COSC 421: Powerpoint Notes for Final- Soils and Geotech Reports

geotechnical report example

We had a small project that was going to be an addition to an existing building. The owner had the original construction documents which showed the existing building was built in the 1980's on drilled piers and provided the allowable bearing pressure and cohesion/adhesion. Regardless, we recommended the owner solicit a geotechnical report, which revealed that due to the low loads on the addition and the soil properties, we could use spread footings in lieu of piers, which saved the owner more than 10x the cost of the geotechnical report.

plasticity index

a measure of the range over which the clay sample will retain its plastic characteristics

minimum design lateral soil loads

lateral soil load... backfill material.... GW, GP, GM, GC, SW, SP, SM, SM-SC, SC, ML, ML-CL, CL, OL, MH, CH, OH

stability against sliding and overturning

provide lateral stability to the superstructure by anchoring it to the ground

expansion index

provides an indication of swelling potential of a soil

providing a level surface

providing a level surface reduces the amount of unknowns in the superstructure to be mitigated as it can be assumed they are starting from a constant elevation

geotechnical reports

"the field of soil mechanics and foundation engineering is diverse and complicated, and since it is not an exact science, its application requires specialized knowledge and judgment based on experience"

purpose of geotechnical report

-Geotechnical (soils) reports are prepared to provide the design engineer and contractor with information regarding the soil conditions at a specific location. -These reports are a wealth of information for the person that can properly interpret the information presented. -Usually the soil report is primarily written to give the structural engineer the specific information needed to effectively design the structural foundations. -Construction methods and contractor concerns are usually addressed as a minor side issue and in gross generalities. -Only after a number of site visits and studying several soil reports will you begin to really understand what is being presented by the reports.

auger cast pile install

-advantageous over drilled shafts in cases where groundwater or loose soil conditions cause sloughing conditions that would require casing -advantageous over piles in urban settings where noise is a concern or conventional pile driving equipment may cause vibration induced settlement of adjacent buildings -in seismic regions, rebar cages are required -regardless of seismic requirements it is generally good practice to provide at minimum 1 central vertical bar the full length of the pile

what if a geotechnical report is not required?

-building owner should be made aware of risks associated with not obtaining a geotechnical investigation --external geologic hazards (sloughing, settlement, soil failure) --financial risk (prescriptive design may be very conservative or vice-versa) --project delays due to unforeseen conditions --operational risk (long term maintenance issues) -since building foundation repairs are expensive and impractical, the liability associated with the integrity and quality of the geotechnical report is critical -the return on investment for geotechnical investigation is well worth the cost even if not technically required

clay

-clay particles are the finest of the soil particles, measuring fewer than .002 mm in size -consists of microscopic and sub-microscopic particles derived from the chemical decomposition of rocks -fine grained cohesive soil -stick together readily and form a sticky or gluey texture when they are wet or dry

sand

-consists of particles of rock and hard minerals, such as silicon dioxide -largest type of soil particles, where each particle is visible to naked

drilled pier "caisson"/piles

-downward axial resistance: end bearing + skin friction/cohesion -upward axial resistance: self-weight + skin friction/ cohesion -shear and moment resistance: passive pressure of soil

expansive soil

-estimated to attribute about 9 billion in damage annually to buildings, roads, airports, pipelines, and other facilities -most states have, but it was reported that certain areas of the US such as Colorado, Texas, Wyoming, and California are more susceptible to damage from expansive soils due to large surface deposits of clay and climates characterized by alternating period of rainfall and drought

expansive soil: ASTM D4829 Classification of Potential Expansion of soils using EI (expansion index)

-expansion index: 0-20 (potential expansion: very low) -expansion index: 21-50 (potential expansion: low) -expansion index: 51-90 (potential expansion: medium) -expansion index 91-130 (potential expansion: high) -expansion index >130 (potential expansion: very high)

