17013

R. S. Ladd t

Preparing Test Specimens Using Undercompaction

REFERENCE: Ladd, R. S., "Preparing Test Specimens Using Undereompaetion," Geotechnical Testing Journal, GTJODJ, Vol. 1, No. 1, March 1978, pp. 16-23.

ABSTRACT: A specimen preparation procedure is presented that offers an improved method of preparing reconstituted sand specimens for cyclic triaxial testing. The method leads to more consistent and repeatable test results. This procedure (1) minimizes particle segregation, (2) can be used for compacting most types of sands having a wide range in relative densities, and (3) permits determination of the optimum cyclic strength of a given sand at a given dry unit weight.

KEY WORDS: sands, compaction, triaxial tests, specimen preparation, percent undercompaction, dynamic testing

Nomenclature

~3c +--Od

D10, D30, D50, and D6o

Cc c. D,

+ tYd/2ff3c epp N

Ne

N/Nf WT

3/dr Wa

Vm WL h.

ht nt n

u. Uni Unt

ni u.

Effective isotropic consolidation stress Cyclic axial deviator stress Soil diameters of which 10, 30, 50, and 60% of soil weights are finer, respectively Coefficient of curvature Coefficient of uniformity Relative density Applied cyclic stress ratio Peak-to-peak axial strain Number of loading cycles Number of loading cycles to obtain a given peak- to-peak axial strain Number of loading cycles to failure Normalized number of cycles Total wet weight of material required Required dry unit weight of test specimen Average water content (as a decimal) of prepared material Final volume of compacted material Weight of material required for each layer Height of compacted material at the top of the layer being considered Final (total) height of the specimen Total number of layers Number of the layer being considered Percent undercompaction for layer being considered Percent undercompaction selected for first layer Percent undercompaction selected for final layer First (initial) layer Average percent undercompaction for layers compacted

1Associate and laboratory director, Woodward-Clyde Consultants, Clifton, N.J. Member of ASTM.

0149-6115/7810003-0016500.40

ID Inside diameter L Cyclic strength index

Au Change in pore ,water pressure Aac Change in cell pressure

16

Introduction

The specimen preparation procedure most commonly described in the literature on cyclic triaxial strength testing [1-3] requires the sand to be saturated, poured into a water-filled forming mold (usually attached to the bottom pedestal of a triaxial cell), and then densified to the required density by some means, usually by vibrations. This method is referred to herein as the wet-pouring (pluvial) method.

Several problems are associated with this wet-pouring method. The two most significant are (1) the segregation of particles when using silty and relatively well-graded sands, and (2) the difficulty of readily preparing test specimens having a prescribed dry unit weight with uniform density. A more precise means of preparing specimens is needed so that cyclic test results will be consistent, repeatable, and less influenced by specimen preparation.

Presented herein is a method of reconstituting cyclic triaxial strength test specimens that minimizes most of the problems outlined previously. In addition, the concepts presented can be applied to the preparation of reconstituted test specimens for other types of tests and materials. It should be noted that there is no inference here that this method of reconstitution results in specimens which are representative of in-situ conditions.

The procedure incorporates a tamping method of compacting moist coarse-grained sand in layers. Each layer is compacted to a selected percentage of the required dry unit weight of the specimen; this procedure differs from the application of a constant compactive effort to each layer required by ASTM Tests for Moisture-Density Relations of Soils, Using S.5-1b (2.5-kg) Ram- mer and 12-in. (304.8-mm) Drop (D 698-70) and ASTM Tests for Moisture-Density Relations of Soils, Using 10-1b (4.5-kg) Rammer and 18-in. (457-mm) Drop (D 1557-70). This new approach was selected since it is generally recognized (especially for loose- to medium-dense sands) that when a typical sand is compacted in layers, the compaction of each succeeding layer can further densify the sand below it. The method uses this fact to achieve uniform specimens by applying the concept of undercompaction. In this case, each layer is typically compacted to a lower density than the final desired value by a predetermined amount which is defined as percent undercompaction U,. The U, value in each layer is linearly varied from the bottom to the top layer, with the bottom (first) layer having the maximum U. value. The method of variation is illustrated in Fig. 1. (See

