introduction to soils testing & mechanics prepared by: marcia c. belcher, p.e. construction...

INTRODUCTION TO SOILS TESTING & MECHANICSPrepared by:

Marcia C. Belcher, P.E.

Construction Engineering Technology

Geotechnical Engineering

Investigation of site conditions

Evaluation of soil properties, ie.: plasticity (shrinkage/swell) bearing capacity permeability shear strength

What is A Geotechnical Engineer? Expert soils consultant for:

Structural engineer Environmental engineer Forensics

What does the Geotechnical Engineer Do?

Visits the site Takes soil borings (crews perform drill) Samples soil Laboratory testing of soil (lab

technicians) Report and recommendations

Soil Defined

Definition of soil:

aggregate of loose minerals and organic particles

“loose” distinguishes soil from rock

Soil Components

Soil is primarily a mixture of gravel sand clay silt organic material

Describing Soil Types

If a soil is a mixture of components, it is given the name of the greatest component

For example: silty clay sandy clay sandy loam

What is “Moisture Content”

Amount of water vs. amount of solids

Notation for moisture content:

w% = (Mw/Ms)x 100%

Why Do We Care?

Water affects soil behavior:

Let’s consider sand dry sands are soft moist sands are stiffer (beach driving) wet sands are “fluid” (quicksand)

Driving on the Beach

Moisture Content of Soil

Let’s consider clay soils: dry clay is very hard and stiff Wet clay is difficult to compact wet clay is soft and sticky shrink and swell

Dry Clay Looks Like This!

Moist Clay Soils Can Look Like This!

Wet Clay Looks Like This!

Why Do We Care?

If we are going to build a structure: We must know the natural moisture content

of the soil (aka “existing conditions”) We must know the “optimum” moisture

content of the soil for proper compaction (“required conditions”)

Many equations used in soils engineering require the moisture content to be known.

SPECIAL IMPORTANCEMoisture Content

For clays, an optimum moisture content exists for maximum compaction

Good compaction means our foundations won’t settle!!

Moisture content is also important during laboratory procedures used to identify engineering properties of soils.

Performing the Moisture Content Determination Test (ASTM D-2216) Apparatus:

1. Soil2. Tin Cups (3)3. Drying Oven 4. Gram Scale (.1g readability)

Minimum Mass of Soil Sample

Max. Particle Size

Sample Size

≤ 2 mm 20g → CLAY

4.75 mm 100g → SAND

9.5 mm (3/8”) 500g

19 mm (3/4”) 2.5kg GRAVELS

37.5 mm (1.5”) 10 kg

75 mm (3”) 50 kg

Sampling Sand

Mix thoroughly using a metal scoop Specimen must represent water

condition of entire amount of material Select representative sample of

adequate amount (100g min for sand)

Sampling Clay

Trim outside edges of the material to remove edges that are dryer than the rest of the sample.

Use a knife or cutting wire to slice away a representative sample. (20 g min. for clay)

Label Three Tins

Your group number followed by “a”, “b” and “c”

Example: 1-a, 1-b, 1-c

THESE WILL BE YOURS ALL SEMESTER LONG!! KEEP TRACK OF THEM!!

Lab Tests (Avg. of 3):

Record mass of empty drying tins/lids (g) Place samples in tins & apply lids Record mass of samples in tin w/lid (g) Dry samples at 110º C ± 5º until

constant weight. Remove from oven and record mass of

dry samples & tin/lid. Complete data sheet in lab manual

(Chapter 4)

Moisture Content Equation

Dry weights will be posted on the class website.

Be sure your cans are labeled accurately.

w% = (Mw/Ms)x 100%