understanding geotechnical report(soil test report)

7/26/2019 Understanding Geotechnical Report(Soil Test Report)

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Understanding the Geotechnical Report as an Engineering andConstruction Reference

Eugene Washington, PE

Course Outline

1. Engineer and contractor requirements.2. Geotechnical report contents

3. Analysis of data given in report

4. Soil boring equipment

. !ecommendations for construction methods and slopes

". Soil boring logs

#. Soil description $classification such as silty%sand&

'. Soil properties $hard( stiff( dense( loose( etc.&

). *oring depth

1+. *oring refusal

11. *lo, counts

12. Soil moisture content

13. Soil dry density

14. -article distribution curves $sieve analysis&

1. Atterburg limits $liquid limits and plasticity inde&

1". /ompaction test $optimum moisture for compaction&

1#. 0irect shear tests

1'. Seismic velocity lines

1). -ermeability

Learning Objective

his course introduces the student to the many components that mae up a thorough Geotechnical report. hevarious observations and tests are eplained so that an inference to the ground properties can be achieved. he

Geotechnical report is a comple scientific document that can be confusing to even highly eperienced people.

,ners( architects( engineers( and contractors must be able to understand the implications of the reports in orderto achieve an on time( on budget and failure free proect.

At the conclusion of this course( the student ,ill learn5

hat maor proects usually have Geotechnical !eports.

6o, to interpret the Geotechnical report for consistency.

6o, to evaluate soils for embanment suitability.

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6o, to determine the presence of ground ,ater and ho, it ,ill effect design and construction.

6o, to interpret seismic velocity diagrams for ecavation methods.

o understand that the Geotechnical !eport is often a subective opinion of an individual geologist.

Course Introduction

he Geotechnical report provides critical and vital information for the o,ner( architect( design engineer( and thecontractor to use and evaluate. he o,ner ,ishes to assess the cost of the proect foundation and earth,or. hestructural engineer is responsible for the design of an economical but sturdy building foundation. he architect maybe forced to arrange building layouts to accommodate varying soil conditions. he contractor ,ants to bid acompetitive but realistic price for the ecavation and embanment ,or. 7or these reasons( all the essentialplayers have a personal stae in the Geotechnical report. All these players must be able to understand theramifications of the soil study. A foundation failure of a maor structure is measured in millions of dollars and ruinedcareers. 8t can even lead to loss of life( such as the eton 0am failure. An ignorant or misinformed contractor canlose millions in unanticipated costs. Such mistaes often lead to maor la,suits that drag on for years and costeveryone ecept the la,yers. *eing able to understand and analy9e the Geotechnical report to avoid costlymistaes is in the best interest of all the proect players.

Course Content

Geotechnical $soils& reports are prepared to provide the design engineer and contractor ,ith information regardingthe soil conditions at a specific location. hese reports are a ,ealth of information for the person that can properlyinterpret the information presented. :sually the soil report is primarily ,ritten to give the structural engineer thespecific information needed to effectively design the structural foundations. /onstruction methods and contractorconcerns are usually addressed as a minor side issue and in gross generalities. nly after a number of site visitsand studying several soil reports ,ill you begin to really understand ,hat is being presented by the reports.

he soil report is usually made available at least in part to the contractor during the bidding stage. 8t is in all theparties; best interest to accurately and unambiguously define the ground conditions. he more definitive the soilsreport the greater the confidence the engineer has in developing a foundation design and the contractor ,ill pricethe ,or more competitively.


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Engineer objectives5

he design engineer ,ants to no, ,hat is under the surface to support the structures. o that end( the engineerneeds to no, several critical soil properties5

1. 2.

3.

4.

.

".

#.

'.

).

1+.

11.

12. /an the native soil be used for bacfill>

13.

14. Are ha9ardous ,astes present>

1. 6o, corrosive are the soils>

1".

