Datos de investigación

A few case histories are offered here to provide food for thought in helping to reduce the number of

"changed conditions" claims. The specific names of projects are avoided to eliminate any embarrassment to anyone and because the point is not who did it, but what happened, why it happened, what was the impact, and what lesson was learned. Not all of the cases ended in court, most were settled by negotiation.

time to do otherwise. He must decide, from your data, what equipment and methods he will use and, most importantly, he must "quantify" into specific dollars to do the job based on your data. Consider if you could do this yourself from your data.

Case 1

A quarry site for protective rock on a major dam was designated by the owner. Drill hole logs and cores were provided but no tests were made on the rock to see if it would perform as specifications required. The owner apparently assumed that the rock as exposed in outcrops was satisfactory.

Unfortunately, the contractor was told that if he did not use the designated quarry, any other one he used would have to meet stringent specific tests before it would be approved. The contractor agreed to use the designated quarry, but it did not perform acceptably and much excess excavation was needed to obtain sufficient rock. The claim was large, requiring 9 years through legal channels to settle. Had the owner made tests such as the contractor would have been required to do, or had even a single test blast been made, the problem may well have been foreseen and avoided.

Case 2

An interstate highway was to be built. Specifications were issued showing the alignment and some designated quarries from which road-base rock and asphalt aggregate was to be taken. After award of contract, the owner belatedly noticed that the alignment passed through a large basalt flow outcrop. Instructions were issued to the contractor not to use the previously indicated quarry, but to use the road cut rock as the quarry for road base. Geologic data made available at the road cut consisted of a previous small road cut in basalt, plus 3 or 4 offline borings ranging from 7 to 21 feet deep, which had been drilled to determine the depth of overburden and its suitability for backfill material. When the contractor opened the road cut, the basalt proved to be a relatively thin flow cap overlying an earlier mudflow. The volume of rock was insufficient and almost impossible to work.

Another pit several miles away had to be used. The ultimate claim paid was between $1 and $2 million.

A few pre-bid borings to determine the real depth of rock and a trial blast, probably not exceeding

$25,000 cost, would have averted this delay and claim cost.

Case 3

A major dam was to be built in a very rugged and difficult access area. The site investigations included a very through job for the dam and appurtenant structures. No geologic study was made of the access road, only an alignment was indicated. The contractor assumed no problems when he bid, but when the access road was constructed much of it encountered sidehill dipping rock layers that were unstable when the road undercut them. The contractor was delayed months in beginning work on the dam because of the difficulty in reaching the site.

Case 4

A cut-and-cover section was to be constructed for a rapid transit system. The material to be excavated was alluvial overburden covering transitional residual soil, grading into hard metamorphic rock. Boreholes were located at approximately 300-foot intervals. All of the boreholes, except one near the middle of the route, indicated bedrock fairly shallow. The one anomalous hole indicated soft soil below the invert and ended in soil. An interpretive bedrock profile in the geologic report seemed to indicate a broad, deep swale in bedrock. During construction, the one anomalous hole proved to be located in a narrow vertical fault zone and the bedrock was found to be at a nearly flat grade through the area. The result was extra rock excavation, time delays and expensive changed condition costs.

Lesson to be learned -- never leave an unsolved anomalous bit of geologic data in your site investigation. Two or three additional check holes around the anomalous hole would have shown the real bedrock surface elevation.

Case 5

A deep cut-and-cover, large-diameter pipeline was to be built. Borings along the route were made from 500 to 1000 or more feet apart and many of them were 50 to 100 feet off the direct trench line.

The materials were described in general terms as lightly consolidated alluvial fan and outwash materials, lenticular in nature and containing lenses of hard cemented caliche. Water table measurements were made and shown on the logs. With one or two exceptions, the water table was shown to be below invert or only slightly above it. Bids for construction were let about two years after the site investigations. Construction encountered high groundwater through much of the trench and a tremendous amount of caliche type rock excavation where soils had been anticipated. A changed conditions claim of considerable magnitude was submitted.

In this case, the borehole exploration was totally inadequate to permit reasonably accurate estimates of the amount of caliche to expect and plan for. Secondly, subsequent to the site investigation, some of the originally open country crossed by the pipeline route was gradually developed for housing.

Thus, an area previously of very low precipitation was being subjected to year round irrigation with consequent rise of the water table. A claim resulted.

Lesson learned is that it is important to convey to bidders what we think they will encounter at the time of actual construction. We cannot expect them to make such predictions or interpretation for themselves.

A major building structure was to be built in a broad alluvial valley. Some twenty or more 4- to 6-inch diameter borings were made for the foundation investigation. From those data it was decided to put the structure on caissons placed into the alluvial soils. All of the boring logs were included with the bid documents. Since no boulders were encountered in the borings, none of the logs showed any boulders. During construction, nearly every caisson drill hole (36" diameter) hit one or more large boulders. The cost for changed conditions adjustment was large.

In this case, the geotechnician for the drilling confined his efforts to accurately logging the small diameter boreholes and did not look around the area surrounding the job site. Had he looked around, he would have seen boulders exposed on the surface within 100 yards and seen that the site lay in the path of discharge from an adjacent canyon with an intermittent stream.

The lesson is that we must always look beyond the site being investigated as well as within it and under it.

Case 7

A long tunnel was to be constructed for a sewer for a large midwestern city. Approximately 28 borings were made. The area is in a broad area of glacial deposits. None of the logs showed any boulders. Subsequently, the tunnel boring machine which was selected for this job (because no boulders were indicated) encountered much trouble and delay from boulders. It is true that the logs were accurate, but they were obviously misleading. In this case the city entered into a contract with a local firm of consultants to make the site investigation. In the contract, the consultants were told that, in addition to the specific factual data, their report should, because of their familiarity with the area, indicate any opinions concerning possible problems a contractor might have in constructing the tunnel, such as water or boulders, etc. The report included no opinions on boulders, although the consultants presumably were aware that the morainal material of the area commonly contained boulders.

In this case the contractor, instead of submitting a differing site condition, filed a "third party, breach of promise suit" alleging that the consultant's failure to fulfill the terms of the contract with the city resulted in damage to the contractor. In this case the contractor lost, but the situation could occur again. We should furnish our pertinent opinions as well as our data to contractors.

Case 8

A submarine launching facility was to be built on the East Coast. A limited number of borings were made to determine the nature of the overburden and the depth of the bedrock. The bidders were given a simple, non-detailed, contour drawing of the bedrock. The structure was to be constructed by setting contiguous sheet-pile coffer cells in rows extending outward from the shore and closed by a connecting row across the ends. The cells were to be filled with soil materials of low permeability.

cells. Excavations began, and before completion, a blow-in occurred under the base of one cell, filling the area with water and driving the workers out. Ultimately, the leak was sealed with tremie concrete plus underwater work with divers. Final cleanup and completion of excavation showed the details of the bedrock topography and the considerable difference from what had been previously indicated.

The nature of the contacts of sheet piles with bedrock was such that it was surprising that more leaks did not occur.

Considering the magnitude and complex nature of this structure, a much more detailed site investigation and analysis of geologic data should have been made.