The following figure shows the minimum number of test holes for common types of bridge structures. Do not space test holes more than 300 ft. apart.

In general, drill test holes 15 to 20 ft. deeper than the probable tip elevation of the foundation. Estimate the probable founding or tip elevation from the results of Texas Cone Penetration tests and correlation graphs in and experience with foundation conditions in the area. Pay special attention to major structures where high foundation loads are expected. If the depth of the boring is questionable, consult the Bridge Division for a detailed analysis of the projected foundation loads and foundation capacities.

Stream Crossings.

Structures over channels less than 200 ft. wide are classified as minor stream crossings. For these crossings, place a boring on each bank as close to the water’s edge as possible. If boring information varies significantly from one side of the channel to the other, a boring in the channel may be necessary.

Major stream crossings require core borings in the channel if no existing data is available. A site inspection by the driller or logger is necessary to evaluate site accessibility and special equipment needs.

Karst Features.

Structures suspected to be in a karst formation may require more borings or geophysical survey.

Grade Separations.

If the structure borings indicate soft surface soils (fewer than 10 blows per foot), additional borings and testing may be required for the bridge approach embankments.

Bridge Field Exploration.

The exploration should include the following:

Test hole spacing. Space test holes near each abutment of the proposed structure plus a sufficient number of intermediate holes to determine the depth and location of all significant soil and rock strata. If you do not get a reasonable correlation between borings (for example, TCP data, stratigraphy), consult with the project engineer to determine the need for additional holes.
Texas Cone Penetration tests. Conduct Texas Cone Penetration tests at 5-ft. intervals beginning at a 5-ft. depth. Standard penetration test data is not acceptable for foundation design using the TxDOT design procedure.
Near surface soil layer test. Test soft near surface soil layers (0 to 20 feet) as directed under the subsection in this chapter titled
Slopes and Embankments
.
Soil and bedrock classification. Fill out a complete soil and bedrock classification and log record for each test hole on the standard log, including all information to complete the form.
Ground water. Include ground water elevation measurements (including date of measurement) as part of the data acquisition. Obtain the groundwater elevation minimum 15 minutes after the initial encounter with ground water. Site conditions may require installation of piezometers to establish a true ground water surface elevation and method of monitoring water surface fluctuations.