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Geotechnical Testing, Observation, and Documentation 2008 American Society of civil Engineers 61

Soil Engineering for the Technician

A main responsibility of the soil technician is to help confirm that the recom-mendations presented in the geotechnical report are implemented in the field during the grading and other construction processes. to properly interpret the recommendations, the technician must be familiar with soil engineering termi-nology. both trainees and more experienced field personnel can use the glos-sary in Appendix A as a quick reference.

Project Preparation

many grading recommendations in the geotechnical report are common prac-tice, such as removing debris, stockpiles, and the stripping of vegetation. mini-mum requirements typically include the removal of undocumented fill, as well as porous (collapsible) and loose or soft soils, followed by the preparation of the exposed soils by scarification, moisture conditioning, and compaction. however, each soil report is based on specific site conditions and must be read carefully. too often, technicians may become overconfident and neglect to read the geotechnical report closely, thus overlooking a recommendation that may not be typical during normal grading operations.

it is good practice for the technician to prepare for a new project by highlight-ing specific recommendations in the geotechnical report. these include the following:

depth of removals (cut or over-ex);

type of materials to be removed (such as loose, soft, porous, expansive, or highly cemented soils);

degree of compaction recommended (which may vary with soil conditions or proposed structure type);

moisture limits to be targeted during compaction (i.e., near optimum, 2 to 4% over optimum, etc.);

5

Geotechnical Testing, Observation, and Documentation

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Geotechnical Testing, Observation, and Documentation 2008 American Society of civil Engineers

62 chapter 5: Soil Engineering for the technician

placement of specific soil or material (including oversized material, expan-sive clays, gypsiferous soils, or other unique materials); and

allotherproject-specific recommendations.

Pay careful attention to the type of soils encountered in the geotechnical report through close review of the boring and/or trench logs. compare mate-rials exposed during excavation and site grading with those described in the soils report. Any discrepancy should be discussed with the project engineer immediately.

Flatland Projects

When grading generally flat sitesafter removal of unacceptable materialthe degree of compaction and percent moisture content are usually the most criti-cal factors. however, prior to placing any fill, the existing ground surface must be prepared by scarification (typically 6 in. deep), moisture conditioning, and then compaction.

When placing expansive soils, the moisture content of the material is as impor-tant as the degree of compaction. many silty and clayey soils are extremely sen-sitive to moisture changes. in some silty soils (diatomaceous soils, for exam-ple), a variation of only a few percent in water content could change the soils dry density by as much as 5 or 10 lb/ft3 during compaction. Also, many plastic soils increase in volume (expand) with added moisture (and, conversely, shrink when dried back); therefore it is standard practice to place expansive soils in a slightly over-optimum condition, and often at a lower degree of compactioncompared with nonexpansive soilsto help limit expansion potential.

Even sandy and silty noncohesive soils generally compact better when placed at or slightly above optimum, thus lubricating the particlesas well as helping to mitigate any future settlement or consolidation caused by increases in mois-ture from landscape watering or heavy rains.

road construction

the final surface of the road sectionthe asphaltdepends on the materials supporting it: the aggregate base and the subgrade. the first step is the prepara-tion of the existing ground surface to create a compact, stable subgrade. often the area must be cut down to reach the proposed finish subgrade elevation, during which time rocks and/or soft pockets are often exposed. to create a homogeneous subgrade, it is important to prepare the cut surface (prior to plac-ing aggregate base) by scarifying, removing cobbles and larger rock, moisture conditioning, and then compacting. improperly prepared subgrade is often the cause of potholes, cracks, and areas of uneven pavement.

During the grading of subgrade and aggregate base course for road construction it is important not only to test for compaction, but also to observe the actions


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chapter 5: Soil Engineering for the technician 63

of the compaction equipment on the road material during the compaction pro-cess. For instance, although a density test taken on the subgrade may indicate that the moisture content of the material is only a few percentage points over optimum (which is generally desirable within building areas), you may notice that the subgrade soil is moving (pumping or rolling) beneath the compaction equipment. Soft and yielding soils are not acceptable. the surface must be firm and relatively unyielding prior to paving. Soils that deflect or move can be det-rimental to pavement.

