Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

Transcription

1 09

2 Soil Compaction - 2

3 Effect of compaction on soil structure Flocculated Direction of increasing dispersion E B A Highly Flocculated High strength, more k, less shrinkage, more swelling C Highly Dispersed D Dispersed Low strength, low k, more shrinkage, less swelling

4 Effect of compaction on soil structure 1. At low water contents, attractive forces between clay particles predominate, creating a more or less random orientation of plate like particles. (results in low density) 2. The addition of water increases repulsion between particles leading them to assume more parallel orientation near OMC. 3. If compacted wet of optimum parallel orientation is further increased leading to what is described as dispersed structure.

5 Compaction equipment In the field, fill soils are typically imported to a site and applied to the existing grade level in layers which are called lifts. When a lift of soil is placed, it will be very loose. Special compaction equipment is then used to compact this lift of the soil. Rollers, Rammers and Vibrators

6 Types of Rollers Smooth-wheeled rollers Vibratory rollers Pneumatic-tire rollers Sheepsfoot rollers Impact compactors

7 Rammers Dropping weight (including piling equipment) Internal combustion type Pneumatic type

8 Smooth Wheeled Rollers 100 % coverage area under wheel with ground contact pressures up to 380 kpa. 1. Conventional three wheel type - 18 tons Tandem rollers Three axle tandem rollers - 1 to 14 tons - 12 to 18 tons Weight increased by ballasting the rolls with water or by a heavy sliding weight. 2. Performance is affected by the load per unit width under the compaction rolls, and the width and diameter of the rolls.

9 Smooth Wheeled Rollers 3. Load per unit width and diameter control the pressure in the surface layer of soil; dimension of the rolls affect rate with which this pressure decreases with depth. 4. Suitable for gravels, sands, hardcore, crushed rock and any material where crushing action is needed.

10 Pneumatic-tyred Roller 80 % coverage area (i.e. 80 % of area is covered by tires) With tire pressures up to 700 kpa Suitable for fine grained soils (closely graded sands). Best performance on cohesive soils obtained when moisture content is 2-4% below PL. Depth of compaction: Light rollers (200kN) Med. rollers (500kN) Heavy rollers (1800kN) 150 mm 300 mm 450 mm

11 Sheepsfoot Rollers Sheepsfoot rollers are most suitable for fine soils, both plastic and non-plastic, especially at water contents dry of optimum. Compaction is by tamping and kneading Area of protrusions range from 30 to 80 cm % coverage, very high contact pressures are possible, ranging from 1400 to 7000 kpa

12 Vibrators Out of balance type Pulsating hydraulic type

13 Out of balance type vibrator

14 Vibrators 1. Vibrators consist of a vibrating unit of either the out-of-balance weight type or a pulsating hydraulic type mounted on a plate or roller. 2. Vibrators give maximum dry density much in excess of the corresponding compaction test value at OMC. 3. Frequencies range cycles/min. (Frequency range within natural frequency of most soils)

15 Impact Compaction 1. It is the transfer of compactive energy into the soil by means of the lifting and falling motion of a non-circular rotating mass. 2. It is thus a process capable of transferring impact loads similar to those found in dynamic compaction on a continuous basis.

16 Features of Impact Compaction Higher Energy 1. The energy rating of the different impact compaction equipments range from 10kJ to 25kJ. 2. Higher energy helps to achieve higher maximum dry density that allows to work over a wider range of moisture contents.

17 Higher Energy

18 Features of Impact Compaction Increased Depth of Influence 1. Contact stress of impact compactor is 300kPa to 1200kPa (exceeding conventional roller), dependent on the soil stiffness. 2. Impact roller s profile radius is not referenced to the centre of the drum greatly exceeding conventional rollers, resulting in a greater contact area. 3. Net result is superior depth of influence, enabling compaction in layer thicknesses exceeding 1m.

19 Depth of Influence

20 Features of Impact Compaction Increased Load Duration The impact compactor s load duration has been measured to be approximately times longer than that of conventional rollers. This allows for a softer soil response and hence enhanced compressibility is attainable.

21 Increased Load Duration

22 Features of Impact Compaction High Operating Speeds Impact Compactors operate at speeds up to 5 times faster and 10 times greater volume per day than conventional compaction equipment.

23 Compaction equipment Equipment type Smooth wheeled rollers Pneumatic rubber tired rollers Sheepsfoot rollers Vibratory rollers Vibratory tampers Soil type Sands and Gravels Silts and clays Silts and clays Sands and Gravels Sands and Gravels To increase the compaction energy applied to the soil in the field: a) Increase the mass/weight of the compaction equipment; b) Decrease the thickness of lift thickness; and c) Increase number of machinery passes.