1978 bythe American Society for Testing and Materials

LADD ON SPECIMEN PREPARATION USING UNDERCOMPACTION 17

Maximum Value

8 ,,=, r~ 5

==

Minimum Value (usually zero) n i = 1

nder m n in la n

~ ~ n t l ~x~ -iler fa~t ilrc=nt ii(ir_ ,on n n t

LAYER NUMBER

compaction

Where: A. Percent under-compaction in layer being considered, U n

pUni- u.tl ] U n = Uni - L n--~-~_ 1 x (n- 1)

B. Average percent under-compaction for layers compacted, O n

_ U n Un= ~

Uni = Percent under-compaction selected for first layer

Unt = Percent under-compaction selected for final layer (usually zero)

n = Number of layer being considered

n i = First (initial) layer

n t = Total number of layers (final layer)

FIG. 1--Concept of undercompaction procedure.

also Appendix A.) If this method of variation is appropriate and the proper/.1, value is selected for the first layer (U,i), the end product is a specimen having a virtually uniform unit weight throughout.

The method used to arrive at this proper U,i value is presented

in this paper. In addition, to illustrate how the cyclic behavior is affected by the Uni value selected, a series of cyclic triaxial strength tests was performed on specimens of Monterey No. 0 sand in which the Uni value was varied.

Material Tested

The particle size distribution curve and the selected index properties of the Monterey No. 0 sand, obtained by Mulilis [4], are shown in Fig. 2 and Table 1, respectively. The sand is a washed uniform medium-to-fine beach sand (SP). The maximum and minimum dry unit weight determinations were performed in general accordance with ASTM Test for Relative Density of Cohesionless Soils (D 2049-69) and Kolbuszewski's method [5], respectively. The specimens tested had initial relative densities Dr of approximately 60%.

Specimen Preparation Procedure

Each test specimen, 74 mm (2.9 in.) in diameter and 152 mm (6 in.) high, had an initial molding water content of approximately 6% and was compacted in eight layers in a split compaction mold not attached to the triaxial cell ("external" split compaction mold). Further details of this method of specimen preparation are given in Appendix A.

After compaction, the split mold was removed and the weight, height, and diameter of the specimen were measured. The specimen was then placed in the triaxial cell and confined with a rubber membrane. The triaxial cell was filled with deaerated water, and a cell pressure o3~ of 36 kN/m z (750 psf) was applied.

Test Procedure

Each specimen was saturated prior to being consolidated by flushing deaerated water through the specimen under a back pressure of between 625 and 960 kN/m 2 (13 000 and 20 000 psi).

COBBLES COARSE I FINE cq,ARSE [ MEDIUM I FINE SILT OR CLAY I

I- -r r~

>. gD

Z

F-

O r r

DIAMETER U,S. STANDARD SIEVE SIZE

6" 4" 3" l'h" 3/4" 3/8 " 4 10 20 40 60 100 200 I- I

1]:1 l [ Ill ] [ IliIl'~ I Illl I J Ill so ! Ill[ I~l 1 1 l l ]l[I[ Ill

70 Igl ~____ [! [ I]lll ]l',I]i [ ]~l ! ~ i!;i so lffl llJ]

[~[ L s0 I~l 1-~ - - i

4o Ig] I . 3o] ! II J ' . . . . . ' " ' .... + ~ ..... III[~I[ I I--

I 20

IIIt:[:t .: [- -I-.. -I . . . . . -- - Ill!Ill I L 200 100 10 1,0 0.1

GRAIN SIZE iN MILLIMETERS

UNIFIED SOIL CLASSIFICATION SYSTEM

II

i i

[I I[ I I I

0.01 I

0.001

FIG. 2--Particle size distribution curve.

