User:Hoskir/Revision Request 4256
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The Geotechnical Section of MoDOT provides geotechnical laboratory and field testing as well as subsurface drilling. This section obtains and interprets the subsurface information needed to design highways and bridges. Soils and Geology personnel also investigate various geotechnical matters including foundation stability and settlement of embankments, slide corrections, subgrade and base stabilization, bridge and wall foundations, sinks and mine subsidence, and the use of new technology and products. The Geotechnical Section serves as a contact regarding geotechnical problems, new products, specifications, training programs and implementation of new products and procedures. This section also participates in proficiency sample programs and inspections by the AASHTO re:source.
Geotechnical functions in MoDOT are performed by personnel assigned to the Construction & Materials Division, both in the District and at Central Office in Jefferson City. Each of the Districts should have a District Geologist or Geotechnical Specialist who typically reports to the Assistant District Construction & Materials Engineer. Within Central Office, these functions are administered through the Geotechnical Section.
The Geotechnical Section has a specialized soil laboratory with technicians and equipment for performing consolidation tests, direct shear tests, and other specialized soil tests. Professional level employees with academic backgrounds in Geology and Civil or Geological Engineering concentrate on one or more specialties. The core drill section assigns equipment and personnel statewide as needed by either District or Central Office staff. The Geotechnical section completes all final foundation investigations for structures prepared by the Bridge Division as well as settlement and stability investigations as necessary. Other duties include slide investigations, work on subgrade and base stabilization, geophysical explorations, research activities and any other geotechnical problems as they arise.
The core drill section employs its own staff of field drilling personnel and drilling support equipment. Drilling equipment includes mobile power augers with pavement cores, CME 45 truck mounted rig, track mounted Acker Renegade, track mounted Acker XLS, and a track mounted GeoProbe 3135T SPT/CPT Rig.
This Geotechnical section typically provides three types of reports, a Preliminary Geotechnical Report (PGR), a Final Foundation Investigation Report (FFIR) and a Special Investigation Report.
The PGR touches on foundations by pointing out possible foundation problems. It also contains basic slope recommendations which affect bridge length, a preliminary bridge foundation boring, soil types and properties for pavement design, depths to rock and type of rock for determining cut quantities and cut slope recommendations for soil and rock.
The FFIR for structures starts with a formal request from either the Bridge Division or District following procedures in EPG 751.1.2.19. The Bridge Division or District will provide a preliminary layout and a suggested boring plan. This boring plan should follow EPG 321.2.2. The suggested boring plan is only a guide which will be modified as deemed necessary in the field by the Geotechnical Section. The basic aim is to furnish the Bridge Division or District with the information needed to develop designs for foundation types practical for a particular site. The Geotechnical Section may make recommendations for specific foundation types if geologic conditions necessitate a change. Rules of thumb as to what is practical have been developed jointly by the Geotechnical Section and the Bridge Division.
Contact Information:
- MoDOT Construction and Materials Division
- 1617 Missouri Blvd
- Jefferson City, MO 65109
- E-mail: Lydia.Brownell@modot.mo.gov
- Zachary.Troesser@modot.mo.gov
copy all 321.2
321.2.1 Overview of MoDOT Practice
General Procedures
The Bridge Division or District will prepare a Request for Final Soundings for Structures Form as per EPG 751. The Geotechnical Section may adjust or modify that plan as necessary to provide the parameters requested, based upon the site conditions and access problems which may be encountered.
The first step after receipt of a request from the Bridge Division or District is a desk top study and review of the PGR completed by the District Geologist. The desktop study should include, but not limited to, GeoSTRAT investigation and review of previous foundation reports and bridge plans for the existing or adjacent structures. A packet of information, which includes plans, correspondence, and prior reports is assembled for field use. Next, the District is consulted for advice as to field conditions, problems with utilities, crops, landowners, etc. If site access conditions are especially bad, someone from the Geotechnical Section may visit the site to determine what equipment may be needed and how the site can be accessed. General knowledge of conditions in the physiographic or geologic area where the work is to be done. This strongly influences the kind of investigation which should be completed and how detailed it should be. For example, in the Springfield area, residual clay over heavily pinnacled rock is likely and the most important thing is to map the rock surface irregularities with a lot of auger borings to rock so that point bearing pile lengths can be determined. In the bootheel area, auger borings are virtually worthless and standard penetration test borings for design of friction piles are most important.
Site access conditions are a very practical consideration. It may just not be feasible to drill a hole in the middle of an urban interstate highway, and it may be extremely difficult and/or expensive to drill one in the middle of a river. That is where judgments must be made about how necessary that specific boring is: can conditions be reasonably extrapolated from offset borings, would geophysical methods work as well, etc. In many cases, the borings can be omitted with little risk. In other situations, considerable expense and trouble may be justified to get on location. This may involve a different type of equipment, hiring a bulldozer, mobilizing the portable barge, or temporarily blocking a lane of roadway. The most extreme access problems involve major river or lake crossings where barges, tugs, and support services must be provided by contract.
Another consideration involves foundation conditions encountered as the investigation progresses. This is a principal reason why all MoDOT foundation investigations are supervised in the field by trained personnel. A Consultant who is completing Geotechnical work for us shall provide, at a minimum, a Geologist Registrant In Training (GRIT) or an Engineer Intern (EI) to log the borings in the field. Their engineer or geologist shall have at least 2 years of experience logging boreholes.
If conditions encountered are different than anticipated, it is expected that the scope of the investigation will be adjusted as necessary to fit actual conditions. As previously noted, the Geotechnical Section rarely makes recommendations for specific foundation types. The basic aim is to furnish the Bridge Division with the information needed to develop designs for foundation types practical for a particular site. Several rules of thumb are helpful in deciding what is practical.
For example:
Spread footings - Spread footings for bridges will not be considered unless foundation material has an unconfined compressive strength of 3 tsf or more. If firm material or top of bedrock is 10 feet or more below final grade line, then piles will be used.
Deep Foundations - Design of friction piles typically requires 30 continuous feet of bearing strata with an N60 value of 20 or greater. It is normal practice to drill half again as deep, or a minimum of 100 feet to determine the depth to rock for a point-bearing option. Point bearing is usually a feasible option almost everywhere in the state except in the southeast lowlands or "bootheel" area where sands extend to depths of up to two hundred feet or more. Even here, however, a careful check must be made for the possible presence of soft clay layers within and just below the range of probable friction pile penetration. Additionally, Cone Penetration Testing (CPT) can be performed in these areas to provide parameters for the design of friction piles.