Contractor questions5

he contractor ,ants to no, ,hat the subsurface conditions are so that an accurate estimate of costs and timecan be entered into a competitive bid. o that end the contractor ,ill search the soils report to determine thefollo,ing5

1. 2. 8s there roc to be drilled and shot $sometimes blasting is not allo,ed&>

3. /an the ground be ripped ,ith a bulldo9er>

4.

. 8s there enough space on the ob to store bacfill materials>

". /an the native material be used for bacfill>

#.

'. 6o, much bacfill must be bought and imported>

). 6o, steep can the temporary ecavation slopes be cut>

1+. /an obstructions be epected>

11.



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12.

13.

14.

@o, that ,e have ased the questions( ho, do ,e glean ans,ers from the soil report> he report is directedmostly to the design engineer and the contractor usually must mae an interpretation of the information to develop

a construction plan. his interpretation is often vital to the success of the proect. 8f the report is ambiguous or failsto properly identify the ground conditions( the result is often a changed condition claim. hese claims can entaillengthy delays( increased cost( disputes and la,suits. 8t is important to read the entire report and understand thateach geologist has a different style of describing the soils( as they are field sampling.

!ecently( a geologist described the soil in one report as dense silty%sand. his classification ,as suspiciousbecause the borehole ,as advanced ,ith a carbide tipped rotary drill and hit refusal above the required ecavationdepth. 7urther investigation revealed that the soil ,as in fact un,eathered massive granite roc. *ecause blasting,as not allo,ed( the ecavation costs ,ere estimated at +.++ per cubic yard versus 3.++ per cubic yard. hisamounted to nearly a million dollars of added ecavation cost to the estimate. A cursory revie, of the report couldeasily mislead the o,ner( engineer and contractor ,ith disastrous results.

Geotechnical Report Contents5

he ,ell%prepared Geotechnical report ,ill be organi9ed along the follo,ing outline5

1. 8ntroduction5 his ,ill identify the proect by location and name. 8t ,ill also briefly outline the scopeof the investigation.

2. -roect 0escription5 his ,ill give an overvie, of the structures( ,ith proposed foundation depths.

3. 7ield Eploration and esting5 his ,ill identify the methods and equipment used to bore and testthe soils.

4. Site conditions5 his ,ill describe the terrain( prior no,n land use( general area geology(ground,ater( fault proimity( seismic shaing( landslides and other concerns such as sin hole orfracturing problems.

. !ecommendations5 he various eplorations and tests are translated into specific loading criteria(settlements( de,atering requirements( seismic accelerations( pavement sections and sitecoefficients.

". Site observations5 his is ,here the appeal is made that it is important to have Geotechnicaleperts revie, the actual ecavation procedures and the ,arning that differing soil conditions maybe found.

#. Baps5 A general area map ,ith geology and faults is usually included. Site maps ,ith boring loglocations are included.

'. Cogs5 his ,ill contain the boring logs ,ith soil densities( blo, counts( ground ,ater elevations(moisture( soil classifications and sample locations.

). est results5 his section includes sieve analysis( optimum moisture plots( direct shear tests( conepenetrometer( contaminants and other various tests that are deemed necessary.

1+. Seismic velocities5 his procedure is used ,hen hard roc is epected. he sound speed throughroc is a good indicator of ,hat methods ,ill effectively ecavate the ground.

he contractor must mae an interpretation of the soils report in order to ans,er the 14 common questions. hereare a number of clues in the report that ,ill assist the contractor in maing decisions to select the appropriateconstruction methods( productions and estimating earthmoving costs. here is no one indicator to sho, ,hat theground conditions really ,ill be. he entire report must be read( absorbed and analy9ed. he report ,ill containobservations( recommendations and test results that must be individually interpreted. Sometimes apparentlyconflicting information ,ill be presented. Also( the soil conditions ,ill be described in such broad generalities that it



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is nearly meaningless to the contractor. ften impractical construction solutions are offered( such as temporaryconstruction slopes that are so flat that the top of slope ,ould etend beyond the easement limits.