18 GEOTECHNICAL TESTING JOURNAL

TABLE 1--Index data for Monterey No. 0 sand.

Unified soil classification system symbol SP Particle size data

Ds0, mm 0.36 Cc~ 0.9 C u 1.5

Dry unit weight data e Maximum, lb/ft 3 105.7 Minimum, lb/ft 3 89.3

aCc= (1)30)2/(960 D10). bcu = D60/D~o. c 1 lb/ft 3 = 16 kg/m 3.

During back-pressuring, an effective confining stress of 36 kN/m 2 (750 psf) was maintained. This low confining stress minimizes unrecorded volume changes during saturation; however, if the specimen has a tendency to swell, higher values should be selected. In addition, a small axial stress, sufficient to maintain the specimen in an isotropic state of stress, was applied. Saturation was assumed when the B factor (ratio of the change in pore water pressure Au to the change in cell pressure Aa~) was equal to or greater than 95%.

The specimen was then consolidated to the required effective stress F3~. Changes in volume and axial height were recorded during consolidation. The relative density of the specimen prior to cyclic loading is based on these measurements.

The specimens were cyclically loaded without drainage by using an eleetrohydraulic closed-loop loading system manufactured by the MTS Systems Corp. The MTS system applied a sinusoidally varying load about an ambient load at a frequency of 1 Hz. Therefore, a cyclic sinusoidally varying axial deviator stress +_ oa was applied to the specimen in which the stress varied between peak compression and peak extension values. During cyclic loading, the cell pressure was kept constant, and the changes in axial load, axial deformation, and pore water pressure were recorded.

Test Results and Discussion

The results of the cyclic triaxial strength tests are summarized in Table 2. A plot of the cyclic strength index versus the percent undercompaction of the first layer of each specimen is given in Fig. 3. The cyclic strength index Ic is defined as the ratio of the number of cycles to obtain a given peak-to-peak axial strain Are to the product of relative density in percent D, and applied stress ratio _+ ad/2F3~, that is, Ne/Dr( +-Od/253c); Ic was used to normalize small differences in relative density and applied stress ratio from one test to another.

The data show that Ic, which is directly related to the cyclic strength, varies with U,i or the uniformity of dry unit weight within a test specimen. For the U,i values evaluated (0 to 18%), the number of cycles to obtain a peak-to-peak axial strain of 10% at an applied stress ratio of 0.26 varied between 16 and 41 (see Table 2). Furthermore, a peak Ic value (optimum cyclic strength) was obtained. The U,i value where this peak occurred is defined as the optimum percent undercompaction.

Another important factor in understanding the cyclic behavior of sand is its strain development characteristics. Axial strain in compression and extension versus the logarithm of the number of loading cycles is plotted in Fig. 4. The shapes of the curves vary considerably, and it was almost impossible to determine trends visually. To determine whether there was a relationship between U,i and the strain development characteristics, as was found with cyclic strength, the cyclic data were normalized. The curves of the normalized peak-to-peak strain versus the normalized number of cycles are plotted in Fig. 5. The normalized peak-to-peak strain e,p/~pp = 10% is defined as the ratio of peak-to-peak strain at a given number of cycles N to a peak-to-peak strain of 10% (selected failure criteria), while the normalized number of cycles N/Nf is defined as the ratio of the number of cycles required to obtain a given e,p to the number of cycles required to obtain an Epp of 10%. This figure shows that as U,~ becomes closer to the optimum percent undercompaction, the normalized strain development curves become more concave.

TABLE 2--Summary of results of tests preformed on Monterey sand No. O.