Culverts with Floor Slab Omitted - Large box culverts may be built more economically if a floor slab can be omitted. The Bridge Division has established feasibility if rock is within five feet of the flowline. So, where rock may be shallow, an attempt is made to drill auger holes every 25 feet or so along each proposed wall. An attempt is made to judge the durability of the rock based on inspection of exposures, rock cores, and knowledge of past performance of particular formations. The rock should have an RQD equal to or greater than 75 and should not be thin bedded. If rock is deeper, only a few auger borings to verify this fact may be sufficient.
Culverts with Compressible Foundations - Culverts, if built over compressible foundations, may require special investigations based on undisturbed sampling to determine need for camber to compensate for settlement and to assess the danger of joints opening due to spreading caused by settlement. In some areas of the state where this is a particular problem, structural collars are sometimes recommended around joints to control spreading and faulting and piping of silty soils into opened joints.
Retaining Structures - For retaining structures, information is obtained for determination of allowable footing bearing pressures and angles of internal friction of the materials to be retained and the foundation material. The latter is done by correlation to Plasticity Index (PI) for walls of low height (using the average correlation m one standard deviation - 321.2.8.5 Correlations of Strength Characteristics), and by drained shear testing for higher and more critical structures. In most cases, it will be necessary to obtain undisturbed samples for testing in order to evaluate overall stability of the slope of which the wall will be a component. By agreement with the Bridge Division, evaluation of global or overall stability is a Geotechnical Section responsibility. If inadequate global stability is likely, possible solutions are evaluated - such as lowering the base of the wall, increasing the width to height ratio and excavation and replacement with rock fill, etc.
Spill and Channel Slopes - The Geotechnical Section attempts to furnish overall guidance on prudent slope selection. This was done first by development of criteria, based on soil type and geologic origin, which is used by the District Geologist in making project slope recommendations. Secondly, a review is made, often by specific request of the Bridge Division, of the adequacy of embankment stability in the vicinity of the bridge ends, particularly at stream crossings. Geotechnical recommendations may affect bridge length, the fill end slopes, and erosion control measures for the channel banks. Often, for example, evidence will be found of channel bank failures which reflect a need for bank stabilization or bridge lengthening.
Philosophy
The boring log’s primary function is to provide foundation engineering data. Any geologic, pedologic, or mechanistic factors influencing or contributing to the desired information are of importance. However, it is difficult to visualize what interest a bridge designer might have in a description such as: "A dark gray to black stratum of Willow Pond shale, fissile, containing lingulas, conodonts and various species of brachiopods." Similarly, describing a soil as "dark blue, moist, alluvial Tippecanoe montmorillonitic clay, mottled rusty red, derived from podsolic soil group" is mostly gobbledygook as far as a bridge designer is concerned. Emphasis should be concentrated on such engineering information as will be of value to the designer, regardless of the engineers interest in micro- or macrofossils, fine distinctions in color shades, etc. For proper perspective, it should be kept in mind that the final aim is to build a bridge. The gobbledygook is reserved for quarry columnar sections!
321.2.2 Technical Guidelines for Geotechnical Investigations
321.2.2.1 Bridges
Deep Foundations (Piling)
A minimum of one sample boring or CPT sounding at every other bent location unless soil conditions are not consistent between borings. Typically, 2 sample borings per single span structure.
Run standard penetration tests (SPT) at 2.5 feet, 5 feet and then every 5 feet until 30 continuous feet of 20 blow count or better of bearing strata is encountered. After bearing is achieved, continue penetration tests at 10-foot intervals until rock is encountered or boring is advanced to 100 feet. Core or rock bit 5 feet of rock or shale. If rock is encountered and adequate bearing is not achieved in the overburden, core 10 feet of rock. (Should adequate bearing be at a deep depth, it may be cheaper to use point bearing piling placed on rock. This is why we need at least one boring to establish rock elevation).
If shale is encountered and adequate bearing is not achieved in the overburden, run SPT on top of shale, core 5 feet, run SPT, core 5 feet and run SPT. SPT’s between core runs may be eliminated if an adequate shale sample to run a Qu test is recovered and recovery of cores is >50% and Pocket Penetrometer (PP) values are > 9tsf.
Unless stated below, a minimum of one auger boring should be completed at the opposite side of the sample boring to confirm depth to bedrock.
- Northern Missouri - One boring should be extended to at least 100 feet if bedrock is not encountered. Other auger borings should be taken down to the depth where bearing is achieved as determined by the SPT. If adequate bearing is at a deep depth, it may be cheaper to use point bearing piling placed on rock. This is why we need at least one boring to establish the bedrock elevation.
- Central Missouri -- Auger borings to bedrock and make pattern holes if the top of rock is uneven (more than 5 feet difference in rock elevation within one bent), especially for spread footings.
- Bootheel – Auger borings are not typically performed for Final Foundation Investigations in the bootheel due to the depth of bedrock. SPT or CPT borings are preferred in this area.
When eliminating bents due to bents falling within the waterway, make sure to penetrate other borings until 20 blow count or better strata is found for 30 consecutive feet below the stream bed elevation.
Shallow Foundations
One sample boring per bent location and one auger boring at the opposite side of the sample boring to confirm depth to bedrock. A minimum of 15 feet of good rock core (RQD 75 or higher) should be obtained. It is common for piling to be used on the end bents except where the bridge end is positioned on a rock bluff. If rock is less than 10 feet at an end bent, complete sampling for the possibility of a shallow foundation.
When eliminating intermediate bents in waterways and bedrock is within 5 feet of the stream bottom elevation, 15 feet of rock core below the stream bottom elevation should be obtained. On major rivers, borings will normally be based on the type of footings that are planned (drilled shafts or spread footings). Always run SPT in the overburden to aid contractor in driving coffer dams or drilling for shafts.
Drilled Shafts (Excluding Major River and Lake Crossings)
One sample boring per bent location and one auger boring at the opposite side of the sample boring to confirm depth to bedrock. SPT sampling should be completed in the overburden to aid in lateral resistance analysis. A minimum of 25 feet of core in rock and 30 feet in shale should be obtained. If the geology is non-uniform or within a karstic prone environment, one sample boring per foundation location should be completed.
How is the rock core depth determined? For end bearing you need rock core a minimum of 2x the diameter of the shaft below the bottom of the planned rock socket. Traditionally, the top 5 feet of rock is ignored in calculations. So, if you have 6-foot diameter shaft with a 5-foot long rock socket then you will need: 5 feet (top portion ignored) +5 feet (planned length of rock socket) +12 feet (2x the diameter) =22' (round to 25 feet). Our normal drilled shafts for typical bridges do not exceed a 6-foot diameter.
