Mechanics of the Test

The test procedure calls for the input of specified impact energy to drive a split tube soil sampling barrel of specified dimensions a required distance. A schematic of the current test is shown in Figure 2.1. The test consists of a falling weight or hammer of mass 63.5 kg (140 lbs) that is allowed to drop (free-fall) a distance of 0.76 m (30 in.) to strike an anvil. The anvil is connected to a set of drill rods that extend to the depth of testing/sampling. A sampling barrel, usually called a split spoon, is attached to the end of the drill rods and is advanced into the soil with each impact of the hammer.

In typical practice, chalk marks are made on the drill string to mark off three 0.15 m (6 in.) increments; the hammer is raised and dropped; the number of hammer blows to advance the spoon each 0.15 m (6 in.) increment are recorded as N₀_₆; N_s_₁₂; and N₁₂__lg. The initial

0.15 m (6 in.) penetration is considered by ASTM as a “seating” penetration and the sum of the hammer blows for the second two 0.15 m (6 in.) increments is called the SPT N value (with units of blows per 0.3m or blows per ft) and is the reference measurement obtained from the test. It is important that the incremental blow count values for each 0.15 m (6 in.) be recorded and reported, which is actually required by ASTM D1586.

In the event the full 0.46 m (18 in.) of penetration cannot be achieved, ASTM D1586 allows the test to be terminated if

Figure 2.1 Schematic of SPT.

• 1. A total of 50 hammer blows have been applied during any one of the three 0.15 m (6 in.) increments;
• 2. A total of 100 hammer blows have been applied; or
• 3. There is no observed advance of the sampler during the application of ten successive hammer blows.

If only partial penetration occurs, the penetration resistance is recorded as the number of hammer blows for the penetration increment achieved, for example, “50 for 2 in.”. In very soft clays, the spoon and drill rods may advance on their own without driving. This is often referred to as “Weight of Rod” (WOR). If the spoon and rods advance after the hammer has been attached, this is referred to as “Weight of Hammer” (WOH). When this occurs, it is important that the water level onside the boring be noted on the boring logs and the size of drill rods being used are recorded. The total mass of the spoon and rods may include partially buoyant weight for that length below the water level.

After driving, the spoon is brought to the surface and opened, and the amount of soil retrieved for that drive is recorded. This is known as recovery. The recovery ratio, R, is defined as

where:

L_s = length of recovered soil in the spoon L_d = length of spoon drive

Recording the recovery is considered a part of the ASTM procedure and should be recorded on the boring logs. The recovery ratio may be used qualitatively to help interpret the SPT results. For example, if the recovery ratio is consistently low in coarse-grained deposits, this may suggest that abundant gravel or cobbles may be present with particles too large to enter the spoon. After recording the Recovery, the soil is usually placed into water-tight containers such as glass jars or bags or is wrapped in plastic and aluminum foil for preservation and then transported back to the office or laboratory.

Equipment

The mechanics of the SPT described in the previous section represent a relatively simple concept; however, because the test is perceived as being so simple, the execution of the test may vary widely. This is a result of the variations in test equipment that have been available and are currently being used in the field to conduct the test. As indicated in Figure 2.1, the test equipment consists of four basic components: the drop weight or hammer with an anvil, a string of drill rods connecting the sampler to the hammer, and a barrel sampler.

Hammer

The purpose of the hammer system is to provide the specified amount of energy to the rods in order to advance the sampler. Because this energy may be created in a number of ways, SPT hammer systems typically represent the largest equipment variation observed in the test. Historically, different styles of drop hammers have been used to perform the SPT. In fact, this has caused considerable consternation among users and represents the largest source of variability to test results. Up to about 1970 pin weight hammers were used, but in the past 40years, three primary hammer types have been used in North America. Figure 2.2 shows the schematics of (1) donut hammer; (2) safety hammer; and (3) automatic hammer. At present, most hand-operated drop hammer systems have largely been retired and are no longer used in routine practice. The automatic hammer is the single most important improvement to the SPT in the past 50years and now considered the preferred and in many cases, the required drop hammer system for use in performing the SPT.