Water Content, Dry Unit Number of Cycles for % Weight, lb/ft 3a Dr, %

Peak-to-Peak Percent After ,After After Initial Strain, % Under- Consoli- Consoli- Consoli- Lique-

Test compaction Initial dation Initial dation Initial dation +_Od/2iY3 c faction 2.5 5 10 20 Remarks b

1 0 6.0 24.7 98.3 99.2 59.2 64.0 0.26 24 24 26 30 54 see Note 1 2 2 8.8 24.8 98.6 99.4 60.8 65.5 0.25 23 22 24 28 42 see Note 1 3 4 6.0 24.6 98.5 99.7 60.3 67.2 0.26 33 33 36 41 67 see Note 1 4 6 5.8 25.6 98.4 99.3 59.8 65.0 0.26 33 33 36 40 57 5 8 5.8 23.8 98.8 99.5 61.7 66.3 0.26 20 19 22 27 62 see Note 1 6 10 6.3 24.9 98.0 98.9 57.2 62.6 0.25 22 22 24 29 47 7 12 5.7 24.2 98.2 99.1 58.7 63.8 0.26 19 18 20 25 44 8 14 6.0 24.6 98.5 99.3 60.3 65.0 0.26 30 28 30 35 64 see Note 1 9 16 6.0 25.1 98.5 99.3 60.1 65.0 0.26 18 18 20 24 130 see Note 1

10 18 6.0 25.3 98.5 99.6 60.4 66.9 0.26 10 9 11 16 43 see Note 1

a 1 lb/ft 3 = 16 kg/m 3. b Notes: 1. A significant (> 10%) decrease in peak-to-peak axial load occurred after a peak-to-peak axial strain of 10% had occurred. 2. Test specimens were 74 mm (2.9 in.) in diameter by 152 mm (6 in.) in height and were compacted in eight layers by using the moist tamp-

ing method presented in Appendix A. 3. Consolidation pressure 03 c equaled 44.6 kN/m 2 (2088 lb/fl 2).

LADD ON SPECIMEN PREPARATION USING UNDERCOMPACT~ON 19

X ,,' 3 E3 z -1- i - L9 z 2 uJ n~

. J ~D >- 1 cD

Peak to Peak Symbol Axial Strain, %

O 5 A lO

Note: 1 KN/m 2= 20.88 Ib/ft 2

Optimum Cyctic Strength Index - ~

/0 \ A --. _A l l \

O

I I 0 2

Test Conditions Relative Density, Dr (%) Stress

After (~-3c Ratio Consoli-

Initial Ib/ft 2 +_ l:Td/2 ~3c

57-62 63-67 2,088 0.25-0.26

Number of Cycles to Obtain a Given

Cyclic Strength Index = Peak to Peak Axial Strain Relative Density (%) x Stress Ratio

A E)

0 0

O ~ 1 Optimum Percent Under-Compaction

I I I I I I I

4 6 8 10 12 14 16 18

PERCENT UNDER-COMPACTION OF FIRST LAYER

FIG. 3--Cyclic strength index versus percent undercompaction of first layer for Monterey No. 0 sand.

Conclusions

A specimen preparation procedure is presented in Appendix A that offers an improved method of preparing reconstituted sand specimens for cyclic strength testing. The method leads to more consistent and repeatable test results and a reduction in the number of uncertainties inherent in presently used procedures. This procedure, termed the undercompaction procedure, (1) minimizes particle segregation, (2) can be used for compacting most types of sands, which have a wide range in relative densities, and (3) permits determination of the optimum cyclic strength of a given sand at a given dry unit weight.

Acknowledgment

Portions of this investigation were sponsored by the Professional Development Program of Woodward-Clyde Consultants (WCC). This support is acknowledged with appreciation. Special acknowledgment is given to P. Dutko of WCC who developed the percent undercompaction equations. Members of the staff of WCC who made considerable contributions are, in particular, K. Hau, H. M. Horn, Y. Kim, and J. H. Wilson. Special thanks are also due to D. Koutsoftas of Dames and Moore, M. L. Silver of the University of Illinois at Chicago Circle, and D. J. Leery of Lan- gan Engineering Associates for their reviews of and comments on this paper.

APPENDIX A--RECONSTITUTED SPECIMEN PREPARATION PROCEDURE FOR COARSE-GRMNED SOILS

A procedure is presented below for preparing coarse-grained specimens for dynamic cyclic testing or static triaxial testing.

