Chapter two Subsurface Exploration and In-Situ Testing

2-1 INTRODUCTION Investigation of the underground conditions at a site is prerequisite to the economical design of the substructure elements. The elements of a site investigation depend heavily on the project but generally should provide the following: 1. Information to determine the type of foundation required (shallow or deep). 2. Information on the allowable bearing capacity and the potential of total and differential settlement of the foundation. 3. Location of the groundwater table in case it exists in site , groundwater table fluctuations may be required.

4. Identification of potential problems (settlements, existing damage, etc.) concerning adjacent property. 5. Identification of environmental problems and their solution. the primary focus of this chapter is on site exploration for buildings and other structures ,many of the methods are applicable to roads; water, sewer, pipe, etc… 2-2 METHODS OF EXPLORATION The most widely used method of subsurface investigation is boring holes into the ground, from which samples may be collected for either visual inspection or laboratory testing. Several procedures are commonly used to drill the holes and to obtain the soil samples.

2-3 PLANNING THE EXPLORATION PROGRAM The purpose of the exploration program is to determine the stratification and engineering properties of the soils underlying the site. The principal properties of interest will be the strength, deformation, and hydraulic characteristics. The program should be planned so that the maximum amount of information can be obtained at minimum cost. The actual planning of a subsurface exploration program includes some or all of the following steps: 1. Assembly of all available information on dimensions, column spacing, type and use of the structure, basement requirements, any special architectural considerations of the proposed building, and These information can be used to estimate foundation loads.

2.Reconnaissance of the field. This may be in the form of a field trip to the site, which can reveal surface and subsurface information without drilling holes or excavating test pits. The kind of information to be obtained : • The general topography of the site, the possible existence of

drainage ditches • Soil stratification from deep cuts • The type of vegetation which may give an indication of the soil

nature • Ground water levels that can be determined by checking nearby

wells • behavior of adjacent structures such as cracks, possibly sticking

doors and windows • Rock outcrops may give an indication of the presence or the depth

of bedrock.

3. A preliminary site investigation. In this phase a few borings (one to about four) are made or a test pit is opened to establish in a general manner the stratification, types of soil to be expected, and possibly the location of the groundwater table. If the initial borings indicate that the upper soil is loose or highly compressible, one or more borings should be taken to rock or competent strata. There are no hard and fast rules for establishing the location, depth and spacing of bore holes , some guidelines for various types of projects are detailed below: For buildings One boring at each corner of the structure and one in the middle Depth beneath the loaded area equals 1.5 to 2.0 times the least

dimension of structure For deep excavations the depth of boring should be at least 1.5 times the excavation depth For Roads Bore holes of depth 2-3 m along alignments with 100-500 m spacing

4. A detailed site investigation The preliminary borings and data are used as a basis for locating additional borings that will be required to delineate zones of poor soil, rock outcrops, fills, and other areas that can influence the design and construction of the foundation. Sufficient additional soil samples should be recovered to refine the design and for any unusual construction procedure required by the contractor to install the foundation. In the detailed program phase it is generally considered good practice to extend at least one boring to competent rock if the overlying soil is soft to medium stiff. This is particularly true if the structure is multiple-storied or requires settlement control. 2-4 SOIL BORING The choice of boring technique depends on the type and extent of information desired from exploration, the general type of soil, and the amount of money and time available. Exploratory holes into the soil may be made by hand tools, but more commonly truck- or trailer-mounted power tools are used.

Hand Tools The earliest method of obtaining a test hole was to excavate a test pit using a pick and shovel. 1- For small jobs, where the sample disturbance is not critical, hand or powered augers (Fig. 2-1) held by one or two persons can be used. Hand-augered holes can be drilled to depths on the order of 2 to 5 m, although depths greater than about 8 to 10 m are usually not practical. Commonly, depths are, as on roadways or airport runways, or investigations for small buildings.