he basic clues that must be understood to mae an educated udgement of the soil conditions are5

1. Soil boring equipment2. !ecommendations for construction methods and slopes

3. Soil boring logs

4. Soil description $classification such as silty%sand&

. Soil properties $hard( stiff( dense( loose( etc.&

". *oring depth

#. *oring refusal

'. *lo, counts

). Soil moisture content

1+. Soil dry density

11. -article distribution curves $sieve analysis&

12. Atterburg limits $liquid limits and plasticity inde&

13. /ompaction test $optimum moisture for compaction&

14. 0irect shear tests

1. Seismic velocity lines

1". -ermeability

1. Boring Equipment

he bore hole or test pit is advanced ,ith a variety of equipment. *ore holes advanced ,ith an augermeans that the ground can be ecavated ,ith normal earthmoving equipment( i.e. bachoes( scraper(bulldo9ers and the lie. *ores that are made ,ith a pneumatic( carbide tipped drill or similar roc drillmeans that blasting or heavy ripping ,ill be required.

est pits are usually dug ,ith a bachoe. he si9e of the bachoe maes a huge difference as to ho,easily the ground can actually be dug. A small rubber tired bachoe%loader ,ill sho, refusal $unable to dig&on soils that can be readily ecavated ,ith large trac mounted bachoes or heavy equipment.

!. Recommendations

8f the report is concerned ,ith settlement( liquefaction and suggests over%ecavationD that usually means,ea clayey soils.


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slopes may be required. =ery high%recommended loading of about '+ pcf indicates ground ,ater ispresent and?or oo9ing clay mud is present. !emember this is only one part of the information available.7urther scrutiny of the report ,ill often reveal that the recommendations are ultra%conservative.

3. "oil Boring Logs

he boring logs ,ill detail the soil layers by depth from the surface or by elevations. he log ,ill containsuch information as5 soil classification( relative denseness of the soil( sampling points( sample recovery(,ater content( dry unit ,eight( blo, counts per foot and ground ,ater depth( drill refusal and if ,ell casing,as needed. ften the soil descriptions are subective by being based on the eperience and udgementof the observing geologist. 6o, dense or hard the soil is often based on ho, quicly the drill can beadvanced. he description is often based on the loo( feel and sometimes smell or taste of the soil. heboring logs are a good place to start to understand the soil properties. he boring logs should be plottedon the dra,ings and on cross sections so the relationship of the ecavation( structures is scaled to thesoils and ,ater table. 8f casing of the hole ,as necessary( it usually means the ground is too ,ea to standon a normal construction ecavation slope.

*elo, is an eample of a ,ell%presented boring log

@ote that the log presents the follo,ing data.

1. he date of the boring. his is useful information.


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3. he ground elevation at the boring is given5 #+3.' $BSC&. BSC is for mean sea level.Barine construction often uses mean lo,er lo, tide elevation as a reference. hedifference bet,een these references can cause mistaes( so be sure of the referenceelevation.

4. he drill used ,as a hollo, stem auger. his means that the ground ,as not roc( sinceand auger ,ill not penetrate roc that requires blasting or heavy ripping.

. he soil is logged and classified by depth from the ground surface. Sometimes the log ,illalso reference elevation.

". he blo,s?foot is number of times the 14+ lbs drive ,eight dropped 3+ inches needs topenetrate one foot. he 3# blo,s per foot confirm the description of dense or ,ell%compacted soil.

#. he moisture content range of 1.2 to .# means the soil is very dry and ,ill require a lotof ,ater to achieve compaction.

'. he dry density in the top 4 feet is 12#.# pound per cubic foot $-/7&. his indicates a ,ell%graded soil ,ith a lo, void ratio. 8t can be epected to have significant cohesion andfriction angle.

). he soil bet,een and ' feet of depth has a dry density of only )".' pcf. his indicatesthe soil is poorly graded and a lo, percentage of fines. his soil ,ill probably have little orno cohesion. hat means it may not stand on a construction slope as steep as 16 to 1=.