24 Field compaction In general, granular soils can be compacted in thicker layers than silt and clay. Granular soils are usually compacted using kneading, tamping, or vibratory compaction techniques. Cohesive soils usually need kneading, tamping, or impact. Soils such as GW, GP, GM, GC, SW, SP, and SM have good compaction characteristics. Other soils, such as SC, CL, and ML, are characterized as good to poor. At any rate, the quality of field compaction needs to be assured by measuring the in situ dry unit weight of the compacted soil at random locations.

25 Field compaction and specifications Two categories of earthwork specifications: 1. End product specifications 2. Method specifications With End product specifications, a certain relative compaction or percent compaction, is specified.

26 Field compaction and specifications Relative compaction is defined as the ratio of field dry unit weight (γ d ) f to the laboratory maximum dry unit weight (γ d ) max., according to some specified standard test, for example, the standard Proctor or the modified Proctor test; R C = γ γ d field d max. Expressed in percentage

27 Field compaction and specifications Difference between Relative compaction and Relative density Relative density applied to granular soils only. If some fines are present, it is difficult to decide ASTM D 2049 suggests that the relative density is applicable if the soil contains less than 12 % fines (passing the No. 200 sieve); otherwise the compaction test should be used.

28 Densification of granular soils Engineering properties of cohesionless soils are primarily a function of Relative Density, D r. D r = [(e max e min ) / (e max e min )] x 100

29 Field compaction and specifications γ d = 0 Dry unit weight γ d min γ d γ d max e = Void ratio e max e e min Relative Density D r [%] Relative compaction R c [%] A relationship between RD and RC

30 Field compaction and specifications In the field the questions that have to be answered are: a) To what dry unit weight must the soil be compacted? and b) How can this be achieved efficiently? For many construction applications involving roadway subgrades and trench backfills, etc., there are typical standards specifying the minimum relative compaction that must be achieved.

31 Relative Compaction Type of project Required R C,% Fills to support buildings or roadways 90 Top 150 mm of subgrade below 95 roadways Aggregate base material below 95 roadways Earth dams 100

32 Dry unit weight vs water content indicating the most efficient conditions for field compaction The range a c indicated the range at which the soil should be compacted to achieve R c at any energy level a c

33 Field compaction and specifications To achieve 95 % relative compaction, the placement water content of compacted fill must be greater than water content a and less than c. These points are found where the 95 % R c intersects compaction curve. line If the placement water content is outside of the range a to c, then it will be difficult, if not impossible, to achieve the required percentage of relative compaction This is why it may be necessary at times to wet or dry the soil prior to rolling

34 Field compaction and specifications Having established the range of placement water content, it is required now to ask: What is the best placement water content to use? Most efficient water content would be OMC (field), where the contractor provides the minimum compactive effort to attain the required 95 % relative compaction. The most efficient placement water contents exist between the OMC (lab) and OMC (field). Range of placement water contents should also be specified along with R c

35 Field compaction and specifications In method specifications, the second category, the type and weight of roller, the number of passes of that roller, as well as the lift thicknesses are specified. This specification requires prior knowledge of the borrow soils so as to be able to predict in advance how many passes of, for example, a certain type of roller will produce adequate compaction performance. Requires test fills before carrying-out actual compaction Method specifications is only justified for very large compaction projects such as earth dams.

36 Field Density Tests Typical specifications call for a new field test for every 1000 to 3000 m 3 or so, when the borrow material changes significantly.

37 Field Density Tests Destructive method involve excavation an removal of some of the fill material, where as non-destructive tests determine the density and water content of fill indirectly. Destructive Method 1. Sand-replacement method/core cutter method 2. Rubber-balloon method Non-destructive method 1. Nuclear density method

38 Field Density Tests Comments Destructive methods are time consuming. Nuclear density method has a high purchase cost. Safety precautions during nuclear density test have to be followed.

39 Sand replacement method 1. Sand with known density is filled in the sand cone jar. 2. Weight of sand-cone apparatus with sand is recorded, W Weight of sand to fill the cone is determined, W 2.

40 Sand replacement method 4. Small hole in the compacted soil is excavated and weighed, W Apparatus is inverted over the hole and valve is opened. 6. Weight of apparatus with remaining sand is determined, W 4.