Drilled Shafts/River Borings (Major River and Lake Crossings)
One sample boring per column except for drilled shaft groups where 5 core holes per bent will be required. A minimum of 40 feet of core in rock and 50 feet in shale will typically be appropriate, but if the shaft size is known, sampling should be revised accordingly.
A barge is typically required for this work and this type of bridge will likely be sent to a consultant for completion.
Samples
Atterberg samples should be obtained per soil strata and moistures should be collected from every SPT sample. A pocket penetrometer reading should be obtained on all cohesive SPT samples.
A minimum of one Qu sample should be obtained per 5 feet of rock core. A pocket penetrometer reading should be obtained on shale samples.
A gradation sample, providing D50 and D90, of stream bank surficial materials is required for Bridge Divisions scour analysis. Check the PGR first, as the District Geologist may have already collected the sample.
Bridges located in seismic zones may require earthquake sampling, the Request for Final Soundings for Structures Form and Bridge Seismic Design Flowchart (EPG 751.9) should be reviewed to verify need for additional sampling requirements (see Earthquake Sampling guidance, below).
321.2.2.2 Walls
MSE (Mechanically Stabilized Earth) Walls
A minimum of one sample boring every 200 feet of wall length with SPT’s and undisturbed sampling with shelby tubes to at least 10 feet below the planned bottom of footing elevation should be completed. Each MSE wall should have a minimum of two sample borings. Sample holes should be located at the highest points of the proposed wall.
If soil is too rocky to push a shelby tube, SPT every 2½ feet to at least 10 feet below the proposed bottom of footing elevation. Obtain Atterberg Limit samples, moisture content samples, and pocket penetrometer readings. If foundation material is too soft to use shelby tubes or an Osterberg sampler, SPT every 2½ feet to at least 10 feet below proposed bottom of footing elevation. If still soft, continue SPT’s at 5 feet increments until bedrock is encountered. Rock bit or core at least 5 feet of good rock or shale (RQD 75 or higher or PP >9tsf). If soft soils are encountered, a cantilever wall on piling may be used or ground improvements may be required.
If rock is encountered above the planned footing elevation or less than 10 feet below the proposed bottom of footing elevation, core a minimum of 5 feet below the proposed bottom of footing elevation for an MSE wall and 10 feet minimum for a cantilever wall.
Auger borings are usually laid out about every 25'. If you are in uniform soil and rock is more than 5 feet below the footing elevation, you can skip every other hole. Auger about 10' below the planned bottom of footing elevation or deeper if you suspect rock is close.
Samples
A Qu, direct shear, consolidation (if soft), Atterberg Limit and moisture content sample should be obtained from each undisturbed sample, as necessary. A PP and torvane (TV) reading should be obtained on all cohesive shelby tube samples. These samples will be used to determine the internal angle of friction for retained and foundation material. If retained material is fill, get an internal angle of friction from the PGR. If sand is encountered, take samples for gradations and Atterberg Limits as appropriate.
Atterberg samples should be obtained per soil strata and moistures should be collected from every SPT sample. A PP reading should be obtained on all cohesive SPT samples.
A minimum of one Qu sample should be obtained per 5 feet of rock core. A PP reading should be obtained on shale samples.
MSE walls located in seismic zones may require earthquake sampling, the Request for Final Soundings for Structures Form and Bridge Seismic Design Flowchart (EPG 751.9) should be reviewed to verify need for additional sampling requirements (see Earthquake Sampling guidance, below).
Cantilever Walls or Spread Footings
Follow the drilling and sampling requirements for an MSE wall. If rock is near the proposed footing elevation or the foundation will be seated on rock, obtain 10 feet of rock core.
Sound Walls A minimum of one sample boring every 200 feet of wall length to 20 feet below the bottom of the wall. Continuous SPT’s and undisturbed sampling with shelby tubes should be completed to 5 feet below the bottom of the wall. After this, continue to run SPT’s at 2½ foot intervals to at least 20 feet below the bottom of the wall.
If rock is encountered within 5 to 10' below the bottom of the wall, core 5 feet. If rock is less than 5 feet from bottom of wall, core 10 feet.
Auger borings are usually laid out about every 25 feet. Auger 25 feet below the planned bottom of footing elevation or deeper if you suspect rock is close.
Samples
A Qu, direct shear, Atterberg Limit and moisture content sample should be obtained from each undisturbed sample. A PP and TV reading should be obtained on all cohesive shelby tube samples. If sand is encountered, take samples for gradations and Atterberg Limits as appropriate.
Atterberg samples should be obtained per soil strata and moistures should be collected from every SPT sample. A PP reading should be obtained on all cohesive SPT samples.
A minimum of one Qu sample should be obtained per 5 feet of rock core. A PP reading should be obtained on shale samples.
321.2.2.3 Box Culverts
Culverts with Rock as the Floor
A minimum of 2 sample holes per culvert depending on the depth to bedrock.
If bedrock is encountered deeper than 5 feet below flow line, drill enough holes to make sure rock does not come up to within 5 feet of flow line. Core a minimum of 5 feet in two sample holes. The remaining borings can be auger borings to confirm depth to bedrock.
If bedrock is encountered within 5 feet of the flow line, core a minimum of 5 feet in two sample holes per culvert. Additional auger borings should be completed every 10' for each wall in the box culvert (i.e., double box: 3 walls; single box: 2 walls, etc.).
Culverts with Compressible Foundations
A minimum of one sample boring on each side of the stream crossing with SPT’s and undisturbed sampling with shelby tubes to the depth of the tallest fill height, then continue SPT’s every 5 feet to top of bedrock. The boring locations should be close to the stream and under the highest part of the proposed fill.
Samples A Qu, direct shear, consolidation (if soft), Atterberg Limit and moisture content sample should be obtained from each undisturbed sample, as necessary. A PP and TV reading should be obtained on all cohesive shelby tube samples. If sand is encountered, take samples for gradations and Atterberg Limits as appropriate.
Atterberg samples should be obtained per soil strata and moistures should be collected from every SPT sample. A PP reading should be obtained on all cohesive SPT samples.
A minimum of one Qu sample should be obtained per 5 feet of rock core. A PP reading should be obtained on shale samples.
321.2.2.4 Light Towers
A minimum of one sample boring per tower to 40 feet below the finished ground line. Continuous SPT’s and undisturbed sampling with shelby tubes should be completed to 30 feet below the finished ground line. After this, continue to run SPT’s at 5-foot intervals to at least 40 feet below the finished ground line.
If the soil is too rocky to push a shelby tube, SPT’s at 2½ feet intervals to achieve a depth of 30 feet below finished ground line and then SPT again at 35 and 40 feet to complete the boring.
If bedrock is encountered within 20 feet of the finished ground line, core 10 feet. If bedrock is more than 20 feet from finished ground line, core 5 feet.