1+. he soil classification descriptions and relative compactions are sho,n as loose( dense(etc.

11. @ote that no ground ,ater is sho,n. 8f ground ,ater is found it ,ill be sho,n at the depthof encounter as an inverted triangle ,ith a line under it. Sometimes this symbol is smalland easy to miss.

#. "oil $escriptions

his ,ill usually identify the soil by classification of particle si9ing such as cobble( gravel( sand( silt( or clay(etc. Bost soils are a combination of these classifications( meaning there is a gradation of material. Sandand larger grains are often referred as cohesionless soils.


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you cannot be sure of ,hat may be encountered at the bottom of the ecavation. 8s there going to be rocor ,ater that must be handled> 0id an obstruction halt the drilling effort>

(. Boring Re)usal

his is usually indicated at the bottom to the boring log( if it occurs. 8f all refusals are ,ell belo, theplanned ecavation depth( it ,ill not be a maor concern. 8f a small percentage of the borings have metrefusal in the ecavation limits it usually means that there are isolated obstructions. 8t is not possible todetermine ,hat is the obstruction unless the soils report or site history reveals the obect. A small isolatedboulder can stop an auger drill and be of little concern. n the other hand( it could be a ridge of solid rocthat ,ill be epensive to remove.

*. Blo+ Counts

his is a very important measure of soil properties. his is a standard penetration test ,here a 14+%pound,eight is dropped and the number of blo,s to advance the point one $1& foot is counted. bviously themore the blo,s the harder the soil. Coose or soft soils ,ill sho, blo, counts of less than 1+. *lo, countsof 1+ to + blo,s per foot usually mean the ground ,ill be fairly easily ecavated.


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1+. $r- "oil $ensit-

@ormal soils ,ill have dry densities bet,een ) and 13+ pounds per cubic foot $pcf&. Solid roc and heavymetallic ores can have densities eceeding 1+ pcf. Coose cohesionless sand ,ill usually have a densityof about )+ pcf. =olcanic cinders can be as lo, as + pcf. Soil density by itself tells little about the soilproperties. =olcanic tuff can have a density of less than 1++ pcf and be some of the most difficult materialto ecavate. 0ense roc that is fractured and?or ,eathered can often be easily ecavated.

11. "ieve nal-sis

his test ,ill sho, the grain si9e distribution of the various soils. A ,ell%graded soil ,ill have a uniformgrain si9e distribution from gravel to clay. A poorly graded soil ,ill have the grain concentrated around asingle grain si9e. 0ry sands and gravels that contain less than about 1+ clay can be almost completelycohesionless( meaning that ,hile they are easy to ecavate( they ravel to a slope flatter than 16 to 1=.ver si9ed rocs such as cobbles and boulders should be indicated. he oversi9ed material may have tobe screened out for the soil to be used as bacfill or embanment. 8f there is more than about 2 clay(the soil may be unsuitable for bacfill or embanment. :nless the soil is very dry( it is difficult to screenclay out of soils because it clogs the screen. /lay can be removed by ,ashing but it is a slo, epensiveprocess generating a lot of very dirty ,ater that can be difficult to get rid of.

*elo, is an eample of a gradation test or sieve analysis5

he gradation result sho, above is an eample of a ,ell%graded material from 1.%inch gravel to silt andclay. 8f the soil is dense( it can be epected to have significant cohesion and friction angle. he finespercentage of about 1' means that it ,ill behave more lie gravel than clay( ,ith ust enough fines toenhance the compactive effort.

*elo, is another soil gradation analysis5



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his gradation sho,n above is a poorly graded or open graded soil. @ote that most of the grain si9es areconcentrated bet,een the F1" and F+ sieve and only 3.) fines. his is clean sand and probably ,illhave very little cohesion and may require a construction ecavation slope of 16 to 1= or flatter.


