Test Boring Soil Sampling Types Commonly UsedTypes Commonly Used

The split-barrel sampler (SS) is used in all soil types for representative samples used for iden-tification and index tests.

Thin-wall tubes are used in soft to firm clays and coring samplers are used in stiff to hard cohesive soils for undisturbed samples used for engineering properties tests.

Required Boring Diameters

Wash or exploratory borings for split-barrel sampling are normally of 2½ in. diameter (cas-ing I.D.). UC bor(cas-ings are normally of 4 in. diameter, but may be larger to improve sample quality. Core borings vary from 2 in. to larger, normally with NX core taken in a hole started with 4 in. diameter casing.

Sampling Interval

Samples are normally prescribed for 5 ft (or 1 m) intervals and a change in strata.

Samples should also be taken at the surface to record the topsoil thickness, and contin-uously from the surface to below the depth of shallow foundations to assure informa-tion at footing depth (5 ft intervals often do not provide informainforma-tion at shallow footing elevations).

Continuous sampling is also important through miscellaneous fills that vary widely in materials and that often overlie a layer of organic soil, which may be thin but significant, and through formations with highly variable strata. In deep borings, sampling depths are often changed to 10 or 20 ft intervals after several normally sampled borings are com-pleted and general subsurface conditions defined.

Factors Affecting Sample Quality Sampler Wall Thickness

A large outside diameter relative to the inside diameter causes deformation by material displacement.

Sampler Conditions

● Dull, bent, or otherwise deformed cutting edges on the sampler cause sample deformation.

● Inside friction, increased by rust, dirt, or, in the case of tubes, omission of lacquer, causes distortions which are evidenced by a turning downward of layers, result-ing in conical shapes under extreme cases.

Boring Operations

● Dynamic forces caused by driving casing can loosen dense granular soils or den-sify loose granular soils.

● Sands may rise in the casing when below the GWL (Figure 1.60).

● Overwashing, jetting, and high fluid pressures also loosen granular soils or soften cohesive materials (Figure 1.60).

● Coarse materials often remain in the hole after washing, particularly in cased borings (Figure 1.60). These “cuttings” should be removed by driving a split bar-rel sampler, by pushing a Shelby tube, or with a cleanout auger. Contamination is common after boring through gravel layers or miscellaneous fills containing cinders, etc.

TABLE 1.16 Sampling Tools and Methods Category-Method and ToolApplicationLimitations Reconnaissance Wash sampleIndication of material type onlyCompletely mixed, altered, segregated Auger sampleMaterial identificationCompletely disturbed Retractable-plug samplerMaterial identificationSlight disturbance, very small sample of soft soils Black sampleLarge undisturbed sample of cohesive materialsTaken from test pits, cohesive soils only Test Boring Sampling (Soils) Split barrel (spoon)Undisturbed samples in soils suitable Samples not suitable for engineering properties testing for identification and lab index testsSampling impossible in very coarse granular soils Shelby tubeUndisturbed sample in firm to stiff Will not retrieve very or clean granular soils cohesive soil. Can be driven into hard soils Standard stationary pistonUndisturbed samples in soft to firm clays and siltsWill not penetrate compact sands, stiff clays, and other strong soils. Will not retrieve sands. Can be overpushed Osterberg piston sampler Undisturbed samples in all soils with cohesion except Usually cannot penetrate strong residual soil and very strong. Less successful in clean sandsglacial till. Some disturbance in sand and often loss of sample. User cannot observe amount of partial penetration Shear-pin piston (Greer and Undisturbed samples in all soils with Usually cannot penetrate strong residual soil McClelland)cohesion except very strong. Often or glacial till. Disturbance in sands recovers samples in sands and can beCannot observe amount of partial penetration used to determine natural density Swedish foil samplerContinuous undisturbed samples in Gravel and shells will rupture foil. Cannot penetrate soft to firm cohesive soilsstrong soils Denison samplerUndisturbed samples in strong cohesive soils Not suitable in clean granular soils, and soft to firm clays such as residual soils, glacial till, soft rock alternating Pitcher samplerSimilar to Denison above. Superior in Similar to Denison above soft to hard layers. Can be used in firm clays