Samples
A Qu, Atterberg Limit and moisture content sample should be obtained from each undisturbed sample, as necessary. A PP and torvane (TV) reading should be obtained on all cohesive shelby tube samples. If sand is encountered, take samples for gradations and Atterberg Limits as appropriate.
Atterberg samples should be obtained per soil strata and moistures should be collected from every SPT sample. A PP reading should be obtained on all cohesive SPT samples.
A minimum of one Qu sample should be obtained per 5 feet of rock core. A PP reading should be obtained on shale samples.
Cohesionless soil (Sand)
- 1. Friction Angle from Bowles 1977 using corrected Blow Count (N1)60
- (N1)60 = CnN60
- (N1)60 = N60 corrected for effective Overburden Pressure
- Cn = correction factor for Overburden Pressure
- (Peck et. al.1974)
- 2. Relative density from either DM 7.1-87 or FHWA/RD-86/102. DM 7.1 probably a better value because it accounts for effective overburden pressure.
Cohesive soils
- 1. Undrained Shear Strength- USS or C from Bowles 1977 using uncorrected blow count N60, preferably Qu/2.
- 2. Friction Angle from correlation of PI to angle of internal friction minus one standard deviation as published in Navdocks DM-7.
- 1. K(f) = slope (variation) of linear subgrade modulus. FromEPG 751.9 Bridge Seismic Design or “Soil Properties (Lpile & Com624P)”
- 2. K(f)cyclic = for cyclic loading
- 3. E50 = strain at 50 % of the maximum difference in principal stresses, unitless, from Qu test and EPG 751.9 Bridge Seismic Design or “Soil Properties (Lpile & Com624P)”
Electro Chemical Parameters
Resistivity is a function of the chloride ion and sulfate ion content and most of the time we will not run this test. To run the test we need about half a materials sack and the sample is entered into Per CM, updated "Site Manger" to "AWP".
321.2.2.5 Building Foundations
Currently being developed. Please contact the Geotechnical Section for more information.
321.2.3 Special Foundation Investigations
Not all special investigations will have a detailed investigation plan that fits every situation. Most will require contacting the Geotechnical Section to plan a site visit to determine the best course of action for that site.
Slide Investigations
Currently being developed. Please contact the Geotechnical Section for more information.
1. Photograph slide and improvements effected by the movement such as culverts, bridge ends, utility poles, guard rail, pavement, fences, etc. Also, a good set of field notes should be kept. All written matter should be appropriately dated and identified.
2. Survey entire area, leave no stone unturned (walk out area).
3. Make notations concerning seepage, type of vegetation, etc. Locate seepage on sketches of plan and profile of slide.
4. Identify slide by station limits. If station numbers are not available, reference slide, soundings, and other pertinent information to the nearest crossroad drainage structure or bridge end. One of the 4 compass points can be used to describe the offsets. Final plans can then be acquired and station numbers assigned to the various field notes. Field notes should include what the offsets are referenced to such as CL lane, CL median, edge of pavement, baseline, etc.
5. Drainage structures should be inspected to determine if slide is deep seated and if structure replacement is necessary. (Measure distance from end of structure to major cracks or to any open joints.)
6. Note leaning fences, trees, and power poles. Note the direction they lean. Rows of steel fence posts may be placed on the slope or slide area to detect further movements.
7. Besides cross sectioning the slide area, at least one cross section should be taken at each end of the slide on the undisturbed slope. The spacing and number of sections taken in the slide area should be determined by the length and uniformity of slope and slide. Sections should extend out far enough to implement possible repairs.
8. Power auger soundings should supply the following information:
- a. Depth of disturbed material, especially for shallow slides. (Deep seated slides may require sampling as well as auger soundings to determine depth of slide.) Enough points should be drilled on the cross section to determine the location of the slide plane with reasonable accuracy. The location of the slide plane may be determined by pushing the auger. The scarp(s) of the slide should be plotted on the cross section as well as the sounding information to determine if a logical slide plane can be drawn. Additional soundings can be made at this time if necessary.
- b. The texture, color, and consistency of all materials should be logged. (The term "till" should be used in the log where appropriate along with the texture and consistency.)
- c. Water table reading should be taken after the hole has been drilled and again the following day, if possible. (For fill slides, water tables should be established on the shoulder, slide, and toe of slide. For backslope slides, water tables should be established back of the cut and on the slide.) It is suggested that 4 inch holes be drilled to establish water tables. Perforated galvanized down spout can be used to hold holes open.
- d. Artesian pressures should be determined where applicable. Well points should be placed in those zones suspected of carrying water. These can be made cheaply by sawing slots in PVC pipe and setting the slotted end section in a sand chamber sealed with bentonite.
9. Undisturbed samples should be taken for the following tests for all slides:
- a. TV and P.P.
- b. Direct Shear
- c. Atterberg Limits
- d. Moisture
10. Water tables should also be recorded for core drill holes. Make a notation if bentonite is used and record dates and times of readings and the date the hole was drilled.
11. Make notes pertinent to possible corrective measures. Some items to be considered are as follows:
- a. Will possible repairs change drainage patterns or should drainage be changed to protect repaired slide area. Will alterations cause damage to private property.
- b. Can adequate materials be obtained nearby to repair the slide. (Can suitable soil be acquired from adjacent backslopes for slope flattening.)
- c. Where can disturbed material be wasted.
- d. If additional right of way is needed, what type of property will be involved.
12. Procedure for investigating a fill slide on an active construction site may be somewhat different especially if the type of slide (fill failure or foundation failure) cannot be determined by visual observation.
- a. Sample through fill and determine elevation of natural ground and compare it with original ground line shown on plans or ground line outside construction limits.
- b. Sample fill and foundation to determine the preconsolidation pressures as well as those tests listed under item number 9. (If foundation soil is over consolidated then failure is probably confined to fill.)
- c. Set hubs near toe of fill and on the distressed fill and record the offsets and the elevations. (If hubs on fill move independent of hubs at toe of fill, this may indicate fill failure. Should all hubs move, this may indicate foundation failure.)
13. Inclinometers may be installed to determine the depth of the failure surface and rate of slide movement.
- a. Inclinometers are typically installed on larger slides or on slides that are not of immediate concern and allow time for monitoring of the slide movement. Inclinometers may also be installed on active construction jobs if potential slope failure is of concern.
- b. Inclinometers should be installed in the slide mass and should extend a minimum of 15 feet below the anticipated failure surface.
- c. Inclinometers should be installed in a hole backfilled with grout although sand backfill is acceptable. The sand backfill is not as sensitive to movements as the grout.
Electrical Resistivity Investigations
Currently being developed. Please contact the Geotechnical Section for more information.