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3. C H Co, -lasticity rganic Soil

4. 6 H 6igh -lasticity rganic Soil

. B6 H Elastic Silt

". /6 H 7at /lay

#. G< H


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*elo, is an eample of a maimum density test5

he optimum moisture is sho,n as 11. and the maimum density is sho,n as 11'. pcf. he testmethod used is ASB 0 ")'. his method requires less compactive effort than other methods to achievethe desired relative compaction in the soil. 7or instance( the method that the /alifornia 0 uses requires

at least t,ice the compactive effort to achieve the same relative compaction. 7ind out ,hat the specifiedcompaction method is because it can mae a big difference in cost.

#. $irect "hear 0est

his is very valuable information. his test sho,s the friction angle and the cohesion of the soil. hefriction angle is a measure of the angularity of the soil particles to resist rolling. /ohesion is the measure ofthe bonding of the soil particles in shear. /ombined ,ith the gross soil density $moisture plus dry density&(friction angle and cohesion a slip circle analysis can accurately predict the stable temporary slope theground can be ecavated. :nfortunately( this is a very difficult analysis to perform by hand. here arecomputer programs available to mae the calculation quicly and easily. Some of these programs areinaccurate at steep temporary construction slopes. he author has created a program that is specifically,ritten for construction slopes. his taes the guess,or out of designing construction ecavations and



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allo,s huge savings in ecavation( bacfill( and shoring costs. he program ,ill indicate ,hen speedshores( shields and shoring are appropriate.

*elo, is an eample of a direct shear test5

he three dots are the specific test points plotted on the graph. he slope of the etrapolated line is thefriction angle $it is usually measured ,ith a protractor&. he sloping line intersection ,ith the left verticalais defines the cohesion in psf. Sometimes the cohesion is given in psi( ,hich for lo, cohesion is virtually,orthless since 144 psf is only 1 psi.

S6A classifies all ground in four $4& categories. he first class is !oc ,hich allo,s a vertical cut. henet class is A such as cemented soils and caliche and the recommended construction slope is 3?46 to 1=. he most common class of soil is class * that is compact dense soil and the recommendedconstruction slope is 1= to 16. he last class is / that is for loose ground and the recommended slope is1.6 to 1=. hese recommendations are very conservative. S6A also requires all ecavations overt,enty $2+& feet deep to be analy9ed by a registered engineer. 8t is also ,ell ,orth your ,hile to obtain aslip circle program and analy9e all significant ecavations.

%. "eismic elocit- Lines


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ground. A computer analyses the time delays that are recorded at each microphone. his ,ill sho, thedepth to each stratum and the speed of sound through the stratums. he faster the sound travels throughthe ground the harder the roc. Soil ,ith sound velocities of less than 3(+++ feet per second $fps& is usuallyeasily ecavated. !oc over 1+(+++ fps cannot be ripped efficiently.

Bost roc that can be effectively ripped lies in the 3(+++ to '(+++ fps ranges. he sound velocity is a guideto ripping only. =ery hard roc( such as granite( that is fractured and or ,eathered can sometimes beeasily ripped. n the other hand( relatively soft roc that is monolithic( such as volcanic tuff and caliche(

can be practically impossible to rip efficiently.

*elo, is a plot of seismic velocities plotted by depth from the surface5

he top t,o feet has a velocity of only 1(134 feet per second $fps&. hat means it is probably topsoil orloose alluvium. he net layer ,ith a velocity of (341 is roc. his can probably be ripped using a /at 0'or larger cra,ler tractor. his roc may also be easily blasted unless it is caliche or volcanic tuff. he rocbelo, 22 feet is definitely drill and shoot hard roc. he high velocity of 1'(4)' fps means that it is veryhard( dense and probably the fracture ointing is ,idely spaced.