Subaqueous Sampling Without Test Boring Free-fall gravity coring tube Samples firm to stiff clays, sand and Maximum length of penetration about 5 m in soft fine gravel in water depths of 4000 msoils, 3 m in firm soils Harpoon-type gravity Samples river bottom muds and silts to Penetration limited to few meters in soft soils samplerdepths of about 3 m Explosive coring tube Small-diameter samples of stiff to hard Sample diameters only 1 7/8 in. Penetration only to 3 m (piggot tube)ocean bottom soils to water depths of 6000 mbelow seafloor Gas-operated free-fall piston (NG)Good-quality samples up to 10 m depth from seafloorPenetration limited to 10 m below seafloor VibracoreUndisturbed samples of soft to firm bottomLimited to soft to firm soils and maximum penetration of sediment, 3 1/2 in. diameter to depths of 12 m12 m. Water depth limited to 60 m Subaqueous Sampling with Test Boring Wireline drive sampleDisturbed sample in soilsPenetration length during driving not known Wireling push samplesRelatively undisturbed samples may be Often poor or no recovery in clean granular soils obtained in cohesive materials Rock Coring Single-tube core barrelCoring hard homogeneous rock where Circulating water erodes soft, weathered, or fractured rock high recovery is not necessary Double-tube core barrel Coring most rock types where high recovery is not Recovery often low in soft or fractured rocks necessary, and rock is not highly fractured or soft Double-tube swivel-type Superior to double-tube swivel-core barrel, aboveNot needed in good-quality rock. Barrel is more costly core barrelParticularly useful to obtain high recoveryand complicated than others mentioned above in friable, highly fractured rock Wireline core barrelDeep hole drilling in rock or offshore because of No more efficient than normal drilling to depths of substantial reduction of in–out times for toolsabout 30 m Oriented core barrelDetermination of orientation of geologic structuresProcedure is slow and costly. Requires full recovery Integral coring methodRecover cores and determine orientation in Slow and costly procedure poor-quality rock with cavities, numerous fractures, and shear zones Calyx or shot coringObtain cores in medium- to good-quality rock Slow and costly. Difficult in soft or seamy rock up to 2-m diameter

● Hole squeezing may occur in soft clays if the drilling mud is too thin.

● Plastic clays may remain along the casing walls if cleaning is not thorough.

● Hollow stem augers can cause severe disturbances depending on the rate of advance and rotation, and the choice of teeth on the bit.

● Hollow-stem augers must be advanced with the plug in the auger prior to sam-pling to prevent soil from entering the auger.

Sampler Insertion

All ball check valves and other mechanisms should be working properly before the sam-pler is lowered into the hole. The samsam-pler should be lowered to the bottom immediately after the hole is cleaned. Measurements of the total length of the sampler and rods should be made carefully to ensure that the sampler is resting on the bottom elevation to which the hole was cleaned and to avoid sampling cuttings. Since complete cleaning is usually not practical, split-barrel samplers are seated under the rod weight and often tapped lightly with the drive hammer; piston samples are forced gently through the zone of soft cuttings.

Soil Factors

● Soft to firm clays generally provide the best “undisturbed” samples, except for

“quick” clays, which are easily disturbed.

● Air or gas dissolved in pore water and released during sampling and storage can reduce shear strength.

● Heavily overconsolidated clays may be subject to the opening of fissures from stress release during boring and sampling, thereby substantially reducing strength.

● Gravel particles in a clay matrix will cause disturbance.

● Cohesionless granular soils cannot be sampled “undisturbed” in the present state of the art.

● Disturbance in cohesive materials usually results in a decrease in shear strength and an increase in compressibility.

Sample disturbance and its effect on engineering properties are described in detail by Broms (1980). See Section 1.4.6 for sample preservation, shipment, storage, and extraction.

Split-Barrel Sampler (Split Spoon) (ASTM D1586-99) Purpose

Split-barrel samplers are used to obtain representative samples suitable for field examina-tion of soil texture and fabric and for laboratory tests, including measurements of grain-size distribution, specific gravity, and plasticity index, which require retaining the entire sample in a large jar.