MASW Site Specific Investigations
Currently being developed. Please contact the Geotechnical Section for more information..
321.2.4 Undisturbed Sampling Techniques
321.2.4.1 Introduction
Currently being developed. Please contact the Geotechnical Section for more information.
Undisturbed Sampling refers to obtaining soil samples using either shelby tubes or the osterberg. We take continuous samples and all usable samples should be wrapped and transported to the lab. The general procedure for the Failings and CME is to push a 5" shelby followed by a 3" shelby. The hole is then cleaned out and the process is repeated. Under no circumstances should continuous 3" shelby's be used since this leads to sample disturbance. If the soil becomes too rocky to use the shelby tube, disturbed samples may be obtained by using the SPT-Spoon. No more than 2 tubes should be pushed before you require the driller to clean out. In swelling or caving soils it may be necessary to clean out after every push. In soft to medium stiff soils, pocket penetrometer 0.5 or less, it will be necessary to use a piston sampler such as the Osterberg sampler.
321.2.4.2 Pocket Penetrometer (PP)
1. Firm, SLOW, constant push.
2. Take more than one (1) reading on a sample (average values).
3. The PP is a useful tool but has definite limitations. It was calibrated by the manufacturer against unconfined compression tests made on "silty clays and clayey soils." Correlation of penetrometer values against unconfined tests of Missouri soils show marked disagreement, with penetrometer values almost always exceeding unconfined values (this is believed, to a large extent, to be due to the effect of structure often found in Missouri soils). As the clay content increases, however, the correlation improves. For certain soil types and particularly in certain areas, more definite correlations are possible and may be of value. PP readings are not taken in non-cohesive soils and should not be recorded.
4. Pocket penetrations should be made on each cut surface throughout a sample and the range in readings indicated on the sampling log. Averages of several readings on a surface are preferable to just one.
5. Bear in mind that the pocket penetrometer is at best a crude instrument. Dirt about the plunger or the spring may influence readings. Springs get old. Checks indicate that various penetrometers deviate by a quarter ton or more in indicated values under the same load.
6. For Missouri soil the Pc values are generally equal to the pocket penetrometer value in KSF + 1 KSF. Ex. (Pocket Penetrometer reading is 1 tsf, the Pc = 1 tsf + 1 ksf = 3ksf). This can be a guide in setting up consolidation tests. If the calculated P2 value at a certain depth under a proposed fill is, say 2.5 KSF, and the soil at that depth has a pocket penetrometer reading of 3.0 TSF, or 6.0 KSF, then that soil is obviously preconsolidated beyond the anticipated P2 loading and a consolidation test would probably be a waste of money and time. Of course, if in doubt, make the test.
321.2.4.3 Field Moistures
1. Field moisture samples should be taken immediately from undisturbed samples and sealed in tared moisture cans with tape. Excess moistures as is found about the wetted circumference of most samples should be trimmed away and the moisture sample should be taken from the interior of the sample. Moisture samples of saturated granular materials are of very little value as the moisture content will change with densification or loosening during sampling, extruding, and handling, as well as by draining freely in the case of sands. Moisture samples of a borderline material such as silt should be taken carefully to avoid disturbed zones.
2. A moisture sample should be taken for each soil type encountered, or for marked changes in consistency within a soil type. Indicate on the sampling log the stratum or depth represented by the moisture can.
3. Volume of sample -- fill container nearly full.
4. Do not allow wax to get on the cans!!!
5. Do not allow them to get dirty!!!
6. Make sure electrical tape is stretched on can to ensure a good seal.
321.2.4.4 Torvane (TV)
1. Must be taken on flat surface of an undisturbed sample.
2. Fingers must not interfere with free rotation of measurement dial.
3. SLOW, constant rate of shear.
321.2.4.5 Sample Length
3" samples -- Trim to be 6" for good Qu test (Qu sample length should be at least 2 x Diameter) and for Direct Shear and Consolidation tests. If a 6 in. sample is unattainable, bring what you can but note on logs short sample.
Qu samples should be taken where a bearing value is needed, (i.e. - footing elevation is at 460 feet. The interval that bearing is needed is from 460 feet to 450 feet. Therefore, a Qu sample should be taken in every 3" push throughout the 10 foot interval. This is the minimum. If soil varies a great deal, more Qu samples should be taken.)
321.2.4.6 3-inch Shelby Tube Samples
3-inch Shelby tube samples are used for Qu, Direct Shear, Consolidation and Triaxial tests.
321.2.4.7 Tube Push Length
1. 2 1/2' Push/1 1/2' Recovery -- Go to 1 1/2' Push.
2. 2 1/2' Push/2' Recovery -- Go to 2' Push.
3. 2 1/2' Push/1' Recovery -- Go to 1' Push.
4. Use Osterberg sampler for (both cohesive and noncohesive) soft and wet samples where shelby tubes are not working.
5. Return to longer pushes at your discretion or when full recoveries are obtained.
321.2.4.8 Sample Wrapping
1. Fold tin foil over on the sides (butcher's wrap).
2. Fold over tin foil on the ends of sample.
3. Shale sample wrapping.
- a. Take at least a 5" sample (2 x D)
- b. Wrap sample in saran wrap
- c. Wrap sample in tin foil
- d. Write sample number on tin foil lengthwise
- e. Dip sample in wax to seal sample entirely
- f. Make sure both ends are sealed
- g. Put sample in marked carton but do not wax it into place (personal choice or optional)
321.2.4.9 Handling Samples
1. Do not press hard enough to imprint sample.
2. Ease sample into waxed carton.
321.2.4.10 Waxing Samples
1. Wait till wax has cooled to the point where it appears milky.
2. Pour wax into empty carton (use best judgment as to how much to put in carton).
3. Set sample gently into carton.
4. Pour wax on sample to seal sides and top.
5. Put lid on carton.
6. Dip top of carton into wax to seal lid to carton.
7. Problems.
- a. Wax temperature too hot causing wax to stick to sample and causing drying of outer sample (scalding).
- b. Samples should be waxed as soon as possible (on-site) even when moving around a great deal (i.e. - shallow holes and SHRP project).
321.2.4.11 Storing Samples
1. Make sure samples do not freeze or get too hot. (If >90 degrees F outside temperature, higher temperature in vehicles could cause sample to dry out rapidly.)
2. Samples should be stored in an upright position.
3. Samples should not be allowed to bounce around.
4. Never put anything on top of samples which might damage samples.
321.2.4.12 Atterberg Limits Classification Samples
1. May be taken from disturbed samples (auger cuttings, split spoon sampler, and giddings tube) or undisturbed samples (shelby tube samples).
