Imagine a building site with a multimillion dollar project in full swing and suddenly the concrete for its foundation starts to disappear into a dark sinkhole with depths unknown. Sound like a rare occurrence? Not entirely. While this an extreme, though true case, sinkholes have wreaked havoc in greater or lesser degrees on seven buildings on the University of California Santa Cruz (UCSC) campus.

Sinkhole-filled topography underlies the UCSC campus. It is called “karst topography” and is created when limestone/or marble bedrock dissolves for many thousands of years and leaves deep holes, winding tunnels and narrow fractures. The reaction between acidic groundwater and the underground lime causes the rock to dissolve into the ground water and simply flow away. A swiss cheese pattern is created underground with holes ranging from tiny to huge and cavernous. This geologic phenomenon has allowed many-a-student to have fun in the caves but also proves quite challenging when it comes to constructing large buildings.

I recently had the opportunity to speak with Dr. Gerald (Jerry) Weber about the karst geology of the UCSC campus. He has served as a local planning advisor, taught the UCSC summer geology field camp for 20 years, and has been investigating geologic problems in California for about 50 years. Jerry’s services are used when sinkholes threaten building projects on campus and he has some great stories to tell.

Sink Hole Spotting

The UCSC campus is peppered with sinkholes only noticed by the trained eye. The 50 plus sinkholes on campus are the most noticeable characteristics of the underlying karst topography. When caves grow so large that the weight of the ceiling material is no longer supported, it collapses; thereby creating a sinkhole (doline). Other ways that sinkholes are created are from a settling of surface material over highly fractured dissolving marble or the sinking of soil that has filled widened cracks in marble below.

The sinkholes leave the imagination to wonder to what depths these caves reach. Geologists conducted a 200 foot core sample and still haven’t a clue of the extent of the karst cavities. At first, the UCSC campus terrain seems solid and continuous. Images of rolling grassy hills and massive redwoods disguise the deep voids that could be just 15 to 30 feet below the soil surface.

It’s San Andreas’ Fault

Most people know of the San Andreas Fault that runs almost the length of California but not many are aware of the 33 plus fracture zones and faults that are spread across the UCSC campus. Disguised as dry creek beds, Chancellor Gulch and Jordan Gulch are the largest fracture/fault zones on campus running nearly the entire length of the campus. Movement along these faults creates areas that are pulverized and more permeable. These porous areas make the perfect outlet for groundwater flow in the rainy season and create underground rivers with no destination in sight. Most of the creeks on campus will appear from springs up river and then disappear underground into a closed sinkhole (swallow hole) into the deep unknown.

Where does the Water Go?

No one really knows where all the water goes. Geologists in 1989 drilled wells into the fault/fracture zones to test the amount of stored water. They have found huge water rich aquifers within the karst producing up to 200-300 gallons per minute with little drop in the water table. Despite the sheer amount of water stored right under campus, the city supplies the campus with water for reasons that mystify me in my investigations. Water in karst systems has a high potential for containing contaminants due to lack of filtering (percolating) before reaching an aquifer. Most drinkable water from aquifers has been filled with water that has filtered through layers of clay and other cleansing particles. Instead, the water in karst systems go from cloud, to ground, to storage system much more quickly and with less natural filtration.

Building on Karst

Avoiding underground caverns is challenging when planning for development. Before building on campus is approved, extensive geologic and geotechnical engineering studies are done to gauge the strength of the underlying bedrock. Exploratory drilling to sample the bedrock for voids is done for every major building project on campus. Voids were found before and during construction of eight known structures at UCSC and the weight of a few of these buildings proved too much for the underlying karst.

Prior to the construction of the Science and Engineering Library a large narrow void filled with a toothpaste consistency clay material was discovered at the exact location of the planned library. Construction began after coming to realization that the only way of building over such terrain was to construct an underground bridge spanning the river of clay. Hundreds of students use the library daily without a clue that a river of mud lies just below the foundation of the building.

There is a very large sinkhole in front of the Baskin Engineering building that gives a clue as to what lies beneath the adjacent building. Baskin Engineering was constructed atop a very deep cavern with unknown depths. When construction began the void was discovered and filled with an enormous amount of cement day after day until workers realized that the hole was not being filled at all. Their efforts to fill the hole failed. Instead, cement pillars 300 feet in length were placed deep into the earth in stable bedrock surrounding the void. Its as if Baskin is hanging over the mouth of a hungry giant.

After the construction of the Earth and Marine Building foundation, the weight of the cement foundation proved too much for the hole-filled karst below. In December 1991, just weeks after completion of the foundation, an enormous rain storm whipped through the campus over the New Years Holiday. Construction workers came back from their holiday to discover that their hard work had fallen victim to the inevitability of the karst topography and rainwater combination.

The foundation had given way and collapsed into an underlying cavern. Geologist, Jerry Weber, stuck a 30 foot rebar into one of the open holes and couldn’t feel the bottom of the cavern. Construction workers pumped cement into the exposed holes until the foundation was stable enough to prevent the cavern from eating any more of the building.

Geek Details: Chemistry of the Karst

Rainwater lands on the campus soil where it flows and accumulates tiny dissolved acids and carbon dioxide gas. Groundwater becomes slightly acidic when the carbon dioxide dissolves, producing carbonic acid.

carbon dioxide (CO2) + water (H2O) = carbonic acid (H2CO3)

When the acidic groundwater finally reaches the marble bedrock (cooked limestone), it finds its way into tiny cracks, joints and fault lines. As carbonic acid laced rainwater drizzles through these cracks, the lime within the marble dissolves. 97% of marble on campus is composed of lime which means that the lime loving carbonic acid has potential to drastically change the structure of underground topography by dissolving a large percentage of the marble.

lime (CaCO3) + carbonic acid (H2CO3) = calcium ion (Ca++) + bicarbonate ion (2HCO-3)

Over time, these tiny permeable cracks enlarge and create tunnels and caves large enough to throw a party in. Despite the severe effect that acidic water has on lime, it is not a risk to humans. It only affects the geology so greatly because it has been working at dissolving lime for the past several hundreds of thousands of years (on a geologic timescale).

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This piece is part of the UCSC Natural History Tour. Download the free app with many tours of the Santa Cruz area and beyond.