Earthwork: Use of Fine Grained & Granular Material

Introduction

 Earthwork consists of: – Material excavations (cuts) – Material embankments (fills)  Common types of work: – Roadways & Transportation Systems – Foundations Systems – Water Management Systems  (ponds, channels, etc.)

Importance of Soil Quality

 If pavement is to remain smooth & stable..

AND  If structures are to remain stable and undamaged due to settlement…..

THE SOIL ON WHICH THEY ARE BUILT MUST ALSO BE STABLE AND MUST FURNISH UNIFORM SUPPORT!

Materials Normally Suitable for Earthwork:

 Natural Soil Material: – Created by nature (except organics) – Material that is mined or excavated  Material must have a laboratory dry weight of at least 90 lbs. per cubic foot  Materials that weight less generally have too much organic matter …this leads to excessive compressibility

Materials Normally Suitable for Earthwork

 Natural Granular Materials: – Includes broken rock, gravel, sand, durable siltstone, durable sandstone that can be placed in 8” loose lifts.

– These are generally mixtures of coarse & fine particle sizes

Engineering Properties of Granular Soils (Sand & Gravel)

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Good foundation & embankment material Not frost susceptible & are free draining May erode on embankment side slopes Identified by particle size Easily compacted when “well graded”

Engineering Properties of Fine Grained Soils (Clay & Silt)

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Often have low strengths  (crush under load) Plastic & compressible Lose strength when wet or disturbed Practically impervious Slopes are prone to sliding & eroding Frost susceptible Shrink & swell with change in water content

Why is particle size important?

 Soil’s performance is largely dependant on its particle size distribution (gradation)  For example: – Backfill behind a retaining wall must be free draining

WHAT MATERIAL TYPE WOULD WE WANT?

Why is particle size important?

For example: We want a free draining compactable soil for a sub base beneath a foundation.

Stated previously: – compactability of soil largely depends on “gradation” or “grain size distribution” WHAT HAPPENS TO YOU WHEN YOU STEP ON A PILE OF GRAVEL OF UNIFORM SIZE?

WHAT IF THE GRAVEL WAS MIXED WITH SAND AND SMALLER PARTICLES?

Load carrying capacity will be affected by particle size distribution

Construction Specifications

 Construction project specifications set limits for particle size based on the required performance of the soil/material  Engineer will require certain soil/material characteristics  Soil/material must be tested in the lab to determine suitability for the project  FOR EXAMPLE:

Soil Classification

 Soils are classified according the grain size distribution  Both ASTM & AASHTO classification systems reference sieve analysis

We Can Also Accurately Describe Soil Texture

   Soil texture refers to the relative proportion of sand, silt and clay size particles in a sample of soil.

Soils that are dominated by clay are called fine textured soils.

Those dominated by larger particles are referred to as coarse textured soils

 Soil Texture Triangle: – Sieve analysis will reveal % sand, silt, clay (see graph) – What is the texture class for a soil having 45% clay, 45% silt, and 10% sand?

Gradation Examples

Sieve Size Information

Lab Procedures

 Particle Size Analysis testing is broken down into 2 parts: – 1. Sieve analysis (Sand & Gravel Portion) – 2. Hydrometer analysis (Silt & Clay Portion)

Today’s Lab: Particle Size Analysis of Coarse Grained Material ASTM D-422

We will determine “gradation”, or also referred to “particle size distribution” of a granular soil

Our Lab Agenda

  Sieve Analysis This Lab Hydrometer Analysis Next Lab