2. Volume of sample: About 800 grams or fill bottom of bag about 2 inches.
3. Too little of a sample does not give the lab enough to run tests.
4. Too much of a sample is cumbersome to store and requires additional time to break sample down.
5. Do not store Atterberg Limits sample bags in material sacks.
321.2.4.13 ASTM Visual Classification (D2488)
1. Loess - Loess is a wind blown deposit typically consisting of silt or lean clay. Loess is to be used as a side note in parenthesis after the field person has logged the material in terms of its makeup.
2. Glacial Till - Over used term that does not describe what the material is comprised of. Glacial till is a "lean to fat clay with rounded gravel." Glacial till is to be used as a side note in parenthesis after the field person has logged the material in terms of its makeup.
3. Sand - Describe in terms of grain size (fine, medium, coarse) and describe whether it is loose, dense, etc...
4. Silt - No more silty clays unless it really is one (this determination will be very hard to make in the field). Silt is basically a material that individual particles can be seen by the naked eye but are smaller than a fine sand.
5. Lean Clay - Replaced silty clay in field descriptions for the most part. By rolling in fingers, material has only minimum of cohesion. Does not create a shiny surface or feel slick.
6. Fat Clay - By rolling in fingers, individual grains cannot be felt, material feels sticky or very cohesive, feels slick and creates a shiny surface.
321.2.4.14 Mark sample sheets for required test and turn in a copy of this sheet and a lab assignment sheet to the lab. The original sample sheet should be returned to the work file, and to avoid confusion it is important that it show what tests the lab is running
1. Walls require a minimum of one shear for the retained insitu material and one for the foundation material. One or two Qu's should be run on the foundation material. Five inch samples are preferred for shears although 3" will work. Qu's must be 3" samples.
2. Slides require a minimum of 1 shear per layer.
3. Compressible foundations require one consolidation per layer, a 3" sample is preferred although a 5" will work.
321.2.4.15 Assign samples for lab testing immediately and do not let samples back up
321.2.5 Field Logs for Undisturbed Sampling
321.2.5.1 Log Sheets
1. Completely fill out log header. (County, Route, Station, Project No., Bridge No., Hole No., Logged By, Date, Type of Drill, Surface Elevation, Water Table Depth, etc.)
2. Logs should be completed in the field. Re-copying should be avoided unless the field log is soiled or illegible.
3. Draw site sketch on sample jobs include north arrow, ponds, springs, seepage, scour, utilities, fences, guardrails, and right of way markers. Right of way markers are particularly useful since they will usually show up on the plans.
321.2.5.2 Locations
1. Usually it is preferred that either a core hole or sample hole not be offset.
2. If a boring is offset, make sure you list the original station and elevation, as well as the new station and elevation and give reason for offset.
3. The location of the sample hole should be determined in the field with the greatest accuracy possible under the circumstances. Cloth tapes, prisms, and pacing may provide accuracy adequate for most purposes. If greater accuracy is required, arrange for a survey party if you do not have the time or equipment to do the job yourself.
4. In certain situations use of hole numbers keyed to a plan or even to aerial photos may be adequate. However, make certain that the index is available to all users of boring information, adequately labeled, and properly filed or located.
5. If you are drilling on a slide or environmental job that has not been staked yet, reference borings to bridge ends, cross road culverts, or some other substantial structure that should show up on the plans. Guardrail is not a good choice, however it should be noted.
6. The space on the sampling log for "General Location" should be used for a descriptive location of the sample hole, as for example, "On C/L south of south bank of Moreau River on Henry L. Morgan farm."
321.2.5.3 Elevations
1. A surface elevation should be determined whenever possible. If it is an approximate elevation determined by hand leveling or from contours, note the elevations in terms consistent with the degree of accuracy inherent in the method used. An elevation such as 403.2' indicates accuracy to 0.1' which is not ordinarily obtainable except without an instrument and a reliable benchmark.
2. Hand leveling, properly done, can provide surprisingly good accuracy over short distances. The instruments do get out of adjustment with mishandling and should be checked periodically. Use of a staff and backsighting can minimize error even with a hand level not in proper adjustment.
321.2.5.4 Water Table Checking
1. Water table observations should always be made. They are worth some time and trouble including extra trips back to the job site after completion of the hole. And, once noted, record them. The information may not be used for several weeks or longer by which time your memory may be fuzzy.
- a. Reading should be taken 1 to 2 minimum per structure.
2. The rate of change of observed readings may be of value in determining if a sand layer is providing drainage. The water level depth and depth of open hole should both be recorded two or more times to make certain that equilibrium has been reached. A convenient expression is in the form of 12.0'/24.3'/6 hours which notes the depth to water, depth to bottom, and time interval since drilled.
3. How to take readings.
- a. If the hole is losing water, the water table may be taken shortly after the hole is completed. For borings in clayey soils it may be necessary to take readings over a period of time until the water level stabilizes.
- b. Keep in mind that the water level in holes where drilling mud is used may not stabilize for several days or longer. It is permissible to use a nearby auger hole for water level information but note its location and elevation.
4. When and how to set well points.
- a. Usually well points are set on special foundation investigations-slides. Slotted pvc pipe wrapped in geotextile is used as the well screen. The pipe is placed in the hole and about 5' of sand is poured in the hole completely covering the slotted pipe. Next, bentonite in the form of pellets or granules is placed above the sand to seal the hole. Usually borings for well points should just be advanced say 5' below the water table.
5. CRITICAL FOR STABILITY ANALYSIS!
- a. Water is usually the cause of most slides.
- b. It is almost impossible to do settlement analysis without water tables.
6. If no water table is found, record time of observation and depth of the open hole.
7. Holes frequently collapse to, or slightly above, the natural ground water level. This is particularly true of sands. In the absence of better information, record the depth to the collapse and note the apparent degree of wetness of soil adhering to the tape weight.
8. The degree of saturation of soil sample should also be noted in the description. This, in addition to the consistency as indicated by the pocket penetrometer, and the color change can serve to approximately locate the water table.
9. Seeps, springs, ponds, and any drainage feature which may influence local ground water levels should be noted and sketched on the back of the sample sheet.
321.2.5.5 Description and Notes
1. Descriptions and Notes on the sampling log should be as detailed as possible. This contrasts with bridge sounding logs where a brief, concise description generally adds to usability. This completeness of description should extend not only to the soil description, but to the drilling operation as well.