Fig.2-1 posthole and helical auger

Mounted Power Drills For numerous borings to greater depths and to collect samples that are undisturbed, the only practical method is to use power-driven equipment. Wash boring is a term used to describe one of the more common methods of advancing a hole into the ground. A hole is started by driving casing (Fig. 2-2) to a depth of 2 to 3.5 m. Casing is simply a pipe that supports the hole, preventing the walls from sloughing off or caving in. The casing is cleaned out by means of a chopping bit fastened to the lower end of the drill rod. Water is pumped through the drill rod and exits at high velocity through holes in the bit. The water rises between the casing and drill rod, carrying suspended soil particles, and overflows at the top of the casing through a T connection into a container, from which the effluent is recirculated back through the drill rod. The hole is advanced by raising, rotating, and dropping the bit into the soil at the bottom of the hole. Drill rods, and if necessary casing, are added as the depth of the boring increases. Usually 6 m or less of casing is required at a hole site. This method is quite rapid for advancing holes in all but very hard soil strata.

Fig.2-2 wash boring

Rotary drilling : is another method of advancing test holes. This method uses rotation of the drill bit, with the simultaneous application of pressure to advance the hole. Rotary drilling is the most rapid method of advancing holes in rock unless it is badly fissured; however, it can also be used for any type of soil. Drilling mud (such as bentonite) may be used in soils where the sides of the hole tend to cave in. Continuous-flight augers with a rotary drill are probably the most popular method of soil exploration at present (Fig. 2-3) . • core drill as a screw conveyor to bring the soil to the surface. • The method is applicable in all soils • Borings up to nearly 100 m can be made with these devices,

depending on the driving equipment, soil, and auger diameter. • The augers may be hollow-stem or solid with the hollow-stem type

generally preferred, as penetration testing or tube sampling may be done through the stem.

• borings do not have to be cased using continuous-flight augers, and this feature is a decided economic advantage over other boring methods.

Fig.2-3 continuous auger

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2-5 SOIL SAMPLING The most important engineering properties for foundation design are strength, compressibility, and permeability. The samples obtained in the field can be classified as “disturbed” or “un-disturbed”. Practically speaking, all samples are disturbed to some degree due to the difficulty of removing soil from great depths. However, with some degree of care and expert field personnel, some samples may yield useful results to determine the following engineering parameters: A) From undisturbed samples: - Consolidation; - Permeability - Shear strength. B) From disturbed samples: - Grain size analysis; - Atterberg limits (plastic and liquid limits); - Specific gravity of solids; - Classification of the soils.

Disturbed Sampling of all Soils In recognizing the difficulty and resulting expense of obtaining undisturbed samples, it is common practice on most foundation projects to rely on penetration in-situ tests and, depending on the method, recovery of disturbed samples for obtaining an estimate of the soil conditions. • The standard penetration test (SPT) • Cone penetration test (CPT) • Vane shear test (VST) • The flat dilatometer test (DMT) • The pressuremeter test (PMT) and many others A simple test pit (Fig.2-4) may also be used to recover a quality sample of cohesionless soil since it’s nearly impossible to recover undisturbed samples,It is very inexpensive, but it provides a visual experience for the geotechnical engineer.

Fig.2-4

2-6 THE STANDARD PENETRATION TEST (SPT) The standard penetration test, is currently the most popular and economical means to obtain subsurface information . The test consists of the following: 1. Driving the standard split-barrel sampler a distance of 460 mm into the soil at the bottom of the boring. 2. Counting the number of blows to drive the sampler the last two 150 mm distances (total= 300 mm) to obtain the N number. 3. Using a 63.5-kg driving mass (or hammer) falling "free" from a height of 760 mm.