Tip: it is a good idea to walk next to the compaction equipment during the final compaction of both the finished subgrade and aggregate base surfaces to closely observe the action of the material for any movement or deflection beneath the tires or roller. before accepting a subgrade or aggregate base as finished, the surface should be proof rolled. A fully loaded water truck works well for proof rolling. beware that some con-tractors will try to proof roll with a partially filled truck!

often, remixing and/or drying back of the material to bring it closer to optimum, followed by recompaction, will stabilize the subgrade. however, sometimes overly wet or soft soils need to be excavated and replaced with compacted aggregate base or other acceptable material. For more severe cases of unsta-ble subgrade, stabilization may first include the placement of woven fabric or geogrid, overlain by compacted aggregate base. (See Fig. 5-1.)

Tip: Woven stabilization fabrics work quite well when used properly. however, all too often the fabrics are placed improperly and are not overlapped enough, or not loaded down with sufficient aggregate base or other material. A minimum overlap is usually 24 in., and experience has shown that less than 18 in. of aggregate base cover is often inad-equate.

the technician should also watch the finished baserock surface for any nesting or segregation of the material. Sometimes surface areas or pockets containing mostly gravel with few or no fines may occur. this segregation (nesting) may be due in part to too much rubber-tire traffic allowed on the baserock prior to paving. Surface areas that are not homogeneous or fail to meet gradation stan-dards should be remixed and recompacted.

Density testing of aggregate base can sometimes be tricky. if a baserock sec-tion is relatively thin, 5 in. or less, better test results may be obtained by using the backscatter mode of a nuclear gauge. many times when the drill rod is being driven in (to prepare for a direct transmission test), the baserock may move and become too disturbed to accurately test. using the backscatter mode will not disturb the aggregate base, and silica sand (or aggregate base fines) may be used to fill in any minor surface voids, thus providing a more reliable test. through experience, a technician will develop a better feel for which test method may work best in a specific situation.


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hillside Grading

on hillside projects, a technician will observe a multitude of conditions. the first stepas alwaysis to become familiar with the project by reviewing the geotechnical report, and then to highlight project-specific recommendations. the approved grading plans should also be reviewed, taking notice of important surface features, such as canyons, landslides, steep slopes, seeps, and the like.

Areas of concern during grading include the following:

Cleanout of soft or otherwise unacceptable materials from swales, canyon sides and bottoms, previously farmed areas, etc. (Fig. c-18).

Adequatecompaction or overbuilding of fill slopes.

Properkeyway construction at the toe of fill slopes that are steeper than 5:1 (Fig. c-16).

Properbenching into competent material as fill is placed against existing slope or canyon sides (Figs. c-16 and c-18).

Figure 5-1 Geotextile products.

(A) this combination of a nonwoven filter fabric overlain by a geogrid was used to stabilize a seasonally wet, soft subgrade for a parking lot at a u.S. Post office in riverside, california. (B) the nonwoven fabric was placed to limit the piping of fines into the overlying aggregate base layer. the strengthening geogrid was placed to limit pumping and deflection of the aggregate base caused by heavy postal trucks and other vehicles. (C) the aggregate base was then compacted to 95% over the geogrid/filter fabric combination. At completion the aggregate base was proof rolled, and no pumping was observed.

(A) (B)

(C)


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Observationofcut slope faces for any loose materials, seeps, slide planes, or out-of-slope bedding planes.

Closeobservationofslide removals or buttress fills (Fig. c-16).

Placementofdrainage systems in canyon and buttress fills (Figs. 5-2 and 5-3 and Fig. c-18).

Areas in which rocks have been blasted (Fig. 5-4), with close observation to help determine that significantly fractured material has been removed.

Full-time observation of rock fillsfill composed of 30% or more mate-rial larger than in. in size (and therefore not testable per AStm D1557).

consider as an example a canyon in lake Elsinore, california. During grading, seepage was observed near the top of the canyon. the canyon was cleaned out, a slot was cut, and a burrito-type subdrain was installed. the drain was formed by placing a few inches of -in. crushed rock atop a woven geotextile filter fabric, on which a 6-in. perforated pipe was laid, covering with more -in. rock, and then finally wrapping the fabric over the top to completely envelop the rock and pipe. A 40-ft length of solid pipe was connected at the outlet end of the drain.

Engineered fill was then placed in the canyon, benching the sides as the fill was placed in level lifts. this particular subdrain continues to run, nearly year-round.