IBC definition of expansive soil

-in areas likely to have expansive soil, the building official shall require soil tests to determine where such soils do exist -soils meeting all four of the following provisions shall be considered expansive, except that tests to show compliance with items 1, 2, and 3 shall not be required if the test prescribed in item 4 is conducted --1. plasticity index (PI) of 15 or greater, determined in accordance with ASTM D 4318 --2. more than 10 percent of the soil particles pass a No. 200 sieve (75um), determined in accordance with ASTM D 422 --3. more than 10 percent of the soil particles are less than 5 micrometers in size, determined in accordance with ASTM D 422 --4. Expansion index greater than 20, determined in accordance with ASTM D 4829

common types of shallow foundations

-isolated spread footing -continuous strip spread footing -mat (raft) foundation -stiffened slab-on-grade (conventionally reinforced and post-tensioned) -raised wood floor

topsoil

-large percentage consists of organic material that will decompose over time -need to be removed to provide proper bearing subgrade

stiffened slab-on-grade

-most economical for foundation on expansive soil -used for light structures on expansive soil (light commercial and residential) -the rides (grade beams) stiffen the foundation to dampen out the cyclic movement of the expansive soil -from proper performance, require moisture management ---positive drainage away from structure ---gutter system with downspouts and extensions ---large trees located far away ---uniform landscaping and irrigation

are geotechnical reports required by the IBC?

-not always -the building official shall be permitted to waive the requirements for geotechnical investigation where satisfactory data from adjacent areas is available that demonstrates an investigation is not necessary... except for the following conditions: --questionable soil --expansive soil --high ground water table --need for deep foundations --where shallow foundations will bear upon more than 12" of compacted fill material --irregular rock strata --restrictive setbacks or clearances --slope stability --liquefaction --seismic design categories C through F

are geotechnical reports part of the contract documents?

-not necessarily, and shouldn't -issues for information only (ref documents): --owner can attempt to limit liability for contractor interpretation and changed conditions --geotechnical reports contain recommendations, not requirements --often have multiple recommendations for foundation system, backfill, etc. --since structural drawings are required to provide specific requirements in mandatory language, structural drawings and geotechnical reports will never completely align, which can invite a claim by contractors citing contradictory information -If included it will generally be within the specifications

shallow foundations

-often selected when the structural load will not cause excessive settlement of the underlying soil layers or where expansive soil is not a concern -in general are more economical to construction than deep foundations

driven piles (bearing pile and friction pile)

-piles can generally be categorized into one of the following depending on the amount of soil displaced during driving: --low displacement (H-section, open steel pipes) --high displacement (prestressed concrete, timber) -end bearing pile, friction pile, and batter piles -combined end-bearing and friction

casing removal

-pull temporary casing with a slow and smooth vertical motion maintaining casing in a plumb position -casing shall not be pulled until concrete has been placed to a minimum of 5 ft above external water or slurry level or level of unstable soil -during pulling maintain concrete level a minimum of 5 ft above bottom of casing -vibrate top 5 ft after temporary casing is removed

silt

-sediment material with an intermediate size between sand and clay -carried by water during flood it forms a fertile deposit on valleys floor -non-plastic or low plasticity material due to its fineness -when wet, it becomes a smooth mud that can form easily into balls or other shapes in your hand and when silt soil is very wet, it blends seamlessly with water to form fine, runny puddles of mud

limitations

-site characteristics as provided are for design purposes and not to estimate excavation cost -any use of our report in that regard is done at the sole risk of the excavating cost estimator as there may be variations on the site that are not apparent in the data that could significantly impact excavation cost. -any parties charged with estimating excavation costs should seek their own site characterization for specific purposes to obtain the specific level of detail necessary for costing. Site safety, and cost estimating including, excavation support, and dewatering requirements/design are the responsibility of others. - This report was written and recommendations were made based on the soil data collected between January 11 and February 2, 2017. If construction is delayed or the proposed area experiences severe weather conditions, please contact the geotechnical engineer prior to construction. No warranties, either expressed or implied, are intended or made. In the event that changes in the nature, design, or location of the project as outlined in this report are planned, the conclusions and recommendations contained in this report shall not be considered valid unless the Geotechnical Engineer reviews the changes and either verifies or modifies the conclusions of this report in writing.