The procedure (1) produces specimens that have a relatively uniform stress-strain response, (2) minimizes the tendency for particle segregation, and (3) can be used to compact most types of coarse-grained soils, with a relative density ranging between very loose and very dense. Although the procedure has been developed for the preparation of cohesionless test specimens, the concepts presented can be applied to the preparation of many different material types for various types of tests.

Specimens can be prepared either by attaching a split mold to the bottom pedestal of the triaxial cell ("internal" split mold), as shown in Fig. 6, or in a split mold which is separate from the triaxial cell ("external" split mold), as shown in Fig. 7. A split mold is required since it eliminates many of the problems associated with the extrusion of the compacted specimen from a non- split mold. Most specimens, especially those containing fines, compacted in an external split mold at relative densities above about 50%, will have sufficient strength as a result of capillary force so that they may be set up in the triaxial cell without significant change in their fabric. However, extreme care is required in transferring specimens to avoid disturbing the specimen.

1. Adjust the water content of the air-dried material so that that initial degree of saturation of the compacted material will be between 20 and 70%. Oven-drying of the material is not recommended. The lower the percentage of fines in the material, the lower the degree of saturation required. A degree of saturation greater than 70% can be used if water does not bleed from the specimen during compaction. The material should be mixed with water about 16 h before use.

2. Determine the average water content of the prepared material using a minimum of t~vo determinations.

3. Assemble and check all the necessary equipment to be used in preparing the test specimen. Determine the inside diameter and the height of the mold to within _+ 0.02 mm ( 0.001

20 GEOTECHNICAL TESTING JOURNAL

Z < eT-

< X <

20

15

10

uJ

o

.~_ s

E 10 8

20

15 g

UJ

~ 5

o . E 8 10

15

i I i

L @

/ ,

O Percent under compact ion value 'for f irst layer

~pp = 10%

10 50 100

NUMBER OF CYCLES

FIG. 4--Axial strain versus number of cycles for Monterey No. 0 sand.

200

in.) and calculate the volume based on these measurements. If an internal split mold is used, correct the diameter measurement for the average thickness of the rubber membrane.

4. Select the number of layers to be used in the preparation of the specimen. The maximum thickness of the layers should not exceed 25 mm (1 in.) for specimens having diameters less than 102 mm (4 in.). Typically, the required number of layers increases as the required dry unit weight increases. Layers having a thickness of about 12 mm (V2 in.) are recommended.

5. Determine the total wet weight of material required for sample preparation:

Wr fT~r x (1 +w=)x Vm

6. Determine the moist weight of material required for each layer:

WL = WT/nt

7. For the first layer to be compacted, select a value of Uni. Typically, this value ranges between zero for the preparation of dense specimens to about 15% for the preparation of very loose specimens. For the preparation of very dense specimens, it has been found that negative values are sometimes required. Each subsequent layer receives a lesser percentage of undercompaction, conforming to the relationship shown in Fig. 1.

The correct (optimum) value of percent undercompaction may be determined experimentally by one of the following methods:

a. Run a series of cyclic triaxial strength tests with the same effective consolidation stress and applied stress ratio, but with different U,i values, to determine the optimum value (see Fig. 8).

b. Observe the behavior of the specimen during cyclic loading. Excessive necking or bulging in a layer or layers, either at


II

Q.

t~

z" < n-

o0 v < LU

0

v < LU a .

E3 LU N . J < n- O z

1.0

0.75

0.5

0.25

I I O Percent-under-compaction

of First iayer / /~ J

0 0 0.2 0.4. 0.6 0.8 NORMALIZED NUMBER OF CYCLES, N/Nf = N/N Cpp = 10%

FIG. 5--Normalized peak-to-peak strain versus normalized number of cycles.

