New EnglandSoils 101

October 8, 2009

New England Soil

Soil is not like concrete or steel

Soil is not always homogenous

Soil is generally reviewed at the surface

Soil is one of the few construction materials with variable design criteria

Need to involve a geotechnical engineer

New England Geology - Soil

Generally glacial soil underlain by shallow bedrock with some marine and post glacial depositsGlacial TillGlacial Lake [glaciolacustrine]Glacial River [glaciofluvial or outwash]Marine Deposit [sand, silt, clay]Post Glacial River [alluvial, fluvial, and organics]

New England Geology - Bedrock

Igneous Granite Schist Basalt

Metamorphic Gneiss Phyllite

Sedimentary Shale Sandstone

Soil Design Criteria

Depends on: Density Grain size [soil type] Moisture content Maximum past pressure

Soil Density Evaluation

Test boring with Standard Penetration Test [SPT]

Cone Penetrometer Test [CPT]

Density Gauge– Nuclear Densometer– Balloon– Sandcone

Estimating Soil Density

Estimate Consistency By:

Soil Condition

Equipment/Visual

Standard Penetration Test (blows/foot)

Cohesionless Cohesive

Loose

Medium Dense

Dense to Very Dense

Hard

Very SoftSoft

Medium

Stiff

Very Stiff

Man standing sinks > 3”

Man walking sinks 2” - 3”

Man walking sinks 1”

Pickup truck ruts ½” – 1”

Loaded dump truck ruts 1” – 3”

Insignificant rutting by loaded dump truck

<2

2-4

4-8

8-15

15-30

>30

Fundamentals of Compaction

Soil compaction is the action of increasing the density of the soil through manipulation, by pressing, ramming or vibrating the soil particles into a closer state of contact

Appropriate soil compaction requires:– Lift thickness– Moisture content– Equipment– Proctor Value

Fundamentals of Compaction Mechanics The mechanics of consolidating fine-

grained soil is very complex involving capillary action, pore pressure, permeability, and other factors.

What are fine grained soils? Impacts of water Past pressure influence

Standard Proctor – ASTM D698

Developed prior to World War II Utilizes a lower compactive effort than the

Modified Proctor 5.5 lb Hammer, 12-inch drop, 25 Blows/lift Typically higher compaction requirements

are recommended (98% Building, 95% Pavement)

Stone correction

Modified Proctor – ASTM D1557

Developed After World War II More energy onto the soil sample than the

Standard Proctor Test 10 lb Hammer, 18-inch drop, 56 blows/lift Stone correction

AASHTO T-180 Method D

Recommended for reclaimed aggregates Similar to Modified Proctor ASTM D 1557 ¾-inch plus material is removed and

replaced with ¼-inch material No stone correction is applied

Moisture Density Relationship [Proctor Test]

116

118

120

122

124

126

128

130

132

134

136

138

140

142

144

146

148

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0

Moisture Content (%)

Dry

Den

sity (p

cf)

Uncorrected Density

Corrected Density100 % Saturation

Moisture Density Relationship [Proctor Test]

RANGE

Moisture Density Relationship [Proctor Test]

Foundation Systems

Shallow foundations Ground improvements Deep foundations

Shallow Foundations

Most common foundation type Minimal engineering [low tech] Generally have the most risk of settlement

Spread Footings

Design based on soil bearing pressure

Typically constructed to frost depth

Shape – square, rectangular, strip

Usually min 3,000 psi concrete

Economical

Reducing Risk

To reduce risk you need to understand the geology and implement recommendations of the geotechnical report

Bearing capacity review– Verify correct soil– Evaluate proofrolling– Evaluate compaction of fill– Appropriate use of geotextiles

Geotextiles

Non-woven geotextile [filter] Woven geotextile [filter and improves

stability] GeoGrid [improves stability]

Shallow Foundation Pitfalls

Frozen subgrades Existing fill conditions Use of crushed stone

Ground Improvements

Preload/surcharge Deep dynamic compaction Rammed aggregate piers Soil stabilization

Preloading/Surcharge Can be used for shallow

and deep cohesive or organic soils

Requires placing fill to design loads before construction

Pre-evaluation of settlement and time

Used with or w/o wick drains to speed settlement

Verify by monitoring settlement

Preload/Surcharge

Deep Dynamic Compaction High energy densification

of soils up to 40 feet deep

More suitable for granular deposits

Systematic dropping weights from 40 to 80 feet. Energy required is a function of depth of improvement and soil conditions

Verify with borings or crater measurements

Rammed Aggregate Piers Compacted aggregate shafts– Patented 1990’s Improved bearing capacity – replace mass excavation

greater than 5 to 6 feet Allows spread footings/soil supported slabs 24 to 30 inch diameter; 10 to 30 feet deep, spacing 8 to

12 feet 20 to 40 ton capacity, verify w/ modulus test

Soil Stabilization

Soil mixed with cementitious materials at surface or in columns

Grouting– Compaction– Jet – Chemical

GeoGrid

Deep Foundations

Driven Piles– Steel HP Sections– Steel Pipe or Shell – Pre-cast Prestressed

Concrete– Timber

Pressure-Injected Footing (PIF)

Drilled Shafts Drilled Mini-Piles

Steel H-Piles

60 to 120 tons End-bearing Full penetration

welded splices Capacity > 50 tons

require load test

Steel Pipe Piles

65 to 125 tons End-bearing typically Welded base plate w/ full

penetration welded splices

Capacity > 50 tons require load test

3,000 to 4,000 psi concrete filled

Pre-cast Pre-stressed Concrete Piles 70 to 135 tons End-bearing or friction Splicing possible but

difficult 4,000 psi concrete 10”x10” to 16”x16”,

square or octagonal cross section

Lengths w/o prestress – 40 to 50 feet

Lengths w/ prestress – 130 feet max

Treated Timber

15 to 25 tons End-bearing or friction Typical length: 35 to

45 ft., max 50 to 55 feet, non spliceable

CCA treated

Pressure-Injected Footings Also known as Frankie

Pile 50 to 150 tons Bottom driven thick

walled drive tube High energy rammed

concrete base 3,000 to 4,000 psi poured

or rammed concrete shaft 10 to 35 feet deep Load test required

Drilled Shafts

100 to 500+ tons End-bearing and friction Often rock-socketed for

high capacity 30 inch to 120 inch

diameter 3,000 to 4,000 psi

concrete Cost: $350 to $450/cy Load test required

Drilled Mini-Piles 20 to 150 tons Friction based,

minor end-bearing Often rock-

socketed for high capacity

4 to 8 inch diameter

4,000 to 5,000 psi grout w/steel center bar

Installed w/ temp steel casing

Questions?