deep foundations types

-straight-shaft drilled piers (caissons) -belled piers -auger-cast piles -drive piles -soldier piles -secant piles

isolated spread footing

-subgrade must have adequate bearing capacity, and ideally note expansive -must bear below the frost depth -deep enough to prevent undermining if running water is present -typically support columns (large axial loads)

unclassified excavation

-the contractor is required to excavate to sub grade elevations specified, regardless of the character of materials or obstructions encountered -no additional costs will be paid by the owner for any underground obstructions encountered -all the risk is on the contractor vs classified is based on unit rates and you will get a change order from that -what would happen to the earthwork contractor if it was unclassified, the contractor would raise his price

continuous strip footing

-typically used to support walls -same limitations as isolated footings

prevent undermining

-undermining removes soil that the foundation is using to transfer loads into the ground -ex: overtopping of dam can lead to downstream scour, undermining of the dam, and instability

mat "raft" foundation

-used to spread the load of the structure over a large base. foundation has to be stiff enough to resist differential settlement -for tall buildings, generally requires deep over-excavation (basement) to obtain the necessary bearing capacity -still considered a shallow foundation

Geotechnical report: minimum requirements for deep foundations

-where deep foundations will be used, a geotechnical investigation shall be conducted and shall include all of the following, unless sufficient data upon which to base the design and installation is otherwise available: 1. recommended deep foundation types and installed capacities 2. recommended center-to-center spacing of deep foundations 3. driving criteria 4. installation procedures 5. field inspection and reporting procedures to include procedures for verification of the installed bearing capacity where required 6. load test requirements 7. suitability of deep foundation materials for the intended environment 8. designation of bearing stratum or strata 9. reductions for group action, where necessary

geotechnical report: minimum requirements: reporting

-where geotechnical investigations are required, a written report of the investigations shall be submitted to the building official by the owner or authorized agent at the time of permit application -this geotechnical report shall include, but need not limited to, the following information: 1. a plot showing the location of the soil investigations 2. a complete record of the soil boring and penetration test logs and soil samples 3. a record of the soil profile 4. elevation of the water table, if encountered 5. recommendations for foundation type and design criteria, including but not limited to: bearing capacity of natural or compacted soil; provisions to mitigate the effects of expansive soils; mitigation of the effects of liquefaction, differential settlement and varying soil strength; and the effects of adjacent loads 6. expected total and differential settlement 7. deep foundation information in accordance with section 1803.5.5 8. special design and construction provisions for foundations of structures founded on expansive soils, as necessary 9. compacted fill material properties and testing in accordance with Section 1803.5.8 10. controlled low-strength material properties and testing in accordance with section 1803.5.9

geotechnical report: minimum requirements for compacted fill material

-where shallow foundations will bear on compacted fill material more than 12 inches in depth, a geotechnical investigation shall be conducted and shall include all of the following: 1. specifications for the preparation of the site prior to placement of compacted fill material 2. specifications for material to be used as a compacted fill 3. test methods to be used to determine the maximum dry density and optimum moisture content of the material to be sued as compacted fill 4. maximum allowable thickness of each lift of compacted fill material 5. field test method for determining the in-place dry density of the compacted fill 6. minimum acceptable in-place dry density expressed as a percentage of the maximum dry density determined in accordance with Item 3 7. number and frequency of field tests required to determine compliance with Item 6

typical design procedure

1. assessment of site conditions in the context of the site and soil investigation report 2. calculation of anticipated structural loading 3. choosing the foundation type, should consider: -soil condition -type of structure -structural loading -economic factors -time factors relative to the proposed contract period -construction problem 4. sizing the chosen foundation in the context of loading, ground bearing capacity and any likely future movement of the building/structure

soil types

1. course-grained --sand --gravel 2. fine-grained --silt --clay 3, high organic soil --peat 4. topsoil

function of foundation

1. distribution of loads 2. stability against sliding and overturning 3. minimize differential settlement 4. safe against undermining 5. provide level surface 6. mitigate effects of frozen soil "Frost Depth"

temporary casings

Where necessary, install watertight steel casings of sufficient length and thickness to prevent entry of soil or water seepage into shaft; to withstand compressive, displacement, and withdrawal stresses and to maintain stability of shaft walls. The use of mud slurry to lubricate casings or seal off water will be allowable only with the prior approval of the Project Geotechnical and Structural Engineers. Casings may be left in place only with prior approval of the Project Geotechnical and Structural Engineers.