The sum of the blow counts for the next two 150-mm increments is used as the penetration count The boring log shows refusal and the test is halted if 1. 50 blows are required for any 150-mm increment. 2. 100 blows are obtained (to drive the required 300 mm). 3. 10 successive blows produce no advance. When the full test depth cannot be obtained, the boring log will show a ratio as 70/100 or

50/100 indicating that 70 (or 50) blows resulted in a penetration of 100 mm.

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Fig.2-5 SPT test

The SPT Split Spoon Sampler. This sampler is used in conjunction with the SPT system. The SPT blow count is the number of blows of the hammer, required to advance the split barrel sampler 12 inches (305 mm) into the ground.

SPT CORRELATIONS The SPT has been used in correlations for • unit weight γ • relative density Dr • angle of internal friction Φ • undrained compressive strength qu • bearing capacity of foundations • the stress-strain modulus Es Table 2-1

TABLE 2-2

In granular soils, the N value is affected by the effective overburden pressure σ’v, For that reason, the N value obtained from field exploration under different effective overburden pressures should be changed to correspond to a standard value of σ’v ; that’s:

Ncorr =CN.N

Where: N corr: corrected N value to a standard value of σ’v CN : correction factor, should be ≤1.0 N :value obtained from the field

CN=9.78√1/ σ’v , σ’v in KN/m2

CN=√1/ σ’v , σ’v in ton/ft2

2-7 CONE PENETRATION TEST7 (CPT) The CPT is a simple test that is now widely used instead of the SPT particularly for soft clays, soft silts, and in fine to medium sand deposits. The test is not well adapted to gravel deposits or to stiff/hard cohesive deposits.

the test consists in pushing the standard cone (see Figs. 2-6) into the ground at a rate of 10 to 20 mm/s and recording the resistance. The total cone resistance is made up of side friction on the cone shaft perimeter and tip pressure. Data usually recorded are • the cone side resistance qs • point resistance qc.

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Fig.2-6

Fig.2-7

The measured point resistance qc and side friction qs are used to compute the friction ratio fr as:

CPT Correlations The CPT test data are used to classify a soil, to establish the allowable bearing capacity of shallow foundation elements, or to design piles.

For cohesive soil

1. One correlation between the cone bearing resistance qc and

undrained shear strength su is based on the bearing capacity equation is as follows:

where po = γz : overburden pressure point where qc is measured as previously defined and used. (if qc is an effective pressure, use p'o).

fr= qs/qc *100

Su= qc-Po/Nk

Nk = cone factor (a constant for that soil). Nk has been found to range from 10 to 30 range ,the Figure is a correlation based on the plasticity index Ip which might be used, where soil sensitivity St=10/fr

Cone factor Nk versus Ip plotted for several soils with range in sensitivity

noted.

Fig.2-8

2.Soil classification

Fig.2-9

Cohesionless soil 1. Relative density (Dr)

Fig.2-10

2.Angle of internal friction Φ

Fig.2-11

Example 2-1. Classify the soil (on Figure 2-7) at the 10- to 12-m depth. Also estimate the undrained shear strength su if the average y = 19.65 kN/m3 and Ip =10.0 for the entire depth of the CPT. It is known that the profile is entirely in cohesive soil. Solution: qc is eye estimated at average depth=11.0 m (from fig.)

2-8 FIELD VANE SHEAR TESTING The vane shear test VST is a substantially used method to estimate the in situ undrained shear strength of very soft, sensitive, fine-grained soil deposits The test is performed by inserting the vane into the soil and applying a torque after a short time lapse, on the order of 5 to 10 minutes

The maximum torque applied T can be related to the undrained strength (Su or Cu) of a clayey soil:

For actual design purpose, the field vane shear values should be corrected as:

Su (corrected) = λ Su (field) where λ is a correction factor function of the soil plasticity index λ = 1.7 - 0.54 log(PI) , PI plasticity index