Sampler Description

Split-barrel samplers are available with and without liners; the components are shown in Figure 1.61. O.D. ranges from 2 to 4½ in. A common O.D. is 2 in. with ¼ in. wall thickness (1½ in. sample). Larger diameters are used for sampling gravelly soils. Lengths are either 18 or 24 in.

A ball check valve prevents drill pipe fluid from pushing the sample out during retrieval. To prevent sample spillage during retrieval, flap valves can be installed in the shoe for loose sands, or a leaf-spring core retainer (basket) can be installed for very soft

clays and fine cohesionless soils. Upon retrieval, the barrel between the head and the shoe is split open (Figure 1.62), the sample is examined and described, removed, and stored.

In some sampler types, brass liners are used for procuring drive samples of strong cohe-sive soils for laboratory direct-shear testing.

Sampling Procedure

The sampler is installed on the hole bottom, then driven into the soil with a hammer (nor-mally 140 lb) falling on the drill rods. The number of blows required for a given weight and drop height, and a given penetration, are recorded to provide a measure of soil com-pactness or consistency as described in Section 2.4.5 (Standard Penetration Test).

Thin-Wall Tube Samplers Purpose

Thin-wall tube samplers are used to obtain UD of soft to stiff cohesive soils for laboratory testing of strength, compressibility, and permeability.

Tube Materials

Cold-drawn, seamless steel tubing (trade name “Shelby tube”) is used for most soil mate-rials; brass tubes are used for organic soils where corrosion resistance is required. Wall thickness is usually 18 gage; heavier gages are available. Lacquer coating can provide cor-rosion protection and reduce internal frictional resistance and sample disturbance.

Tube diameters and lengths range from 2 to 6 in. in diameter, 24 to 30 in. in length. Tubes 2 in. in diameter are used in 2½ in. exploratory borings, but 2 in. diameter samples have a large ratio of perimeter disturbance to area and are considered too small for reliable labo-ratory engineering-property testing.

Tubes 3 in. (2.87) in diameter are generally considered the standard type for laboratory test samples. The tube should be provided with a cutting edge drawn in to provide about 0.04 in. inside clearance (or 0.5 to 1.5% less than the tube I.D.), which permits the sample to expand slightly upon entering the tube, thereby relieving sample friction along the walls and reducing disturbance.

Tubes 4 to 6 in. in diameter reduce disturbance but require more costly borings. A 5 in.

tube yields four samples of 1 in. diameter from the same depth for triaxial testing.

Operations

Thin-wall tubes are normally pressed into the soil by hydraulically applied force. After pressing, the sample is left to rest in the ground for 2 to 3 min to permit slight expansion

Hardened shoe Ball check

Hardened shoe Outer split tube Split tube

Thin wall liner (a)

(b)

Ball check

FIGURE 1.61

The split-barrel sampler or split spoon: (a) without liner; (b) with liner. (Courtesy of Sprague and Henwood, Inc.)

and an increase in wall friction to aid in retrieval. The rods and sampler are rotated clock-wise about two revolutions to free the sampler by shearing the soil at the sampler bottom.

The sample is withdrawn slowly from the hole with an even pull and no jerking. In soft soils and loose granular soils, the sampler bottom is capped just before it emerges from the casing fluid to prevent the soil from falling from the tube.

Shelby Tube Sampling

A thin-wall tube is fitted to a head assembly (Figure 1.63) that is attached to drill rod. An

“O ring” provides a seal between the head and the tube, and a ball check valve prevents water in the rods from flushing the sample out during retrieval. Application is most satis-factory in firm to hard cohesive soils. In firm to stiff soils, the tube is pushed into the soil by a steady thrust of the hydraulic system on a rotary drilling machine using the machine weight as a reaction. Care is required that the sampler is not pressed to a distance greater than its length.

FIGURE 1.62

Split-barrel sample of sand.

Soft soils are difficult to sample and retain because they have insufficient strength to push the column of fluid in the tube past the ball check valve. In stiff to hard cohesive soils, samples are often taken by driving heavy-gage tubes.