maximum settlement

absolute max downward movement of any part of the building

wood structural floor with deep foundations

advantages and disadvantages of structural floor with crawl space and deep foundations

USCS (United Soil Classification System)

coarse grained soils and fine grained soils

depth of seasonal moisture change

depth in which the soil moisture content is relatively unchanged between the summer and winter periods

shallow foundations: load distribution

distribute the loads from the superstructure to an area of the soil in which intensity of load at its base does not exceed the safe bearing capacity of the soil

expansive soil (edge drop and edge lift)

heave: when the expansion soil gains moisture and rises. loss or gain in moisture in soil

deep foundations: load distribution

loads are transmitted either through end bearing, side friction/cohesion, or both

differential settlement

max difference between two point in a foundation

layman definition

near to at-bear surface clays that dry out during periods of drought but then expand during the rainy season or when they get wet by irrigation water or water from leaky pipes

deep foundations

often selected for large structural loads, or where settlement or expansive soil are a concern

prescriptive masonry foundation walls

plain masonry foundation walls

prescriptive load-bearing values

presumptive load bearing values: crysalline bedrock, sedimentary and foliated rock, sandy gravel and/or gravel (GW and GP), sand, silty sand, clayey sand, silty gravel, and clayey gravel (SM, SP, SM, SC, GM AND GC), clay, sandy clay, silty clay, clayey silt, silt and sandy silt (CL, ML, MH, and CH)

stiffened slab-on-grade: conventionally reinforced

section cut

stiffened slab-on-grade: post tensioned

section cut

minimize differential and maximum settlement

settlement is the vertically downward movement of a structure due to the compression of the underlying soil because of a load

types of foundations

shallow and deep

types of load distribution

shallow and deep foundations

structural slab on carton forms w. deep foundations

structural slab with void space and deep foundations: advantages and disadvantages

classified excavation

the following classifications of excavation will be made when rock excavation is encountered in the work: -earth excavation: includes excavation of pavements and other obstructions visible on ground surface; underground structures, utilities, and other items indicated to be demolished and removed; together with earth and other materials encountered that are not classified as rock or unauthorized excavation -rock excavation in trenches and pits: includes removal and disposal of materials and obstructions encountered which cannot be excavated with a 1.0 cubic yard (heaped) capacity, 42" wide bucket on track-mounted power excavator equivalent to Caterpillar Model 215, rated at not less than 90HP flywheel power and 30,000lb drawbar pull. Trenches in excess of 10'-0" in width and pits in excess of 30'-0" in either length or width are classified as open excavation

friction pile

typical for use in soft clays where the end bearing is small due punching shear at the pile tip

end bearing pile

typical when a soft upper layer is underlain by a dense or hard stratum

building earthwork excavation types

unclassified and classified

batter pile

used for high lateral resistance

importance of frost depth

when soil freezes it expands, if the foundation is bearing above the depth in which the soil freezes, it will cause heave

geotechnical RFP: information geotechnical engineers should provide

• Clear and unambiguous recommendations for foundation system •Clear testing recommendations for fill materials •Response to checklist of items the structural engineer has provided •Additional testing recommendations if needed

geotechnical RFP: information structural engineers should provide

•Accurate loading •Site plan with proposed boring locations •Preliminary plan including column locations •Type of structural system •Info on use of proposed structure / adjacent facilities •A specific list of items needed from the geotechnical recommendations, including multiple foundation options if desired. •Requirement for Geotech to review completed foundation drawings

geotechnical report contents

•Specific information required by the structural engineer to design the foundation. •Construction methods and contractor concerns •Typically very vague and includes many generalities since ultimately means and methods is left up to the contractor unless it will cause detrimental effects