Rock Fill (Oversize Material Placement)

Population increase and the need for more housing in many areas has led to con-struction being pushed into plots of land that were previously deemed unbuild-able, oftentimes owing to the hilly and rocky nature of the landscape. in recent years the construction of homes and industrial complexes over rock fills has become common practice. Proper placement and observation of these fills are mandatory to help minimize unacceptable settlement that can be caused by nesting, voids, or lack of adequate densification.

A fill can be considered oversized (or rock fill) when more than 30% of the material (by weight) is larger than -in. in size, and therefore according to AStm standards a Proctor (AStm D1557) cannot be performed on the material. because rocks do not tend to fit together flush when placed by themselves in a fill, it is critical that the matrix material (soil and material finer than -in.) is able to infill the voids between the rocks. For this reason predominantly granu-lar material must be used as matrix soil.

the geotechnical report will often indicate a criteria for the matrix soil, such as an SE (sand equivalencyAStm D2914) of 32 or greater or a nonplastic Pi (per AStm D4318). the report will also describe acceptable placement techniques and require full-time observation by an engineering firm during placement.


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Figure 5-2 Subdrain installation.

Figure 5-3 Canyon fill and working subdrain.


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Blasting, breaking, and placement of rock. Figure 5-4

this hard granite formation in riverside, california, could not be broken up with conventional grading equipment. holes were drilled and packed with explosives (A), then blasted to prede-termined depths (B). remaining boulders were then broken down by an excavator with a rock hammer attachment (C). the oversize material was then placed in shallow basins. Dozers and loaders spaced the large material out, then relatively clean sand was flooded and compacted around the boulders (D).

(A)

(B)

(C)

(D)


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Placement and Observation of Rock Fill

it is generally not practical to take density tests in a rock fill (with the nuclear gauge or 6-in. sand cone). therefore it is important that full-time observation be made during placement. During rock fill placement, primary concern should be given to the following:

Isthematrix material granular enough?

Isthetypeofcompaction equipment used heavy enough?

Isadequate water being added to help lubricate the matrix soil into the voids between the rocks during compaction? often a water content criterion is recommended, such as 2% to 8% over optimum for the matrix soil.

Placement of rock fill is performed by pushing the rock and matrix mate-rial out as a blanket-type fill, with a water hose or truck continually wetting the material as it is spread, and then the compaction equipment rolling over the top. Placing the rocks in windrows and pushing soil into the rock from the sides is not an acceptable method; compaction is harder to achieve by pushing from the side (as compared to an applied load from the weight of the equipment on top), and the rock is more likely to nest when placed in windrows.

Helpconfirmyourobservationsbyhavingthecontractorexcavateanobser-vation pit into each compacted lift of rock fill. closely observe that the contact between the rocks and the matrix material appears well densifiedvoids or loose material cannot be seenand that moisture is well blended throughout the matrix soil.

Reworking areas of rock fill that do not appear sufficiently densi-fied by the contractor (moisture conditioned, remixed, recompacted, etc.) as necessary. upon completion of the reworked area another observation pit should be excavated and observed.

Cut, Fill, and Transition Pads

Another extremely important step is to daylight the grading plans. this involves observing the elevation contours across the site, and then highlight-ing the cut/fill contacts (e.g., highlighting cut areas in red and fill areas in blue). tracing the daylight line across building pads may be particularly important in determining whether the lot should be treated in a special manner because of a transition (cut/fill contact).

Differential support conditions are a concern where foundations span cut and fill soils or when foundations cross native rock and engineered fill. review of the geotechnical report will indicate whether over-excavation or other methods (extra steel placement, use of a floating slab, etc.) are necessary to help mitigate differential settlement.


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if there is no contact or daylight line across the pad, a determination should be made as to whether the pad will be cut or fill. if material needs to be placed to raise the existing ground to finished pad grade, then it is a fill pad. When mate-rial must be excavated to lower the ground to finished grade, the pad is consid-ered a cut pad.

Deep Foundations

Driven piles, drilled piers, and caissons all require special observation, and this work is usually carefully coordinated with the project engineer. Although shal-low foundations may depend wholly on the bearing material, deeper founda-tions may gain support from friction and/or bearing. therefore during the drill-ing of shafts for deep foundations it is critical to log the soil and rock strata accurately.

Some important areas of observation include the following:

Log the strata of the material as it is drilled into and confirm that it matches geotechnical recommendations. if different soil conditions are observed, inform the project engineer or geologist immediately.

Thestraightness of the excavated shaft should be checked; it should be vertical with no overhanging material.