Fig.2-12

Undisturbed Sampling in Cohesive Soils Recovery of "undisturbed" samples in cohesive soils is accomplished by replacing the split spoon with thin-walled steel tubing commonly referred to as shelby tubes(610 mm in length X 51 to 89 mm in diameter) , which should be pushed, but is sometimes driven. Although sample disturbance depends on factors such as rate of penetration, whether the cutting force is obtained by pushing or driving, and presence of gravel, it also depends on the ratio of the volume of soil displaced to the volume of collected sample, expressed as an area ratio

Where: : outside diameter of the tube

: inside diameter of the tube Ar should be less or equal 10 percent

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Another term used in estimating the degree of disturbance of a cohesive or rock core sample is the recovery ratio Lr:

Lr ≈1.0 Rock of good quality Lr≤ 0.5 Badly fissured or soft rock Example 2-2. What is the area ratio of the 51-mm diam. thin walled sample tube? Solution. Using dimensions from a supplier's catalog, obtain OD = 50.8 mm and ID = 47.7 mm. Direct substitution into Equation gives:

2- GROUNDWATER TABLE (GWT) LOCATION Groundwater affects many elements of foundation design and construction, so if The GWT is encountered in a borehole during a field exploration it should be established as accurately as possible if it is within the probable construction zone. The GWT is generally determined by directly measuring to the stabilized water level in the borehole after a suitable time lapse, often 24 to 48 hr later. This measurement is done: • by lowering a weighted tape down the hole until water contact is

made. In soils with a high permeability, such as sands and gravels, 24 hr is usually a sufficient time for the water level to stabilize

• install a piezometer (small vertical pipe) in soils with low permeability such as silts, fine silty sands, and clays, where it may take several days to several weeks (or longer) for the GWT to stabilize.

For silty soil: The following technique (Hvorslev 1949) may be used to determine the ground water level:

1. Bail out water in the borehole to a level below the estimated GWT 2. Observe the water levels in the borehole at times

t=0;t=t1;t=t2;t=t3 note that t1-0=t2-t1=t3-t2=∆t 3. Calculate ∆h1, ∆h2 and ∆h3 4. Calculate :

5.Plot the values h0,h2 and h3 above the corresponding water levels And determine the average water level in the borehole

Fig.2-13

2- Geophysical Exploration Several types of geophysical exploration techniques are now available for a rapid evaluation of subsoil characteristics. they permit rapid coverage of large areas and are less expensive than conventional exploration by drilling, however in many cases, definitive interpretation of the results is difficult for that reason, these techniques should be used for preliminary works only. Geophysical methods include: • Seismic methods: seismic refraction, seismic reflection method… • Electrical Resistivity method • Gravimeter Surveys • Ground Probing Radar surveys Seismic refraction survey Seismic refraction method is the most useful of these methods in obtaining preliminary information about the thickness of the layering of various soils at a given site , the location of ground water table, the depth to rock… It’s conducted by : • Impacting the surface at point A (in the fig.2-14)by hammer blow

or by a small explosive charge

• Observing the first arrival of the disturbance (stress P waves)at several points away from point A (B,C,D)

Knowing the velocities of P waves in various layers give an idea of the types of soil or rock that are present below the ground surface

Fig.2-14

The velocity of P waves in a medium can be given by the equation:

To determine the velocity (v) of p waves and the thicknesses of those layers, the following procedure is used: 1. Obtain the times of first arrival , t1,t2,t3 …at various distances

x1,x2,x3… from the field 2. Plot a graph of time (t) against distance (x).the graph will be

like the one in fig.(2-15)

3.Determine the slopes of the lines ab,bc,cd

4.Determine the thickness of the top layer as :

5.Determine the thickness of the second layer:

Where Ti2 is the time intercept of the line cd in fig.

Two limitations in refraction surveys need to be kept in mind: 1. The previous equations are based on the assumption that

v1<v2<v3 2. If the presence of ground water table is not known, the p wave

velocity through water(about 1500m/sec) may be erroneously interpreted to indicate a stronger material than what’s present in-situ

Fig.2-15