2. Note recovery on samples, difficulty in keeping the hole open, water loss, mud use, casing used, difficulty penetrating boulders, types of bits used, etc. All of this detail can be important - in future drilling operations, in predicting difficulty of drilling caissons or driving piling, as well as in estimating foundation consolidation and stability problems. 23
321.2.5.6 Question Marks on Logs are Unacceptable
If you must use a question mark or are uncertain as to what you need to do - write down every possible bit of information, state reasons for uncertainty, and in some cases make a phone call. The reason for this section is that logs are coming to the office with question marks and no explanation. If the field personnel can't figure out what they have, how is the office supposed to?
321.2.5.7 Note anything that would affect the constructability of the proposed structure
Note old bridge piers, rock dikes, wells, cisterns, springs, underground storage tanks, old landfills, and abandoned gas stations. Note location of obstacles. Make an attempt to sound wells and cisterns. Underground storage tanks and gas stations should be referred to the Environmental Section of Design.
321.2.8 Diaries
Introduction
Diaries will be required for both field and office work.
Format The following format is suggested:
Sample Log
Diaries should have sample log in back of diary.
1. Format
- The following format will be used:
321.2.9 Earthquake Sampling
321.2.9.1 Introduction
The bridge unit has pretty much drawn a line right down Route 141 with everything to the east of 141 and including 141 requiring earthquake samples. We need 1 to 2 holes per structure.
321.2.9.2 Sampling Procedure
- 3" Sample, 2.5'
- Split Spoon, 1.5'
- Clean Out to 5' Soil
- 3" Sample, 2.5'
- Split Spoon, 1.5'
- Clean Out to 10'
- 3" Sample, 2.5'
- Split Spoon, 2.5'
- Clean Out to 15'
The above sampling procedure to a minimum of 50 feet. If the soil becomes too hard for undisturbed sampling or sand is encountered, continue penetrating every 5' to a minimum depth of 50 feet. The depth should be increased if loose sands or soft cohesive soils are encountered. Samples should be taken form the penetrations. Water tables are important. This sample hole may also count as a bridge boring since you are penetrating.
321.2.9.3 Samples Needed
1. One Qu- one per layer.
- Soil (typically 3" diameter and 8" minimum length).
- Rock (typically 2" diameter and 5" minimum length).
2. Moistures-one per layer, may be taken from split spoon (minimum 100g).
3. Atterberg limits-one per layer, may be taken from split spoon (minimum of about 500g).
4. Direct Shears-one per layer (typically 3" to 5" diameter and 6" minimum length).
5. Gradations-one per layer for silts, sandy, and gravelly soils. Additional samples are necessary if the stiffness in the case of silts or density in the case of sandy soils changes. Sieves commonly used are 3/4",3/8",No.4,No.10,No.16, No. 40, No. 50, No. 100, and No. 200. (Normally a 1000 grams is required, but since the normal procedure is to obtain samples with a split spoon, 400g is acceptable.)
321.2.9.4 Reporting
1. You may use regular bridge logs if you are not concerned with liquefaction.
- a. Report Poisson's ratio in cover letter as published in FHWA/RD-86/102 Seismic Design of Highway Bridge Foundations.
- i. 0.45 for CL, CH, and ML
- ii. 0.35 for sand
- b. Report % Passing the # 200 sieve for English units or 75um (micro meters) for metric.
- Ex. Soil Classification Test Data
- Depth,m LL PI ASTM Class %Passing #75um
- c. Report horizontal acceleration due to gravity as published in the most recent AASHTO Standard Specifications for Highway Bridges. St. Louis is 0.1g. 25
2. If liquefaction is a concern, use earthquake summary sheet form (see Appendix E). Liquefaction is a concern when you have cohesionless soils such as sands and some silts.
- a. Dr= Relative Density (see handouts)
- b. Undrained Shearing Strength (U.S.S.) kPa
- U.S.S. = Qu/2=Torvane
- c. Resisting Stress Ratio (R. S. R.)
- Found from correlating blowcounts to charts (see handouts)
- d. Factor of Safety for Liquefaction (F. S. Liqu)
- Fs=Ri/Rf
- Ri= Earthquake induced shearing stress ratio (see handouts)
- Rf= Resisting Stress Ratio (see handouts again)
- e. Vs (m/s) = Shear Velocity
- S or shear waves cause shearing deformation in a material during seismic events. Shear waves can be measured directly with a seismic cone penetration test or by crosshole tests. Shear wave velocity can be calculated from the shear modulus G (Vs=Square Root (G/ρ ), ρ=soil density or from correlations to standard penetration tests.
- f. Gmax (kpa) = Maximum Shear Modulus
- Gmax =ρVs2
- Gmax can also be attained from correlations of overburden stress or from standard penetration tests.
- g. G (kPa)= Shear Modulus
- Shear modulus is used in calculating stiffness values for footings. As the shear strain of the soil increases during seismic events, the shear modulus decreases. Shear modulus is calculated from seismic response analysis programs such as SHAKE91.
- h. Es (kPa) = Youngs Modulus of Elasticity Es can be obtained from cone penetration tests and/or flat plate dilatometer tests, or calculated if the shear modulus and Poisson's ratio are known.