Standard Stationary Piston Sampler

A thin-wall tube is attached to a head assembly. The tube contains a piston (Figure 1.64), which is connected to a rod passing through the drill rod to the surface. When at the bot-tom of the tube, the piston prevents soil from entering the tube as it is lowered into the hole and permits seating through soft cuttings. The rod connected to the piston is held fixed at the surface, while the hydraulic system on the drilling machine presses the tube past the piston into the soil. With light rigs, the reaction can be increased by using Earth anchors. In properly fitted piston samplers, a strong vacuum is created to hold the sample in the tubes during withdrawal from the hole. The stationary piston sampler is used to retrieve very soft to firm cohesive soils.

Osterberg Hydraulic Piston Sampler

A thin-wall tube contains an actuating and a fixed piston (Figure 1.65). An opening in the head assembly permits applying fluid pressure to the actuating piston at the top of the tube. Fluid pressure is applied to the actuating piston, which presses the tube past the fixed piston into the soil. The actuating piston eliminates the cumbersome rods of the standard stationary piston as well as the possibility of overpushing. The sampler is commonly used for very soft to firm cohesive soils.

Shear-Pin Piston

This device is similar to the Osterberg sampler except that the tube is attached to the pis-ton with shear pins, which permit the fluid pressure to build to a high value before it

“shoots” the piston when the pins shear. The sampler can be used in soft to stiff cohesive soils and loose sands. In loose sands disturbance is unavoidable. The “apparent” density, however, can be determined by measuring the weight of the total sample in the tube and assuming the volume calculated from the tube diameter and the stroke length. With the shear-pin piston, the sampling tube will almost always be fully extended because of the high thrust obtained.

Double-Tube Soil Core Barrels Purpose

Double-tube soil core barrels are used to obtain UD in stiff to hard cohesive soils, sapro-lite, and soft rock.

30-in. brass tube O-ring

Screws Rubber

seal

Bronze ball Retaining pin

Head assembly

FIGURE 1.63

Thin-wall “Shelby tube” sampler. (Courtesy of Sprague and Henwood, Inc.)

Denison Core Barrel

The Denison core barrel is used in materials such as hard clays and cemented sands. It includes a rotating outer barrel and bit containing a fixed inner barrel with a liner, as shown in Figure 1.66. The cutting shoe on the inner barrel can extend below the cutting bit. Liners range from 28-gage galvanized steel to brass and other materials such as phe-nolic-resin-impregnated paper with a 1/16 in. wall. Various bits are available for cutting materials of varying hardness, to obtain samples ranging in diameter from 2 3/8 to 6 5/16 in. Sample tubes range from 2 to 5 ft in length. The extension of the cutting shoe below the cutting bit is adjustable. The maximum extension is used in relatively soft or loose mate-rials, whereas in hard materials the shoe is maintained flush with the bit.

During drilling, pressure is applied by the hydraulic feed mechanism on the drill rig to the inner barrel, while the bit on the outer barrel cuts away the soil. The sampler is

Buckets

(leather) Piston assembly

30-in. steel tube Screws Piston

Stationary piston sampler. (Courtesy of Sprague and Henwood, Inc.)

Drill rod

Operation of the Osterberg piston sampler. (a) Sampler is set on cleaned bottom of borehole. (b) Hydraulic pressure propels sampling tube into the soil. (c) Pressure is released to through hole in piston rod. (From ENR, Engineering News Record, 1952. Reprinted with permission of McGraw-Hill.)

retrieved from the hole, the cutting bit removed from the barrel (Figure 1.67), and the thin liners, retained in the inner tube by wall friction or a basket-type retainer, are removed.

Pitcher Sampler

The operation of the Pitcher sampler is similar to that of the Denison core barrel except that the inner barrel is spring-loaded and thus provides for the automatic adjustment of the distance by which the cutting edge of the barrel leads the coring bit (Figure 1.68).

Because of adjusting spring pressure, the Pitcher sampler is particularly suited to sam-pling deposits consisting of alternating soft and hard layers.

1.4.3 Miscellaneous Soil-Sampling Methods