CABRILLO Historic Structures Report

Figure 18. Metal shutters in the open position allow visual access through observation slots.

BASE-END STATION, BATTERIES CALEF & WILKESON (Preservation)
BATTERY COMMANDER STATION, BATTERIES CALEF & WILKESON (Preservation)

Location

These structures are located on a bluff above the Bayside Trail. The base-end station is the lower of the two structures, while the battery commander's station is the higher.

History

These structures were among the first base-end stations built at Fort Rosecrans. Base-end stations were part of a remote network that tracked targets and relayed data to their associated guns via a plotting room. Recommended in 1914, they first appear on a 1915 map of the fort's defenses. The original temporary structures were replaced c.1920 with concrete walls and tar and gravel roofs. During World War II the tar and gravel roofs were replaced with earth-covered rough aggregate concrete roofs. Cemented boulders on the roofs provided additional camouflage. The stations were abandoned in 1942.

Description

Exterior

These one story, eight foot square reinforced concrete structures sit partially into the hillside (figure 18). The rough concrete roofs are camouflaged with earth and edged with eight inch diameter stones embedded into the concrete. On each structure, four metal awning shutters cover observation slots which pierce the smooth concrete walls on three sides. Square roof hatches with metal covers resting on concrete curbs access the structures. Lush vegetation conceals these structures from view.

Interior

The single, square rooms each feature a concrete slab floor, and reinforced concrete walls and ceilings with visible formwork lines. Steel rung ladders, embedded into the concrete walls, lead from the roof hatches into the spaces below. These spaces contain corner cable pits; low, concrete, octagonal DPF (depression position finder) bases with surmounting wood swivel benches; and miscellaneous conduit attachments (figure 19). The survival of the wood swivel benches is rare. The base-end station features a vertical electrical raceway at the south end of the west wall, and a horizontal chase in the south and east walls. The battery commander's station has a metal raceway at the east and south walls connecting to an electrical box in the southwest corner. Military graffiti remaining on the base-end station walls presumably marked orientation data for the operators.

Condition: Base-End Station

Exterior

Curing cracks from the initial construction and corrosion jacking mar the camouflage roof. Weather and exposure has widened these cracks, causing additional damage. Concrete spalling is typical at the concrete headers. Metal corrosion covers the top hatch hardware, especially the bolts and edges. The concrete hatch curb is badly spalled. The metal shutters have surface corrosion and all hardware is deteriorated. Vegetation covers the roof drainage channels.

Interior

Concrete walls and ceilings are solid. Water seepage has contributed to failing paint and small attachment spalls at the south wall. The wooden bench and concrete DPF base are in good condition.

Condition: Battery Commander Station

Exterior

The large aggregate concrete walls are cracked and spalled. The camouflaged roof edges have missing stones, which have resulted in corroding exposed rebar. The top hatch, overgrown with vegetation, features minor metal corrosion and a seriously spalled and cracked concrete curb. The observation slot sills and headers are in good condition, but the metal shutters have spot corrosion. The south shutter is severely corroded at the east end. Light lichen growth covers some of the roof.

Interior

The interior features numerous concrete spalls: spalls caused by rusted ferrous metal near the concrete surface; eight minor spalls at metal attachments at the south wall; and ceiling spalls where conduit is attached. The ladder rungs at the west wall are corroded and are staining the adjacent wall. Hairline cracks and efflorescence occur at the north concrete wall. The wooden bench and concrete DPF base are in good condition.

Structural Analysis

As noted above, the concrete surfaces have significant cracking and spalling, particularly at the front and side areas of the roof, where the large stones were cast into the concrete. The square reinforcing rod is exposed in several places. This deterioration does not appear to compromise the capacity of the structure at this time, however it does reduce the capacity of the anchors which support the steel shutters. The deterioration at the front and sides of the roof appears primarily as a result of shrinkage over time of the concrete surrounding the large stones. This shrinkage would have caused cracks around the stones, which would have then allowed water, salts and air to penetrate down to the rebar. The reinforcing rod would have then begun to corrode, and the resulting expansive forces would then have spalled the stones. As noted above, the cracking at the hatch curb at the battery commander station is reasonably advanced. Other than supporting the hatch, this curb does not have a significant structural function. The remaining cracks and spalling at the interior concrete surfaces are relatively minor and do not at this time appear to compromise the structure's capacity.

Figure 19. Embedded metal ladder rungs provide access from the hatch opening above.