- Es = 2(1+υ)G
- G = shear modulus
- υ= Poisson's ratio
321.2.10 Appendix
321.2.10.1 Soil Classification Guide (Cohesive Soil)
Plasticity Chart
321.2.10.2 Soil Classification Guide (Non-cohesive Soil)
321.2.10.3 Rock Classification Guide
| Grain Size | Composition | Name | |
|---|---|---|---|
| Mostly coarse grains | Rounded pebbles in medium grained matrix. | Conglomerate | |
| Angular coarse rock fragments. | Breccia | ||
| More than 50% of medium grains | Medium quartz grains | Less than 10% of other minerals | Sandstone |
| Appreciable quantity of clay minerals | Argillaceous sandstone | ||
| Appreciable quantity of calcite | Calcareous sandstone | ||
| Over 25% feldspar | Arkose | ||
| More than 50% fine grain size | Fine to very fine quartz grains with clay minerals, gritty feel | Siltstone (if laminated silt shale) | |
| More than 50% fine grain size | Microscopic clay minerals and very fine quartz, <10% other minerals | Mudstone or claystone(if laminated clay shale) | |
| Grain Size | Composition | Name |
|---|---|---|
| Microscopic | Calcite fragments and calcite cement. White or gray or bluish in color. Fizzes strongly with dilute HCL. | Limestone |
| Carbonate almost completely transformedto dolomite. Often yellowish or pinkish in color. Fizzes weakly with dilute HCL. | Dolomite | |
| Variable | Recrystallized silica | Chert |
Micaceous - Appreciable mica, Calcareous - Limey appreciable calcite, Carbonaceous - Appreciable carbon material, Siliceous - Appreciable silica, Argillaceous - Appreciable clay minerals
| Intrusive (Course Grained) | Essential Minerals | Common Accessory Minerals | Extrusive (Fine Grained) |
|---|---|---|---|
| Granite | K-feldspar, Quartz | Plagioclase, mica, amphibole, pyroxene | Rhyolite |
| Diorite | Plagioclase | Mica, amphibole, pyroxene | Andesite |
| Gabbro | Plagioclase, Pyroxene | Amphibole | Basalt |
| Very thick bedded | Greater than 3' thick (>1m) |
| Thick bedded | 1' to 3' thick (0.3 to 1.0m) |
| Medium bedded | 4" to 1' thick (0.1 to 0.3m) |
| Thin bedded | 1 1/4" to 4" thick (30mm to 100mm) |
| Very thin bedded | 1/2" to 1 1/4" thick (10mm to 30mm) |
| Thickly laminated | 1/8" to 1/2" thick (3mm to 10mm) |
| Thinly laminated | 1/8" or less (paper thin) (<3mm) |
| Term | Field Identification | Approximate Unconfined Compressive Strength, kg/cm2 (tsf) |
|---|---|---|
| Extremely Soft | Can be indented by thumb nail. | 2.6 - 10 |
| Very Soft | Can be peeled by pocket knife. | 10 - 50 |
| Soft | Can be peeled with difficulty by pocket knife. Small, thin pieces can be broken by finger pressure. | 50 - 260 |
| Medium Hard | Can be grooved 2mm (0.05") deep by firm pressure of knife. | 260 - 520 |
| Moderately Hard | Requires one hammer blow to fracture. | 520 - 1040 |
| Hard | Can be scratched with knife or pick only with difficulty. Hard hammer blows required to detach hand specimens. | 1040 - 2610 |
| Very Hard | Cannot be scratched by knife or sharp pick. Breaking of specimens requires several hard blows of the pick. | >2610 |
| Slightly Weathered | Rock generally fresh, joints stained and discoloration extends into rock up to 25mm (1in.),open joints may contain clay, core rings under hammer impact. |
| Weathered | Rock mass is decomposed 50% or less, significant portions of rock show discoloration and weathering effects, cores cannot be broken by hand or scraped by knife. |
| Highly Weathered | Rock mass is more than 50% decomposed, complete discoloration of rock fabric, core may be extremely broken and gives clunk sound when struck by hammer, may be shaved with a knife. |
| Description | Diameter (mm) | Field Identification |
|---|---|---|
| Very Coarse Grained | >4.76 | - |
| Coarse Grained | 2.0 - 4.76 | Individual grains can easily be distinguished by eye. |
| Medium Grained | 0.42 - 2.0 | Individual grains can be distinguished by eye. |
| Fine Grained | 0.074 - 0.42 | Individual grains can be distinguished by eye with difficulty. |
| Very Fine Grained | <0.074 | Individual grains cannot be distinguished by unaided eye. |
| Pit | Voids barely seen with the naked eye to 6mm (0.25in.) |
| Vug | Voids 6 to 50mm (0.25 to 2in.) in diameter |
| Cavity | 50 to 600mm (2 to 24in.) in diameter |
| Cave | >600mm |
| Rock Mass Description | RQD |
|---|---|
| Excellent | 90 - 100 |
| Good | 75 - 90 |
| Fair | 50 - 75 |
| Poor | 25 - 50 |
| Very Poor | Less than 15 |
321.2.10.3.1 Field Identification System for Rock Classification
321.2.10.3.2 Rock Quality Designation
Rock Quality Designation, RQD, is based on a modified core recovery procedure which, in turn, is based indirectly on the number of fractures and amount of softening or alteration in the rock mass as observed in the rock cores from a drill hole. Instead of counting the fractures, an indirect measure is obtained by summing up the total length of core recovered but counting only those pieces of core which are 4 in. long or longer, and which are hard and sound.
If the core is broken by handling or by the drilling process (i.e., the fracture surfaces are fresh irregular breaks rather than natural joint surfaces), the fresh broken pieces are fitted together and counted as one piece, provided that they form the requisite length of 4 inches. Some judgment is necessary in the case of sedimentary rocks and the foliated metamorphic rocks, and the limestones, sandstone, etc. However, the system has been applied successfully even for shales although it was necessary to log the cores immediately upon removing them from the core barrel before air-slaking and cracking began.
An example is given below from a core run of 60 inches. For this particular case the total core recovery was 50 inches, yielding a core recovery of 83%. On the Modified basis, only 34 inches are counted and the RQD is 57%. It has been found that the RQD is a more sensitive and consistent indicator of general rock quality than is the gross core recovery percentage.
The procedure obviously penalizes the rock where recovery is poor. This is appropriate because poor recovery usually depicts poor quality rock. It is not always true, however, because poor drilling equipment and technique can also cause poor recovery. For this reason double-tube core barrels of at least NX-size (2 1/8 in. diameter) are usually specified and proper supervision of the drilling is imperative.
As simple as the procedure appears, it has been found that there is a reasonably good relationship between the numerical values of the RQD and the general quality of the rock for engineering purposes. This relationship is below:
321.2.10.3.3 Core Handling and Labeling
 |
 |
Rock core from geotechnical explorations should be stored in structurally sound core boxes made of wood or corrugated waxed cardboard. Wooden boxes should be provided with hinged lids, with the hinges on the upper side of the box and a latch to secure the lid in a closed position. Waxed cardboard boxes should be protected from the elements and in no instance should they be exposed to rainfall or placed directly upon damp ground.
Cores should be placed in the boxes from left to right, top to bottom. The core should read like a book left to right. When the upper compartment of the box is filled, the next lower compartment (and so on until the box is filled) should be filled, beginning in each case at the left-hand end. The depths of the top and bottom of the core and each noticeable gap in the formation should be marked by a clearly labeled wooden spacer block. Spacers should be placed in the core box to immobilize the core and to keep the core in the correct position. Spacers are necessary due to core loss and when unconfined compression samples are removed from the core. The spacers should be labeled and in the case of core loss the spacer should be placed at the depth of the core loss if known or at the end of the run if not known. Spacers may be wooden blocks, pvc pipe, cardboard tubes, etc. Core box labels and spacers labels should be completed using indelible black marking pens.
Cores should be handled carefully during transfer from barrel to box. Cores should freely come out of the core barrel tube. In the case of shales that tend to swell, it may be necessary to extrude the core from the core barrel. In no case should the core barrel be allowed to be beaten on or thumped against a wooden block. Deliberate breaks of the core are allowed in order to fit the core into the core box.
321.2.10.3.4 Symbols for Rock and Soils
321.2.10.4 Atterburg Limits and Expected Erosion Potential
321.2.10.5 Correlations of Strength Characteristics
321.2.10.6 Example Forms
