of displaced people
• Geological investigations

Tunnel

• Type and strength of the rock or soil
• Presence of groundwater and its flow characteristics
• Possibility of seismic activity
• Potential for geological hazards

Tunnel

1.Rock or soil type
2.Rock mass quality
3.Geological structures
4.Groundwater
5.Seismicity
6.Slope stability
7.Environmental considerations