'. &ermeabilit-

his test is run to determine ho, quicly ground ,ater ,ill travel through the soil. /lean sands and gravelscan transmit huge volumes and require massive pumping to lo,er the ,ater table enough to ,or at thebottom of an ecavation. /lays are impermeable and release the ,ater slo,ly. ften the ground ,ater istransferred through aquifers that are open%graded and very permeable. hese aquifers can be capped ,ithimpermeable layers. here have been cases ,here de,atering ,ells ,ere unnecessarily etended into



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aquifers and forced ecessive pumping. hese tests aid the eperienced de,atering specialist indesigning an appropriate de,atering system.

Soils that contain more than 3+ to 4+ clay ,ill behave lie clay because the clay ,ill more than fill thenatural voids of the larger particles. his means that the larger particles do not have direct bearing on eachother and the failure plane is mostly through the clay fines. /lays can be some of the most difficult andundesirable soils to try to ,or ,ith. -lasticity inde and liquid limit are used to define /lays. Epansive orfat clays have plastic and liquid indees about + or more. hese materials ,ill significantly s,ell and

contract ,ith the change in moisture content. his is often the cause of pavement failure( side,albucling and foundation cracing. /lays can be plastic in that they can continue to settle under afoundation over a long term( causing cracs to develop long after the structure is completed.

/lays vary from being a viscous fluid to dense and stable material.


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nly through study of the site%specific geotechnical report and a site visit can any real udgement ofconstruction methods be made. 8t is also recommended that potholing ,ith a bachoe be performed in thepresence of the estimator for the proect. !evie, the specifications to determine if the soil is suitable forbacfill. he sieve analysis may sho, there is oversi9e that must be screened out. here may be too muchclay fines. he Atterburg properties or sand equivalent may mae the in%situ soil unsuitable and thematerial must be over ecavated and?or disposed of offsite and suitable material bought and imported.

7ailure to understand the geotechnical report has caused many a financial disaster. he contractor must

bid aggressively to ,in ,or. 8t is better to no, the real conditions and avoid costly mistaes that cancause a proect to lose money out of pocet by ignoring or misinterpreting the information contained in thereport. Each geologist ,ill present his or her findings in different ,ays. Some reports are more thoroughthan others for several reasons. 8t is common to find that underground conditions vary from ,hat ,asdiscovered by the test borings. At rare times the o,ner ,ill even deliberately misrepresent the facts to gainlo,er bids. his usually results in a long epensive la,suit over changed conditions that only the la,yers,in.

Each soils report ,ill contain a great deal of information. here is no one test or observation that ,ill tellthe contractor ,hat is the best construction method for each site. Each test and observation is a clue.here are al,ays do9ens or even thousands of bits of information that must be scrutini9ed( analy9ed( andcorrelated. his process can tae several people ,ees to prepare an ecavation plan for a large proectsuch as a dam. nly then can methods and productions be reasonably estimated.

Geotechnical reports al,ays include disclaimers and ,arnings that the ground conditions may vary fromthose found in the borings and test pits. here is good reason for the disclaimers. Geotechnical ,or is stillas much an art as it is a science. Any etrapolations derived form the reports are individual interpretationsof ,hat that person might epect to find. @o one is able to see eactly ,hat is under the ground surface.

All the tests and observations only indicate probable trends that are often subect for debate bet,eenepert geologists. he contractor must be an epert is his o,n right. nly the contractor can and ,illdecide ,hat methods and costs are to be bid for the ecavation and embanment ,or. 8f the contractormaes a udgmental mistae( no else ,ill offer to pay for any of the additional costs.

Once you finish studying the above course content, you need to take a quizto obtain the P! credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part ofD!online.or" or any other person#or"ani$ation named herein. The materials are for "eneral information only. They are not asu%stitute for competent professional ad&ice. Application of this information to a specific pro'ect should %e re&iewed %y a re"isteredprofessional en"ineer. Anyone ma(in" use of the information set forth herein does so at their own ris( and assumes any and all

resultin" lia%ility arisin" therefrom.
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understanding geotechnical report(soil test report)

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