,ncreasingCyc,,cStrength

1.0

6-in. T ravel ---..._ .~.~ Vertical D al ~[~

I

I -

~Tamping Rod

~ Z ~ ) Rae fmep~n~ eG ~i~lla rA sse m b i y

~ Bushings

J I Membrane Protection Collar

Rubber Membrane

Compaction Foot~ I--] (Diameter=V2 ID ~L]TJ Vacuum Applied of Mold) - ' ~

Porous Stone ~ ~ Split Mold

v"/////'~ n l ~ O-Ring

1 I I n =~[. ~ [~Bot tom Drainage [l Valves Line

Triaxial Cell / ~ Top Drainage Line

FIG. 6--Split compaction mold attached to triaxial cell ("internal" split mold).

22 GEOTECHNICAL TEST ING JOURNAL

Reference-Collar ~i

I /',

6-In. Travel Vertical Dial

? ~j~ Ihitial'Vertieal S~tting R, Inches

I-~ -Bushing. X ~,f--- Tamping Guide Assembly

I~" ~ Co l la r -~ / ' I

- - . I

E=

/

~/Spacer'/~ / / /~- / 1 / / 2" IX /

Vertical Dial Setting-h n, Inches

? i\ ,I I

I1 lI' l _l Compaction Foot ~ (Diameter='A ID of Mold)

Bottom Porous Stone I I ~/,Spacer// /1 K / / / / ' / / / / / / /

____~

1 .,.,%

/ J

SYSTEM PRIOR TO COMPACTION SYSTEM DURING COMPACTION

~ Air Outlets t~-~ I f ~ ~ Spacer-Disk Assembly

' I I I

, / II , ! i ; l I;i II v , rCo ar I I ' 11 ' ' I I .~ L / II l,, Ill I I I . / ~ II Hi IP , i i iv"

"1 I / /A / / / / X / / / /V / ,I "~

I1 ~-~ Sintered Brass Disk

SYSTEM FOR COMPACTION OF FINALLAYER

FIG. 7--"External" sp l i t compact ion mold .

the top or at the bottom of the specimen, indicates a specimen with an inappropriate value of U.z.

c. Observe the behavior of the specimen during unconsoli- dated-undrained loading. Nonuniform vertical strains indicate an inappropriate value of U.z.

d. Observe the fabric of the specimen. A honeycomb struc- ture at either the top or the bottom of the specimen indicates an inappropriate value of U.i.

e, Measure the dry unit weight of the prepared test speci-

men as a function of its height. A dry unit weight not uniform with height indicates an inappropriate value of U.i.

8. Calculate the required height of the specimen at the top of thenth layer:

h


cl

i 15

~8 ~ o u ~g

/ I

I I

I I

"',\

~- Material sensitive to percent under compaction \ \ \ \

\ F Material relatively insensitive to percent under compaction

PERCENT UNDER COMPACTION OF FIRST LAYER

FIG. 8--Expected relationship between strength index and percent undercompaction of first layer.

9. Weigh the amount of material required for the layer, as determined in Step 6, and place it into a closed container. If each layer requires a weight greater than about 80 g, it is usually easier to weigh the amount of material required for each layer and place it into small closed containers.

10. Adjust the reference collar on the tamping rod to obtain the proper h, . Weigh, if you have not already done so, the amount of material required for the layer, and place it into the mold. During weighing, care must he taken to lose as little moisture as possible. Using the tamping rod, guided by the tamping guide assembly, compact the surface of the material (after it has been leveled) in a circular pattern starting at the periphery of the mold and working toward the center of the mold. Initially, a light tamping force should be used to distribute and seat the material uniformly in the mold. The force should then be gradually increased until the reference collar uniformly hits the top of the tamping rod guide assembly. For the last few cov- erages, it may be necessary to hit the tamping rod with a rubber mallet in order to compact the material into a dense state. Next, scarify the compacted surface to a depth equal to about one tenth of the thickness of the layer.