In other parts of the world, similar “free-fall” hammers are used. In European countries, these include the Pilcon trip monkey and the Borros AB drop-hammer. In Japan and other parts of Southeast Asia, the Tombi method (e.g., Shi-Ming 1982) is used.

A typical automatic hammer is a self-contained, totally enclosed device that operates using a hydraulically powered chain lift mechanism. The drive weight-lifting and dropping sequence is actuated by the operator using a hydraulic valve switch. This makes the hammer fully automatic, and the test is essentially operator-independent. This type of hammer appears to give the most reproducible results and avoids problems associated with the friction losses of the rope, cathead, pulley, etc. and the variations of hammer drop height. The author recommends that, when possible, only automatic hammers be used to conduct the SPT.

Figure 2.2 SPT drop hammers.

Drill Rods

Rods used to conduct the SPT are usually standard drill rods used for routine test borings. ASTM D1586 requires that flush joint drill rods having a stiffness (moment of inertia) greater than that of A rods be used. Drill rods are available in a wide variety of lengths, diameters, and thread designs. Drillers have traditionally used square thread drill rods that have been in use for many years. However, in more recent years, tapered thread drill rods have become increasingly popular since they can be assembled and disassembled more quickly.

Split Barrel Sampler

The split barrel or spoon used for the SPT is an open-ended thick-walled drive sampler. The barrel is designed in two halves and split along its longitudinal axis, hence the name split spoon. The two halves are held together by a hardened drive shoe threaded onto the lower end and a drill rod adapter head threaded onto the upper end. A ball check valve and ports are contained in the head of the spoon so that if the spoon is lowered into a borehole filled with water or drilling fluid, the fluid will exit out the top end. This allows the soil to enter the spoon during driving and prevents fluid from becoming trapped in the spoon.

Most split spoons are available from suppliers in lengths varying from 0.46 to 0.61 m (18 to 24 in.). A longer spoon allows some room for loose soil cuttings that may be at the bottom of the borehole to enter the spoon and not affect the penetration resistance. The configuration of the split-spoon is shown in Figure 2.3. Typically, the sample is held in the spoon with a small metal or plastic catcher that is equipped with spring action “fingers” and positioned just inside the shoe. To remove the sample from the spoon, the drive head and shoe are removed (usually with a pair of pipe wrenches), and the two halves of the spoon are separated.

The standard size split spoon sampler recommended for use with the SPT in the U.S. has an outside barrel diameter of 50.8 mm (2in.) and an inside barrel diameter of 38.1 mm (1.5 in.). In the U.S., the drive shoe has an inside diameter of 34.9 mm (1.375 in.). This I.D. is smaller than the I.D. of the barrel for a standard 50.8 mm (2 in.) spoon and creates a slight relief inside the barrel. This relief of the barrel is thought to have originated sometime

Figure 2.3 Schematic of the SPT split spoon sampler. (After ASTM DI586.) around the latter 1960s to allow the use of a series of brass liners to be added to the spoon. (Note that Fletcher (1965) indicated that the inside diameter of the barrel was the same as the inside diameter of the shoe as shown in Figure 2.1.) Ireland et al. (1970) also described the “standard” barrel sampler as having a constant internal diameter of 34.9 mm (1.375 in.),

i.e., the same I.D. as the shoe. This appears to be a routine practice in the U.K.

It is important to know whether the spoon is manufactured with this relief or if liners are being used during the SPT since this can affect the N-value by changing the frictional forces as the soil enters the spoon. The international reference test procedure for the SPT shows the

I.D. of the sampler barrel to be the same as that of the shoe, i.e., 34.9 mm (1.375 in.). In the UK, Japan, and other parts of the world, it appears that samplers with a constant I.D. equal to that of the drive shoe are more common.