Ensurethatthetip depth and elevation are per plan.

Measurethehole diameter, and confirm that it is per plan.

Checkthecleanliness of the hole; note any caving, water seepage, etc.

Notethetime and date of completion. holes should only remain open a limited amount of time before placing steel and pouring concrete. (check with the project specifications or the project engineer for time constraints.)

Confirmthatsteel placement is correct: the cage must have proper clear-ance from the walls and bottom of the drilled shaft.

Watchthat theconcrete is tremied to the bottom of the hole during the pour.

Compare the theoretical volume with the actual volume of concrete placed. (too much concrete may indicate a hole blow-out, whereas too little may indicate hole caving.)

Shallow Foundations

technicians are often called on to observe foundation excavations. the usual areas of concern are the depth of the foundations and the density of the bearing


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material. Excavations for continuous and spread footings should be probed (with a hand probe) for loose or unsuitable material and checked to determine whether the footings are free of water and debris. the width and depth of the footings should also be measured to see if they conform to the project plans.

retaining Walls

there are many types of retaining walls, with design determined not only by structural needs but also by financial and even aesthetic reasons. most retain-ing wall construction requires good bearing material at the wall base or footing. Similar to continuous footings, the wall footings should be founded in dense undisturbed soil, relatively unfractured rock, or compacted fill. in all cases the technician must take time to review the wall foundation recommendations from the geotechnical report, as well as the plan details. A few commonly used retaining wall types, along with some important construction criteria to watch for, are described in the following.

Gabion Baskets

As shown in Fig. 5-5 gabion baskets are often used as both erosion protection and slope support. the baskets may be pre-formed or constructed on site. the baskets are usually made from twisted heavy steel wire mesh to create the desired size baskets; these are then filled with rock. the tops of the baskets are then wired closed. these baskets are then wired together end to end and/or stacked on each other.

Interlocking Block Walls

block walls are still one of the most common wall types, are easily constructed, and can be built to many configurations and for varied uses (see Fig. 5-6). the following list notes the important areas to observe.

Checkthefootingsforplannedwidthanddepth.

Probethefootingbottomsforsoftorlooseareas.Recompactorreplacewithacceptable soil or concreteif approved by the project engineer.

Testthelevelingpadmaterial(typicallyaggregatebase)forcompaction.

Setblocksflushagainsteachotherwithnogapsbetweenthem.

Watchthatalignmentpinshavebeeninstalledcompletely.

Tip: confirm that the pins have not been cut (lengthwise) in half; some contractors have been known to do this to save money.

Confirm that the proper geogrid is used; often the size of opening in thegeogrid is determined by the backfill material.


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Gabion baskets. Figure 5-5

Block retaining and wing wall. Figure 5-6

Checktheplansforpropergeogridinstallation;usuallytheelongateddirec-tion is perpendicular to the wall. the geogrid should be pulled taut across the compacted horizontal surface.

Checkthattherearenoloosezonesbetweenthedrainrockandtheadjoiningbackfill; use your hand probe to help verify this.

Rockery Walls

these types of walls are becoming more common. they are especially advanta-geous in developments in which the grading process (sometimes blasting) has generated large quantities of angular rock of varying size. rockery walls are not only cost effective but can be aesthetically pleasing (Fig. 5-7).

Pay attention to the following during rockery wall construction:

Checkfootingdepthforminimumembedment(typicallyaminimumofonefoot); the footing should be wide enough (front to back) to allow for a flush


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fit at the face of the base rock or rocks. check to see that the back cut is free of loose material and is laid back at a safe gradient prior to placing the filter fabric.

Ifaperforateddrainpipeisplacedalongthebottombackofthewall,checkto see that the perforations are facing downwardallowing the water to flow up into the pipe, but limiting the silting-up of the pipe with fines.

As the wall is being built, check for the proper batter (incline into theslope).

Watchforthespecifiedsizeofrocksandthattheyareangularandplacedaccording to recommendations. the long dimensions of the rocks should be placed perpendicular to the wall, and each rock should bear on two rocks below. if rocks are double stacked, the larger rock shall be at the face of the wall.

Thedrainrockplacedbetweenthebackofthewallandthefilterfabricshallbe free draining and is usually 1.5 to 6 in. in size.

Reinforced Concrete Walls

For reinforced concrete walls, the following are of importance:

Checkfootingsforplannedwidthanddepth.

