Exploratory Borings Methods

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Questions and Answers

What is the maximum depth that hand augers can typically reach?

Up to 3 to 5 meters (correct)

Up to 5 meters

Up to 10 meters

Up to 7 meters

Which of the following is a type of hand auger?

Electric auger

Diamond core drill

Posthole auger (correct)

Gas-powered auger

What is the primary use of hand augers in construction?

Drilling through concrete

Removing rocks from the ground

Installing foundations

Exploratory soil work (correct)

What type of project is most suitable for using hand augers?

Highways and small structures Signup and view all the answers

Which auger type is specifically mentioned as a form of hand auger?

Helical auger Signup and view all the answers

What type of information should be collected regarding bridges during preliminary investigations?

Lengths of their spans and loading on piers Signup and view all the answers

Which of the following sources would NOT typically provide information for evaluating a site for construction?

Agricultural market reports Signup and view all the answers

During a visual inspection of a construction site, which evidence might indicate expansive soil conditions?

Deep, wide shrinkage cracks Signup and view all the answers

What aspect of a site investigation can be informed by nearby well measurements?

Groundwater levels Signup and view all the answers

Which factor is NOT considered during the reconnaissance phase of a site investigation?

High flood level records Signup and view all the answers

What minimum number of boring locations is typically recommended for most buildings during site investigations?

One at each corner and one at the center Signup and view all the answers

How is the spacing of boreholes determined according to soil conditions?

Fewer boreholes are needed in homogeneous soil strata Signup and view all the answers

Which of the following factors may dictate changes in the approximate required minimum depth of boring during the drilling?

Encountering bedrock Signup and view all the answers

According to the rules established by the American Society of Civil Engineers (ASCE), which depth needs to be determined to assess the effective stress increase under a foundation?

The depth at which the effective stress increase is equal to (1/10) of the estimated net stress on the foundation Signup and view all the answers

What is the recommended approximate spacing of boreholes for a multistory building?

10–30 meters Signup and view all the answers

Study Notes

Exploratory Borings in the Field

Auger boring is the simplest method for creating exploratory boreholes.
Two types of hand augers are used: the posthole auger and the helical auger.
Hand augers are limited in depth, reaching a maximum of 3 to 5 meters (10 to 16 feet).
These augers are practical for soil exploration on highways and smaller structures.

Preliminary information

The type of structure, intended use, and general design requirements should be determined.
Approximate column loads, spacing, and local building codes should be known for buildings.
For bridges, span lengths, loading on piers and abutments should be established.
Topographical understanding requires considering terrain, drainage features, and existing materials.

Reconnaissance

Conduct a visual inspection of the site to gather information about the topography and possible issues.
Signs of slope creep or deep, wide shrinkage cracks may indicate expansive soil.
Soil stratification can be observed in deep cuts made for highways or railroads.
Vegetation types could indicate soil condition.
Assess nearby wells for groundwater levels.
Examine nearby constructions for cracks in walls (settlement indication) or other issues.
Review soil exploration reports for nearby structures to understand soil stratification and physical properties.
Note high-water marks on nearby buildings and bridge abutments.

Site Investigation

Planning: Define steps for site investigation and future vision for the site.
Test Boreholes: Conduct test borings to gather soil samples.
Soil Sampling: Collect samples at various depths for visual observation and laboratory tests.

Borehole Spacing

There is no standard rule for determining the number of boreholes, but at least one at each corner and one in the center is recommended for most buildings.
Boreholes should be spaced closer together for nonhomogeneous soil strata.
Table 2.1 provides approximate borehole spacing guidelines for different types of structures.

Borehole Depth

Determine the approximate minimum depth of the boreholes.
Depth can be adjusted during drilling depending on subsoil encountered (e.g., bedrock).
ASCE (1972) provides rules to estimate approximate required minimum depth:
- Determine the net increase in effective stress under a foundation with depth.
- Estimate the variation of vertical effective stress with depth.
- Determine the depth where the effective stress increase equals (1/10)q (q is the estimated net stress on the foundation).
- Determine the depth where (Δ𝛔′/𝛔𝐨′ ) = 0.05.
- Determine the distance from the lower face of the foundation to bedrock.
- Choose the smallest of these depths as the approximate minimum depth of boring.

Soil Sampling

Disturbed Samples:
- Representative but not undisturbed.
- Used for grain size analysis, liquid and plastic limits, specific gravity, organic content, and soil classification.
- Not suitable for consolidation, hydraulic conductivity, or shear strength tests.
- Main tool used for collecting these samples is the split spoon.
Undisturbed Samples:
- Preserves the sample's natural structure.
- Used for consolidation, hydraulic conductivity, and shear strength tests.
- Suitable for clays, but difficult to obtain in sands.
- Main tool used for collecting these samples is the thin-walled tube.

Split-Spoon Sampling

Used to obtain disturbed but representative samples.
It consists of a steel driving shoe, a split tube, and a coupling.
The sampler is driven into the soil with a hammer; the number of blows needed for penetration is measured.
The standard penetration number (N) is used to assess soil strength.
The degree of disturbance is calculated based on the area ratio.
Split-spoon samples are typically taken at intervals of 1.5 meters.
Several factors can influence the standard penetration number, including hammer efficiency, borehole diameter, sampling method, and rod length.
Correlations between the standard penetration number and other soil properties are approximate and can be used to estimate the consistency of clay soils, unconfined compression strength, and overconsolidation ratio.

Standard Penetration Test (SPT)

This test is performed to assess the strength and density of soil.
Measurements taken during this test can be used to estimate soil properties like consistency, unconfined compression strength, and overconsolidation ratio.

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Description

This quiz covers the basic methods of exploratory borings, focusing specifically on auger boring techniques. You will learn about the two types of hand augers used in soil exploration and their limitations in terms of depth. Ideal for those interested in civil engineering and construction.