Subsurface Exploration Program

With regard to the scope of the subsurface program for a structure, one must carefully consider the small cost of a boring in relation to the foundation cost. A 65-mm (2½-in) diameter drill hole will cost less than one 0.3-m (12-in) diameter pile. Yet the knowledge gained from that boring will permit the use of proper design techniques that may allow the cost-effective use of shallow foundations and elimination of all piles for that structure.

The number, depth, spacing and character of tests to be made in any individual exploration program are so dependent upon site conditions and the type of structure and its requirements that no rigid rules may be established. Conceptually, the exploration program must provide sufficient information to allow the geotechnical engineer to assess the strength and compressibility of the soils within the zone of influence of the foundation. To evaluate these design considerations, the geotechnical engineer will need site-specific data at each pier, abutment and retaining wall, as to the material type, the density or consistency of the material, and the location and depth of ground water.

The scale of the exploration and testing program usually depends on the importance and criticality of the structure, the magnitude of the loading, the availability of the budget and funding and the limitations of project schedule. The exploration and testing programs for foundations supporting less critical structures will be less extensive than those for critical structures such as bridges, elevated highways or official buildings, etc.

The following program will produce the minimum foundation data for a typical structure site.

Soft ground conditions may require undisturbed sample explorations or in situ testing.

1. As a minimum, advance one drill hole at each pier or abutment that measures over 9 m (30 ft) in length. The drill hole pattern should be staggered at the opposite ends of adjacent footings. Piers or abutments over 9 m (100 ft) in length require one drill hole at the extremities of each element. The drill holes should be advanced using techniques that will allow in situ tests to be conducted in accordance with the test standards and that produce a hole that is stable. Appropriate drill techniques may include mud rotary with or without casing and continuous flight augers. Cone penetration tests (CPT) may be substituted for drilled test holes but should not be relied on solely as a method of obtaining subsurface data at a given bridge site. The CPT is not appropriate for all ground conditions, such as very dense soils or soil profiles with cobbles and boulders. Also, the CPT recovers no samples, so soil descriptions are based on empirical correlations, and no soil classification is possible.

However, the readout of data is essentially continuous, so definition of soil layers is better than drilling and split-spoon sampling alone.

2. Estimate the boring depth from existing data obtained during the terrain reconnaissance phases in conjunction with minimum requirements that are

established to help ensure that the borings are not terminated too soon or in a soft soil layer. The borings should also extend past the estimated depth of influence (Section 3.1.5), unless rock is encountered first. Minimum requirements may include criteria such as these:

• The borings for structure foundations shall be terminated when a minimum SPT resistance of 20 blows per 0.3 m (1 foot) on the sample spoon has been achieved for 6 continuous meters (20 feet) of drilling, or

• The boring shall extend 3 m (10 feet) into rock having an average recovery of 50 percent or greater.

Such minimum criteria should be developed based on the design requirements of the local area and the expected subsurface conditions. More stringent criteria, such as higher blows for longer depths of penetration, should be specified where high lateral load conditions may be present (e.g., impact or seismic loads).

3. Obtain standard split-spoon samples at 1.5-m (5-ft) intervals or at changes in material.

Split-spoon samples at 0.75-m (2½-ft) intervals are recommended for a minimum

4.5-determination of strength or consolidation parameters, but they can be used for general index tests (e.g., moisture content, Atterberg limits and grain-size analysis).

Undisturbed (e.g. thin-walled or Shelby tube) samples should be obtained at 1.5-m (5-ft) intervals in at least one boring in cohesive soils. For cohesive deposits greater than 9 m (30 ft) in depth, the tube sample interval can be increased to 3 m (10 ft). In soft clay deposits, in situ vane shear strength tests are recommended at 1.5- to 3-m (5- to 10-ft) intervals for measurement of intact shearing resistance.

4. Record the Standard Penetration Test data on each drill hole in accordance with ASTM D-1586. This is the most economical method presently available for procuring useful data, regardless of the frequently cited drawbacks of the test.

5. Instruct the drilling crew to perform a rough visual analysis of the soil samples and record all pertinent data on a standard drill log form. The disturbed split-spoon samples must be carefully sealed in plastic bags, placed in jars and sent to the soil mechanics laboratory for analysis. Undisturbed tube samples must be sealed and stored upright in a shockproof, insulated container normally constructed from plywood and filled with cushioning material.

6. Observe the water level in each boring, then record the depth below the top of the hole and the date of the reading on the drill log for the following:

a. Water seepage or artesian pressure encountered during drilling. Artesian pressure may be measured by extending drill casing above the ground until flow stops. Report the pressure as the number of meters (feet) of head above ground.

b. Water level at the end of each day and at completion of boring.

c. Water level 24 hours (minimum) after hole completion. Long-term readings may require installation of a perforated plastic tube before abandoning the hole.

A false indication of water level may be obtained when water is used during drilling and adequate time is not permitted after hole completion for the water level to

stabilize. In low-permeability soils, such as clays, more than 1 week may be required to obtain accurate readings. In this case, the boring should be completed as a

piezometer so that water levels can be measured over time.

The reasons for obtaining this minimum data are clear; the engineer must have adequate data to determine the soil type and relative density or consistency, as well as the position of the static water level. Methods such as driving open-end rod without obtaining soil samples or taking water-level readings after the last soil sample was removed must be discouraged. Good

communication between the driller and the foundation engineer is essential during all phases of the subsurface investigation program.