41 Density Calculations Weight of sand to fill hole, W s = W 1 (W 2 + W 4 ) Volume of hole, V Weight of dry soil, W d Dry unit weight, γ d = W s / γ d, sand = W 3 / (1+w) = W d / V

42 Relative compaction of fill containing oversize particles Oversize particles will be defined as rock that is retained on the 19mm sieve. The soil matrix is material passing the 19 mm sieve. Three methods: i) Elimination method ii) Adjusted maximum dry unit method (DM 7.2) iii) Substitution method Suitable when weight of oversize particles is less than 60% by weight

43 Elimination method Perform field density test and determine total volume and total weight of the soil Sieving on 19 mm sieve separates the oversized particles from the soil matrix Knowing the weight and G s of the oversize particles, the volume of oversized material can be calculated. Assuming a fill that must be compacted to 90% relative compaction, use of the elimination method would require the highest field dry-density values

44 Relation between total dry unit weight of the fill to the fraction of oversize particles γ total = γ w G o ( γ d ) (1 F) Rcγ w + F( γ max d ) max R After Day(1989) c F = the fraction of oversize particles by dry weight G o = Specific gravity of oversize particles As the fraction of oversize particles increases, γ total also increases. This is because the more oversize particles in a fill, then the higher γ total must be in order to keep the soil matrix at a desired relative compaction.

45 Nuclear Density Method Uses a low-level radioactive source that is inserted, via a probe, into the center of a newly compacted soil layer. The source emits rays through the compacted soil that are captured by a sensor at the bottom surface of the nuclear density device. The intensity of the captured radioactivity is inversely proportional to soil density. The apparatus is calibrated using the sand cone/replacement method for various soils, and it usually provides reliable estimates of moisture content and dry unit weight. The method provides fast results, allowing the user to perform a large number of tests in a short time.

46 Nuclear Moisture Density Methods 1. Principle elements Nuclear source emitting gamma rays. Detector to pick the gamma rays passing through the soil. Counter to determine the rate gamma rays reaches detector. 2. Common nuclear sources Radium-Beryllium combination Cesium-Americium-Beryllium combination

47 Density determination-principle 1. Gamma rays penetrate in to soil, some are absorbed and some reach the detector. 2. Amount of radiation reaching detector is inversely proportional to soil. 3. Nuclear count rate received at the detector compared with the calibration curves provided by manufacturer.

48 Moisture Determination-Principle 1. Moisture content is obtained from thermal neutron count. 2. Alpha particles are emitted by the source. Americium or Radium source bombard a beryllium target emitting fast neutrons.. 3. Fast neutrons lose velocity if they strike hydrogen atoms in water molecules. 4. Resulting low velocity neutrons are called thermal neutrons.

49 Result Moisture results are provided as weight of water per unit volume of soil tested Dry weight is obtained from subtracting moisture determination from the wet density determination Significant error occurs if soil contains iron, boron, or cadmium.

50 1 st Method Direct Transmission Method Provides more accurate results Radiation source placed in to the test material by using a punched or drilled hole. Depths between 50 mm and 300 mm can be tested. Information surrounding the source is obtained.

51 Direct Transmission Method

52 2 nd Method Back Scatter Method Radioactive source and detector located on the surface of soil. Gamma rays directed in to the soil and some reflect back to detector. Accuracy suffers if gap exists between bottom of device and soil surface. Information about soil nearest to surface is obtained.

53 Back Scatter Method

54 Air Gap Method Less common method and is used when the composition of the near surface materials adversely affects the density measurement.

55 If the lift thicknesses are too large, then the following will occur: Soil at the top of the lift will be well-compacted Soil at the bottom of the lift will not be compacted.

56 Lift Thickness

57 Field dry density vs Number of passes mm

58 For most compaction equipment, lift thicknesses should typically be on the order of 150 mm

59 Approximate method for determining lift height required 0.3 m d 0.3 max m d d max. 1.2 m d should be small enough so that loose layer is not trapped near the interface 2.4 m between lifts After D appolonia (1969)

60 Illustration Compaction 1.1 Given data: water content vs Dry unit weight G s = 2.70; Standard Proctor compaction w [%] γ d [kn/m 3 ] a) Plot 80 % and 100 % Saturation lines, b) If it is proposed to secure a relative compaction of 95 % in the field, what is the range of water content that can be allowed, and c) Would the 20 % air voids curve be the same as the 80% saturation curve?