11. Repeat Steps 9 (if required) and 10 until the last layer is in place. During the compaction of the last layer, the tamping rod should be used until the surface of the compacted material is about 0.4 mm (1/64 in.) higher than required. Then, for specimens prepared in an external split mold, place the spacer disk assembly into position and lightly strike it with a rubber mallet until it is seated; see Fig. 7. For specimens prepared with the internal split mold, place the top cap and the porous stone directly on the specimen. The top cap should be attached to the loading piston, which, in turn, should be guided by the bushing located in the top of the triaxial cell. Then lightly strike the loading piston with a rubber mallet until the compacted material reaches the prescribed height. This procedure ensures that there is proper alignment and seating of the top cap in relation to the specimen and the loading mechanism of the triaxial cell.

12. For specimens compacted in an external split mold,

remove the specimen from the split mold (using extreme caution to prevent disturbance) and obtain its weight, height, and diameter. The weight should be determined to the nearest 0.01 g; however, for specimens weighing greater than 1000 g, measuring to the nearest 0.1 g is adequate. The height and diameter should be determined to the nearest 0.02 mm (0.001 in.) using a dial gage comparator. The dial gage contact points on these instru- ments should have a flat surface with a minimum diameter of about 5 mm (IA in.).

For specimens compacted in an internal split mold, the initial weight cannot be directly checked. Therefore, the oven-dry weight of the specimen should be checked after the test. How- ever, the height and diameter of the compacted specimen should be measured after a slight vacuum is applied and the mold is removed. A pi tape (Pi Tape, Lemon Grove, Calif.) is recommended for measuring the diameter.

The author has also used this procedure, with some modifications, for compacting fine-grained soils and found that appropriate specimens are obtained much more readily than when the Harvard compaction apparatus [6] is used. In the latter case, one must determine experimentally the appropriate compactive effort (number of layers, number of tamps per layer, and the tamping force) required to obtain the prescribed value of ~/dr"

A brief description of the required modifications is as follows:

a. A U,i value of zero should be used. b. The compaction of each layer is initiated by using a Har-

vard tamping device [6], having a spring force of 18 kg (40 Ib) and with a compaction foot having a diameter equal to about 1/4 the diameter of the specimen. The compaction is continued by using this tamper until the surface of the material is relatively level. The tamping force should be reduced if the compaction foot appears to penetrate below the proper h, value. Then the tamping rod, as mentioned in Step 10, is used to compact the material to the proper h, value.

References

[1] Finn, W. D. L., Picketing, D. J., and Bransby, P. L., "Sand Liquefaction in Triaxial and Simple Shear Tests," Journal of the Soil Mechanics and Foundations Division, Proceedings of the Ameri- can Society of Civil Engineers, Vol. 97, No. SM4, April 1971, pp. 639-659.

[2] Lee, K. L. and Fitton, J. A., "Factors Affecting the Cyclic Loading Strength of Soil," in Vibration Effects of Earthquakes on Soils and Foundations, STP 450, American Society for Testing and Materials, Philadelphia, 1969, pp. 71-95.

[3] Lee, K. L. and Seed, H. B., "Dynamic Strength of Anisotropically Consolidated Sand," Journal of the Soil Mechanics and Foundations Division, Proceedings of the American Society of Civil Engineers, Vol. 93, No. SM5, Sept. 1967, pp. 169-190.

[4] Mulilis, J. P., "The Effects of Method of Sample Preparation on the Cyclic Stress-Strain Behavior of Sands," Ph.D. dissertation, Univer- sity of California, Berkeley, 1975.

[5] Kolbuszewski, J. J., in Proceedings of the Second International Con- ference on Soil Mechanics and Foundation Engineering, Rotterdam, 1948, Voi. 7, pp. 47-49.

[6] Wilson, S. D., "Suggested Method of Test for Moisture-Density Relations of Soils Using Harvard Compaction Apparatus," in Special Procedures for Testing Soil and Rock for Engineering Pur- poses, STP 479, American Society for Testing and Materials, Phila- delphia, 1970, pp. t01-103.

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17013