Probethebottomstofindanysoftorlooseareas.Deepenfootingsthroughpoor soil into acceptable material per the geotechnical report.

Closelyobservethebottoms(andsides)offootingexcavationsforexpansivesoils (which are typically unacceptable).

Figure 5-7 Two-level rockery wall.

Photo courtesy of robert Delk


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Watchforfootingbottomsthatspancutandfillsoils,orforfoundationsthatcross native rock and engineered fill. in these cases consult the project engi-neer to determine whether over-excavation or other methods are necessary to mitigate the potential of differential settlement.

Priortoplacingsteel,ensurethatfootingsarefreeofwater,ice,snow,loosematerial, and any other debris.

the technicians Steps to Success

Geotechnical technicians gain much of their on-the-job experience from other senior technicians and engineers, as there are currently no graduate degrees tailored for the engineering technician. by following these guidelines a techni-cian can be respected and effective on the job.

1. Be safe: never sacrifice job safety for job expediency.

2. Always prepare: one way to gain the respect of the contractor and your associates is to thoroughly understand the project.

Readandunderstandthegeotechnicalreport.

Studytheplans.

Meetwiththeprojectengineertoreviewtheproject.

Setupafieldfile.

Walktheproject:Readthegradestakes,lookatsoilandrockincutareas,visualize how the contractor might grade the site, and note any areas of concern.

ObtainProctorsamples.

Haveapre-jobmeeting(preferablyonsite)andtakegoodnotes.

3. Be professional:

Beontimetotheprojecteveryday.

Make your decisions based on the plans and specifications and beconsistent.

Bereadybyhavingthenecessarysafetyandtestequipmentandsupplieson hand.


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4. Document:

Filloutadailyfieldreport,anddocumentallworkperformed;documentsite meetings and personnel present and all related job activities.

Keepapersonaldiaryofyouractivitiesonsite.

Takepictures(andvideosifnecessary).

Reviewpreviousdocumentation(checkwhethertestfailureshavebeenretested, whether lab test information is current, etc.).

5. Communicate:

Discussthejobwithyoursupervisororprojectengineerdaily.

Communicatewiththejobforemanorsuperintendentdaily.

Beproactive;trytoforeseeproblemsanddiscusstheminadvance.

Shouldyouspotaproblemthatwasmissedearlierinthejob,donotignoreit. take corrective measures; unresolved problems should not be put off.

Planahead,andaskforhelpwhenneeded.

Shouldyouhavetoleavethejobtoworkonanotherproject,meetwiththe replacement technician and do a complete update and review.

6. Take pride in your work:

Develop your career with certification programs (federal, county, andlocal) and continuing education. Some important certification programs include those offered by

NICET(NationalInstituteforCertificationinEngineeringTechnologies),

ACI(AmericanConcreteInstitute),and

ICC(InternationalCodeCouncil;formerlyICBO).

Maintainyourtestingandsamplingequipmentingoodworkingorder.

Washyourvehicleitisthefirstthingseenwhenyouarriveonsite.


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chapter Questions

1. Minimum requirements in a geotechnical report often include the following two recommendations:A) the over-excavation of all sandy soilsb) the scarification of all existing surfaces prior to placing any fillc) removal of all undocumented fillD) the placement of fill in lifts no thicker than 12 in.

2. Expansive soils are often compacted at lower densities and higher moistures.A) trueb) False

3. Pumping or deflection of clayey soils is acceptable in roadway subgrade.A) trueb) False

4. The backscatter method of testing with a nuclear gauge should not be used when testing a thin layer of aggregate base.A) trueb) False

5. Which two conditions are not desirable across a footing bottom?A) Dense native soilb) compacted fill contacting bedrockc) cl/ch soil at under-optimumD) bedrock

6. A caisson was drilled to a depth of 20 ft, and upon completion water had seeped in and filled up 5 ft of the hole; what is not the proper action to take prior to pouring concrete?A) remeasure the hole, and then redrill it to remove slough/sediment if

necessary.b) confirm that the contractor will place a tremie to the bottom of the cais-

son during the pour.c) Pour low-slump concrete from the top of the caisson, making sure to

vibrate from the bottom of the hole during the pour.D) calculate the amount of concrete necessary to fill the caisson, with no

adjustment made for the 5 ft. of water.


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7. When constructing a rockery wall, the long dimension of the rocks should be perpendicular to the wall face.A) trueb) False


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