61 Dry unit weight [kn/m 3 ] Illustration Compaction Compaction curve 80 % Saturation line 100 % Saturation line γ d γ d, max Gsγ w = wg 1+ S = kn/m 3 OMC =15.17 % r s Water content [%]

62 Illustration Compaction 1.1 Range of water content that can be allowed in the field is 10 to 17 % Using R c = γ γ d, field d,max x100 γ d, field = 0.95 x = kn/m 3 γ d = Gsγ w(1 n 1+ wg a ) For w = 8.5 %; n a = 0.2 γ d =17.22 kn/m 3 s Which is different from kn/m 3 for S r = 80% and w = 8.5 % Hence the 20 % air voids curve is not the same as the 80 % saturation line. γ d Gsγ w = wg 1+ S r s

63 In-place densification of granular soils Several new techniques such as Terra-Probe method Vibroflotation Building sand compaction piles Blasting Dynamic compaction -have been successfully used for compaction in in-situ soils (especially granular soils).

64 Terra-Probe method Works best for shallow water table Activated vibrodriver causes the probe to vibrate in the vertical direction. To achieve soil compaction, the probe is vibrated to the planned depth of penetration. Vibrodriver S = 1.5 m

65 In-place densification of granular soils Vibroflotation When conventional rolling type compaction equipment works the surface of an area, the improvement in density is limited to the surficial zone of the deposit; typically, depths below a meter or two, experience a little or no increase in density. Vibroflotation equipment operates from a sites ground surface, but it can densify the full depth of granular deposits which are as deep as about 12 m.

66 Vibroflotation After Mitchell (1970) Range of particle size distribution suitable for densification by Vibroflotation.

67 Vibroflotation It involves the use of a device called a VIBROFLOT, which is a cylindrical piece of equipment about 2 m long, 400 mm in diameter, and weighing about 17.8 kn. The eccentric weight inside the cylinder develops a centrifugal force of about 89 kn at 1800 rpm. The device has water jets at the top and bottom; these have a flow rate of 0.23 to 0.3 m 3 /min at a pressure of 415 to 550 kn/m 2.

68 Vibroflotation The vibroflot sinks into the ground at the rate of 1 to 2 m/min. When the desired depth is reached, the top jet is turned off. The device is then withdrawn at the rate of 0.3 m/min and sand is added from the top. In a regular working day, a compaction of 2550 to 5100 m 3 is not uncommon.

69 Vibroflotation Granular soils Compacted soil

70 Vibroflotation 1. At start, lower jet is opened fully. 2. Water is introduced more rapidly than it can drain away. This creates a momentary quick condition ahead of the equipment, which permits the vibrating machine to settle of its own weight to the desired depth.

71 Vibroflotation 3. The water from the lower jet is transferred to the top jets and the pressure and volume are reduced just enough to carry the sand to the bottom of the hole.

72 Vibroflotation 4. Actual compaction takes place during intervals between 0.3 m lifts which are made in returning the vibroflot to the surface. The vibrator is first allowed to operate at the bottom of the crater until the desired density around the lower part of the machine is attained. By raising the vibrator step by step and simultaneously backfilling, the entire depth is compacted.

73 In-place densification of granular soils Blasting or by Explosives The range of soil grain sizes suitable for compacting by blasting method is the same as for Vibroflotation. In this method, the compaction is achieved by successive detonations of small explosive charges in saturated soils. Relative densities of 70 to 80 % upto a depth of 20 to 25 m can be achieved.

74 Blasting or by Explosives Explosive charges (60 % dynamite, 30% special gelatin dynamite, and ammonite are most commonly used) are placed at about 2/3 times the thickness of the stratum to be densified. The spacings of the charges vary from 3 to 8 m. Three to five successive detonations of several spaced charges are usually required to achieve the desired compaction. The shock waves due to blasting cause liquefaction of the saturated sand, followed by densification. Practically no compaction is achieved in the top 1 m and so this zone usually needs recompaction by rollers.

75 Blasting or by Explosives The relation for the weight of charge and the sphere of influence for compaction can be given by: W = CR 3 Where W = Weight of charge R = Sphere of influence C = Constant ( for 60 % dynamite) If blasting is used in dry or partially saturated soils, preflooding is desirable.

76 Compaction by Pounding, Dynamic compaction or High energy impact Used for improving surface and near surface zones of soil and fill material whose existing condition is considered marginal or inadequate foundation support The method consists of dropping a heavy weight from a relatively great height; Weights ranging: 2 tons to 15 tons, and drops have ranged from 10 to 30 m. Pounding Repeated heavy blows

77 Dynamic compaction Usually, a closely spaced grid pattern is selected for the pounding locations, and the multiple poundings are required at each drop location (typically 5 to 10 drops). Can densify loose cohesion-less soils, fracture and densify buried building rubble such as that which exists at old building sites, consolidate fine grained soils, and compact buried garbage fills.

78 The pounding creates a depression at each drop location and also produces an areal settlement.